CRT Safety Issues
Electrical Safety
TVs and computer or video monitors are among the more dangerous of consumer electronic equipment when it comes to servicing. (Microwave ovens are probably the most hazardous due to high voltage at flesh frying and cardiac arresting high power.)There are two areas which have particularly nasty electrical dangers: the non-isolated line power supply and the CRT high voltage.
Major parts of nearly all modern TVs and many computer monitors are directly connected to the AC line - there is no power transformer to provide the essential barrier for safety and to minimize the risk of equipment damage. In the majority of designs, the live parts of the TV or monitor are limited to the AC input and line filter, degauss circuit, bridge rectifier and main filter capacitor(s), low voltage (B+) regulator (if any), horizontal output transistor and primary side of the flyback (LOPT) transformer, and parts of the startup circuit and standby power supply. The flyback generates most of the other voltages used in the unit and provides an isolation barrier so that the signal circuits are not line connected and safer.
Since a bridge rectifier is generally used in the power supply, both directions of the polarized plug result in dangerous conditions and an isolation transformer really should be used - to protect you, your test equipment, and the TV, from serious damage. Some TVs do not have any isolation barrier whatsoever - the entire chassis is live. These are particularly nasty.
The high voltage to the CRT, while 200 times greater than the line input, is not nearly as dangerous for several reasons. First, it is present in a very limited area of the TV or monitor - from the output of the flyback to the CRT anode via the fat red wire and suction cup connector. If you don't need to remove the mainboard or replace the flyback or CRT, then leave it alone and it should not bite. Furthermore, while the shock from the HV can be quite painful due to the capacitance of the CRT envelope, it is not nearly as likely to be lethal since the current available from the line connected power supply is much greater.
Safe Discharging of Capacitors in TVs and Video Monitors
It is essential - for your safety and to prevent damage to the device under test as well as your test equipment - that large or high voltage capacitors be fully discharged before measurements are made, soldering is attempted, or the circuitry is touched in any way. Some of the large filter capacitors commonly found in line operated equipment store a potentially lethal charge.This doesn't mean that every one of the 250 capacitors in your TV need to be discharged every time you power off and want to make a measurement. However, the large main filter capacitors and other capacitors in the power supplies should be checked and discharged if any significant voltage is found after powering off (or before any testing - some capacitors (like the high voltage of the CRT in a TV or video monitor) will retain a dangerous or at least painful charge for days or longer!)
The technique I recommend is to use a high wattage resistor of about 100 ohms/V of the working voltage of the capacitor. This will prevent the arc-welding associated with screwdriver discharge but will have a short enough time constant so that the capacitor will drop to a low voltage in at most a few seconds (dependent of course on the RC time constant and its original voltage).
Then check with a voltmeter to be double sure. Better yet, monitor while discharging (not needed for the CRT - discharge is nearly instantaneous even with multi-M ohm resistor).
Obviously, make sure that you are well insulated!
- For the main capacitors in a switching power supply which might be 100 uF at 350 V this would mean a 5K 10W resistor. RC=.5 second. 5RC=2.5 seconds. A lower wattage resistor can be used since the total energy in not that great. If you want to be more high tech, you can build the capacitor discharge circuit outlined in the companion document: Capacitor Testing, Safe Discharging, and Other Related Information. This provides a visible indication of remaining charge and polarity.
- For the CRT, use a high wattage (not for power but to hold off the high voltage which could jump across a tiny 1/4 watt job) resistor of a few M ohms discharged to the chassis ground connected to the outside of the CRT - NOT SIGNAL GROUND ON THE MAIN BOARD as you may damage sensitive circuitry. The time constant is very short - a ms or so. However, repeat a few times to be sure. (Using a shorting clip lead may not be a bad idea as well while working on the equipment - there have been too many stories of painful experiences from charge developing for whatever reasons ready to bite when the HV lead is reconnected. More below.) Note that if you are touching the little board on the neck of the CRT, you may want to discharge the HV even if you are not disconnecting the fat red wire - the focus and screen (G2) voltages on that board are derived from the CRT HV.
WARNING: Most common resistors - even 5 W jobs - are rated for only a few hundred volts and are not suitable for the 25kV or more found in modern TVs and monitors. Alternatives to a long string of regular resistors are a high voltage probe or a known good focus/screen divider network. However, note that the discharge time constant with these may be a few seconds. Also see the section: Additional Information on Discharging CRTs.
If you are not going to be removing the CRT anode connection, replacing the flyback, or going near the components on the little board on the neck of the CRT, I would just stay away from the fat red wire and what it is connected to including the focus and screen wires. Repeatedly shoving a screwdriver under the anode cap risks scratching the CRT envelope which is something you really do not want to do.
Again, always double check with a reliable voltmeter!T
Reasons to use a resistor and not a screwdriver to discharge capacitors:
- It will not destroy screwdrivers and capacitor terminals.
- It will not damage the capacitor (due to the current pulse).
- It will reduce your spouse's stress level in not having to hear those scary snaps and crackles.
Additional Information on Discharging CRTs
You may hear that it is only safe to discharge from the Ultor to the Dag. So, what the @#$% are they talking about? :-).BTW, don't wash your CRTs even if the Maid complains about the filth until you have confirmed that your 'Dag isn't water soluble (maybe that's why it has 'aqua' in the name!). It may all come off! Wipe off the dirt and dust with a cloth (and stay away from the HV connector or make sure it is discharged first!).
(From: Asimov (mike.ross@juxta.mnet.pubnix.ten).)
'Dag' is short for Aquadag. It is a type of paint made of a graphite pigment which is conductive. It is painted onto the inside and outside of picture tubes to form the 2 plates of a high voltage filter capacitor using the glass in between as dielectric. This capacitor is between .005uF and .01uF in value. This seems like very little capacity but it can store a substantial charge with 25,000 volts applied.
The outside "dag" is always connected to the circuit chassis ground via a series of springs, clips, and wires around the picture tube. The high voltage or "Ultor" terminal must be discharged to chassis ground before working on the circuit especially with older TV's which didn't use a voltage divider to derive the focus potential or newer TV's with a defective open divider.
About Charge Reappearing on Discharged CRT
(From: Joseph Gwinn (JoeGwinn@comcast.net).)The issue with CRTs is that the glass dielectric, having been kept at many tens of kilovolts for years, will store charge deep in the glass, and this charge cannot be eliminated quickly. The phenomena is called "dielectric adsorption" or "soakage". One can short such a CRT for a week, remove the short, and see the voltage magically spring back. So leave it shorted.
Big capacitors can do this as well, especially the big oil-paper capacitors used in HV power supplies. These can store a lethal jolt.
This same phenomena is used in Electret microphones, where a thin layer of Teflon stores the HV charge needed to make the microphone work.
Warning about disconnecting CRT neck board
Some manufacturers warn against powering a TV or monitor CRT without the CRT neck board connected. Apparently, without something - anything - to drain the charge resulting from the current flow due to residual gas ions inside the CRT, the shortest path may be through the glass neck of the tube to the yoke or from the pins outside the CRT to whatever is nearby. There aren't many ions in a modern CRT but I suppose a few here, a few there, and eventually they add up to enough to cause a major disaster at least on some CRTs.This is probably not a problem on small CRTs but for large ones with high high voltages and high deflection angles where the glass of the neck is very thin to allow for maximum deflection sensitivity, the potential does exist for arcing through the glass to the yoke to occur, destroying the CRT.
There is really no way to know which models will self destruct but it should be possible to avoid such a disaster by providing a temporary return path to the DAG ground of the CRT (NOT SIGNAL GROUND!!) via the focus or G2 pins preferably through a high value high voltage rated resistor just in case one of these is shorted.
This probably applies mostly to large direct-view TVs since they use high deflection angle CRTs but it won't hurt to take appropriate precautions with video and computer monitors as well.
CRT Implosion Risk?
Also see the section: Disposing of Dead TVs or Monitors (CRTs and Charged HV Capacitors).(From: Jeroen Stessen (Jeroen.Stessen@philips.com).)
I have checked with our CRT expert and he thinks that any 'normal' type of scratch does not pose any danger. Usual disclaimer applies ... (what is 'normal'?)
The front of the tube is much thicker and stronger than the rear. It has to be, to withstand the air pressure, because the curvature radius is so much larger. You won't break it by throwing a slipper at it. The neck is in fact very easy to break, usually without causing injuries to anyone.
Normally, if the tube should implode, the rimband (the tensioned steel band around the rim of all modern CRTs of any size) prevents the glass from flying outward too far. Every tube type has to pass tests in which it is deliberately imploded and it is checked whether any large shrapnel flies too far out.
What *is* very dangerous is a CRT with its rimband missing, or a CRT which never had a decent rimband in the first place (like some dubious Russian-made samples we once saw). Such a tube should not be handled at all. NEVER ever attempt to remove the rimband for and reason!
I just saw a picture tube that was broken due to dropping the (entire) TV on one corner. In the cone (the backside) there are open cracks of some 3 feet length in total. Nevertheless all the glass is still in its original place and it looks as if no glass has flown outward. The faceplate is still intact. So in this case nobody would have got hurt. I remember reading about Americans (who else?) who tried to shoot CRT's with smaller rifles, with little or no success.
Does this comfort you? Get out the shotgun and have a go at it!
Or, perhaps, the following:
(From: Ren Tescher (ren@rap.ucar.edu).)
Our 6 month old 20" SGI color monitor (model GDM-20D11) lost a fight with a fork lift. The case is intact, the CRT probably still has a vacuum, but the outer layer of glass on the screen is shattered.
Picture Tube Implosion IS Possible - But You Really Need To Work at It!
As noted elsewhere in this document, picture tube implosion is a hazard but under normal conditions, quite unlikely. Someone wrote:"I heard somewhere that in the early days of TV, the tubes had a tendency to implode at the drop of a hat. (Due to poor design?) In order to prevent flying glass, the sets had a plastic sheet in front of the screen. Obviously, modern sets no longer have this. How safe are modern CRT screens in terms of impact damage etc?"Well, it isn't quite as simple as that..... However, even if CRT implosion is one of those highly unlikely events, the downside is that should it occur in just the wrong way, the consequences can be disastrous. So, I wouldn't depend on the experiences below to guide you! Treat a CRT about the same way you would an armed nuclear bomb. OK, well maybe just 10 sticks of dynamite. :-)
(From: Dan Evens (dan.evens@hydro.on.ca).)
In high school, our electronics teacher did a demo for each class. He saved out an old black-and-white tube for each class and set up a place to break it. Put the tube on the ground by a brick wall, with a hammer suspended on a wire from the top of the wall. Did it on the driveway so that the glass would be easier to pick up. The tube was placed image-side down.
First he pulled the hammer back about 20 feet and just let it go. It bounced off the tube. This was to show that such tubes are pretty tough. Then he pulled the hammer back and gave it a pretty good shove, turning his back to the tube and moving quickly away from it. (Let's face it, the guy could probably have found a safer way to do this.)
Palm sized chunks of glass flew 50 feet. The noise was quite impressive. The thickness of the image plate of the tube was also quite impressive. Kind of looked like a porthole on a submarine. This was from the tube of a small black-and-white TV, about 14 inches or so. One of the larger colour models might be a LOT more violent.
If I was handling these things in such a way as to have the possibility of dropping one, I'd insist on body armor and face protection. And if it involves a picture tube, I insist on competent trained professionals for service.
(From: Matthias Meerwein (Matthias.Meerwein@rt.bosch.de).)
They ARE quite safe. I've got several TVs and computer monitors in for repair that had been dropped. None of them had an imploded CRT. The damage encountered ranged from:
- Broken circuit boards, often around the flyback transformer (the most heavy weight part on the board) - This is quite easily repairable.
- Shadow mask inside the tube knocked out of position (mostly in trinitron tubes due to their heavy aperture grille construction) - this renders the tube (and thus usually the set) a dumpster candidate.
- Neck of tube broken of (usually when the set hit the floor back end first) - obviously junk.
- 26 inch color TV with back panel removed placed face-down under a bridge. Dropped a ~10 pound brick from top of the bride (about 10 ft high) into the glass funnel of the tube. Result: Funnel of tube shattered, faceplate intact. All glass shards (most of them rather large) were lying inside the set's cabinet - no flying glass.
- 14 inch B/W computer monitor tube dropped from the second story onto concrete floor, hitting the ground faceplate-first. Result: tube shattered into thousands of small glass particles (the largest ones were about one inch in size), but all debris was located on one heap - none of them traveled farther than about three feet.
(From: Clifton T. Sharp Jr. (agent150@spambusters.ml.org).)
With today's tubes, that's more or less true (although walking through a picture tube plant can be instructive as you hear the exploding tubes). With older tubes it was a hazard. With pre-1960 tubes it was a big one. My old boss in the TV service, who I trusted not to exaggerate about such things, told me stories of setting a picture tube near a second-floor window, having them fall to the sidewalk and literally blow a hole in the sidewalk. I can tell you factually and first-person that although he took few precautions with other things, when he had to "pop" a picture tube in the dumpster he never ever ever did it without safety glasses, a shield and a six-foot piece of heavy pipe. (I stopped working there around 1973.)
Risks from CRT Scratches?
A really deep long scratch or gouge on the CRT face should be considered a serious safety hazard as it may reduce the structural integrity and increase the risk of implosion. However, you would likely need a hammer and chissel or diamond tipped tool to make scratches that deep. It is very unlikely that such scratches could come from any reasonable normal use. Dropping it from a cliff, deliberate use of a glass cutter, the use of a really really BIG hammer, or 12 gauge shotgun, might perhaps be sufficient.This is more of a concern for modern CRTs that usually have 'integral implosion protection' - that steel rimband around the outside near the front. Older CRTs used either (1) a separate safety shield - that laminated glass plate in front of your grandmom's TV - or (2) a second contoured glass panel bonded to the actual tube face. In both of these cases, the second panel is protective and cosmetic but is not part of the structure of the CRT. Therefore, any damage to it does not significantly compromise the tube. In the case of modern CRTs, the steel band in conjunction with the basic tube envelope is used to maintain the integrity of the overall CRT. In addition should implosion occur as a result of catastrophic damage, the rimband will reduce the range and velocity of flying debris.
Also see the section: CRT Implosion Risk?.
BTW, scratches in the CRT have absolutely no effect on X-ray emission. X-rays are blocked long before they come anywhere near the surface and glass has very little effect on their direction. Any scratch deep enough to have any detectable effect on X-ray emission (actually, it would need to be an inch deep gouge) would have caused the tube to implode.
Disposing of Dead TVs or Monitors (CRTs and Charged HV Capacitors)
I don't know what the law says, but for safety, here is my recommendation:Treat the CRT with respect - the implosion hazard should not be minimized. A large CRT will have over 10 tons of air pressure attempting to crush it. Wear eye protection whenever dealing with the CRT. Handle the CRT by the front - not the neck or thin funnel shaped envelope. Don't just toss it in the garbage - it is a significant hazard. The vacuum can be safely released (Let out? Sucked in? What does one do with an unwanted vacuum?) without spectacular effects by breaking the glass seal in the center of the CRT socket (may be hidden by the indexing plastic of the socket). Cover the entire CRT with a heavy blanket when doing this for additional protection. Once the vacuum is gone, it is just a big glass bottle though there may be some moderately hazardous materials in the phosphor coatings and of course, the glass and shadow mask will have many sharp edges if it is broken.
In addition, there could be a nice surprise awaiting anyone disconnecting the high voltage wire - that CRT capacitance can hold a charge for quite a while. Since it is being scrapped, a screwdriver under the suction cap HV connector should suffice.
The main power supply filter caps should have discharged on their own after any reasonable length of time (measured in terms of minutes, not days or years).
Of course around here, TVs and monitors (well, wishful thinking as I have yet to see a decent monitor on the curb) are just tossed intact which is fortunate for scavengers like me who would not be happy at all with pre-safed equipment of this type!
(From: Jeroen Stessen (Jeroen.Stessen@philips.com).)
We have a procedure for disposing of used CRT's. The vacuum must be broken to avoid future implosion, like when it will be crushed by the dumpster truck press. That's NOT funny! One method is to punch or drill a small hole in the anode contact, which is made of a soft metal. But take care of the electrical discharge of the aquadag capacitance first!!!
The other method is to break the stem in the centre of the socket pins. This is the stem through which the tube was pumped empty during manufacturing. It breaks off easily (after you have removed the plastic part around the pins).
You want to avoid making too large holes, like for example from chopping off the entire neck in one blow with a hammer.
Go there Read Allot More...http://www.repairfaq.org/sam/crtfaq.htm#crtcsa
Basic Capacitor Testing
Testing Capacitors with a Multimeter
Some DMMs have modes for capacitor testing. These work fairly well to determine approximate uF rating. However, for most applications, they do not test at anywhere near the normal working voltage or test for leakage. Normally, this type of testing requires disconnecting at least one lead of the suspect capacitor from the circuit to get a reasonably accurate reading - or any reading at all. However, newer models may also do a decent job of testing capacitors in-circuit. Of course, all power must be removed and the capacitors should be discharged. This will generally work as long as the components attached to the capacitor are either semiconductors (which won't conduct with the low test voltage) or passive components with a high enough impedance to not load the tester too much. The reading may not be as accurate in-circuit, but probably won't result in a false negative - calling a capacitor good that is bad. But I don't know which models are better in this regard.CAUTION: For this and any other testing of large capacitors and/or capacitors in power supply, power amplifier, or similar circuits, make sure the capacitor is fully discharged or else your multimeter may be damaged or destroyed!
However, a VOM or DMM without capacitance ranges can make certain types of tests.
For small caps (like 0.01 uf or less), about all you can really test is for shorts or leakage. (However, on an analog multimeter on the high ohms scale you may see a momentary deflection when you touch the probes to the capacitor or reverse them. A DMM may not provide any indication at all.) Any capacitor that measures a few ohms or less is bad. Most should test infinite even on the highest resistance range.
For electrolytics in the uF range or above, you should be able to see the cap charge when you use a high ohms scale with the proper polarity - the resistance will increase until it goes to (nearly) infinity. If the capacitor is shorted, then it will never charge. If it is open, the resistance will be infinite immediately and won't change. If the polarity of the probes is reversed, it will not charge properly either - determine the polarity of your meter and mark it - they are not all the same. Red is usually **negative** with (analog) VOMs but **positive** with most DMMs, for example. Confirm with a marked diode - a low reading across a good diode (VOM on ohms or DMM on diode test) indicates that the positive lead is on the anode (triangle) and negative lead is on the cathode (bar).
If the resistance never goes very high, the capacitor is leaky.
The best way to really test a capacitor is to substitute a known good one. A VOM or DMM will not test the cap under normal operating conditions or at its full rated voltage. However, it is a quick way of finding major faults.
A simple way of determining the capacitance fairly accurately is to build an oscillator using a 555 timer. Substitute the cap in the circuit and then calculate the C value from the frequency. With a few resistor values, this will work over quite a wide range.
Alternatively, using a DC power supply and series resistor, capacitance can be calculated by measuring the rise time to 63% of the power supply voltage from T=RC or C=T/R.
Ray's Notes on Capacitor Testing
(This section from: Raymond Carlsen (rrcc@u.washington.edu)The best technique depends on what the cap is used for. A lot of electrolytics are said to be "leaky" when they are really partially open and just not doing their job. Electrolytics that are actually electrically leaky are not as common. You can take each capacitor out of circuit and test it with a cap checker or even a VOM, but in-circuit testing is faster. I don't like to grab for a soldering iron unless I'm pretty sure the part is bad. Time is money.
I first do a visual inspection and see if any electrolytics are bulging (they -are- leaky and usually get hot), or physically leaking (corrosion around terminals). Bulging caps in a switching power supply are a dead giveaway, but can point to leaky diodes as well. Next, if the unit will power up, I look for signs of filter caps open... hum bars in picture, hum in audio, flickering displays, low B+ but nothing gets hot, etc. You can tell quite a lot by just being observent and a makling a few simple checks. Try all controls and switches... you may get other clues. What works and what doesn't?
If you have an obvious fault... like a reduced vertical scan on a TV set or monitor for example, to find the cap that is starting to open up, you can bridge each of them with another cap, one at a time and see if it corrects the problem. (Experience has taught me that bad electrolytics will not -usually- kill vertical sweep completely.) In a TV set that is several years old or more, there could be more than one cap dried out (open). Check them all.
"Popping" filters (as it used to be called) by bridging the original with a like value is not good practice with solid state electronics. The shock to a live circuit is likely to damage other components, or it could shock the circuit into working again... for awhile. Then you get to sit there like a fool and wait for it to act up again... minutes or weeks later. For small electrolytics, I use a trick of bypassing each one with a small 0.1 to 0.47uF capacitor while the set is running. If I see -any- change in the performance, I KNOW the original is not doing its job (greatly reduced in value or open). Of course if you hit the timing caps, it will upset the vertical oscillator a bit... that's normal. For bigger electrolytics like the one used to feed the yoke or power supply main filters, the only effective way to check them is by substitution with the same or larger capacitance. Turn the set off, connect the new cap into the circuit and power it up again.
As I stated before, leaky caps are actually quite rare... but it does happen. They usually upset a circuit a lot more than open ones. Things tend to get hot quickly if the cap is a filter in a power supply. Shorted tantalums and electrolytics in power supplies can literally explode. Obviously, leaky caps must be removed from the circuit to substitute them for test purposes.
Most of the other types of small capacitors: mylar, disc ceramic, etc. are pretty rugged. It is rare indeed to find them bad. It happens just often enough to keep a tech humble.
Gary's Comments on Capacitor Testing
(From: Gary Collins (collgra@preferred.com).)All an ohm-meter tells you is if the cap is shorted or not if it is an electrolytic of fairly large value it can tell you if a cap is open. I am a tech in a large industrial controls company in the factory service center. We consider any electrolytic cap to be suspect if it's code date is over five years old. We have a Fluke 97 and it is useless for in circuit tests. All a meter like a Fluke 97 can tell you is if the Cap is on the way to being open from electrolyte loss or if it is shorted. Actually not all you need to know. Several other facts you need to know are what is the conductance (internal leakage resistance), it sometimes varies with voltage. You also need to know what a caps power factor is in some cases. That is its ability to pass A.C. This is especially important in computer equipment that has to pass harmonics and noise to ground. Switching power supplies like are found in almost all PC's these days use high frequency voltage converters to regulate voltage. The harmonics and noise produced by this rapid switching heats DC filter caps and causes them to loose moisture from their imperfect seals. This effect causes the capacitor to gradually open or drop in capacitive value.
If you are talking about other types of capacitors you can test their value with a meter but I have seen caps that look good with a meter but break down under voltage. Special cap meters exist that test all these parameters and let you judge whether the cap is good or not but the best test short of that is to replace the cap and see if it works or not. Feel free to ask if that isn't what you wanted to know.
Actually sometimes the best test is to use a oscilloscope to look at what the cap is doing in the circuit.
What About Capacitance Meters?
Simple capacitance scales on DMMs just measure the capacitance in uF and do not test for leakage, ESR (Equivalent Series Resistance), or breakdown voltage. If the measurement comes up within a reasonable percentage of the marked value (some capacitors have tolerances that may be as much as +100%/-20% or more), then in many cases, this is all you need to know. However, leakage and ESR frequently change on electrolytics as they age and dry out.Many capacitance meters don't test anything else but are probably more accurate than a cheap DMM for this purpose. A meter of this type will not guarantee that your capacitor meets all specifications but if it tests bad - very low - the capacitor is bad. This assumes that the test was made with the capacitor removed (at least one lead from the circuit - otherwise other components in parallel can affect the readings.
To more completely characterize a capacitor, you need to test capacitance, leakage, ESR, and breakdown voltage. Other parameters like inductance aren't likely to change on you.
ESR testers, which are for good for quick troubleshooting, are designed to just measure the Equivalent Series Resistance since this is an excellent indicator of the health of an electrolytic capacitor. Some provide only a go/no go indication which other actually display a reading (usually between 0.01 and 100 ohms so they can also be used as low-ohms meters for resistors in non-inductive circuits). See the section: What is ESR and How Can It be Tested?.
Note: always place the test probes on the capacitor terminals themselves if possible. Any wiring between your meter and the capacitor may affect the readings. Although your user manual may state that you can test capacitors in-circuit, other components in parallel with the capacitor can screw up the readings - usually resulting in an indication of a shorted capacitor or excessively large uF value. Removal is best. Unsoldering only one of the pins is adequate if you can isolate it from the circuit.
Substitution is really the best approach for repair unless you have a very sophisticated capacitance meterd.
The March 1998 issue of Popular Electronics has plans for a digital capacitance tester with a range from 1 pF to 99 uF.
The May 1999 issue of Popular Electronics has plans for an "Electrolytic Meter" which will accurately measure the capacitance and allow the determination of some of the other characteristic of large value capacitors - up to several hundred thousand uF. This is basically a time constant based tester using a constant current source.
More About Capacitor Testing than You Probably Wanted to Know
(From: John Whitmore (whit@hipress.phys.washington.edu).)First, you need an AC ripple current source. Then, you tune to the frequency of interest (120 Hz for rectifier power supply filter capacitors is usual) and apply both the AC current and a DC voltage bias. Measure the phase shift between the current and the voltage (for a perfect capacitor, this is 90 degrees) and measure the induced voltage (for a perfect capacitor, this is I*2*pi*f*C).
Take the tangent of the difference of the phase shift and 90 degrees. (This is 'tan(delta)' and appears on the spec sheet for the capacitor...)
Then remove the AC, and crank the DC bias up to the voltage surge rating; measure leakage current. Ramp the DC bias down to the working voltage rating; measure leakage current.
Raise temperature and repeat the capacitance, phase shift, and working-voltage measurements at the max temperature the capacitor is rated for.
Yes, it DOES sound rather elaborate, but that's the test that the manufacturers use.
Back to Capacitor Testing Table of Contents.
Safe Discharging of Capacitors in TVs, Video Monitors, and Microwave Ovens
Why This Matters
It is essential - for your safety and to prevent damage to the device under test as well as your test equipment - that large or high voltage capacitors be fully discharged before measurements are made, soldering is attempted, or the circuitry is touched in any way. Some of the large filter capacitors commonly found in line operated equipment store a potentially lethal charge.This doesn't mean that every one of the 250 capacitors in your TV needs to be discharged every time you power off and want to make a measurement. However, the large main filter capacitors and other capacitors in the power supplies should be checked and discharged if any significant voltage is found before touching anything - some capacitors (like the high voltage of the CRT in a TV or video monitor) will retain a dangerous or at least painful charge for days or longer!
A working TV or monitor may discharge its caps fairly completely when it is shut off as there is a significant load on both the low and high voltage power supplies. However, a TV or monitor that appears dead may hold a charge on both the LV and HV supplies for quite a while - hours in the case of the LV, days or more in the case of the HV as there may be no load on these supplies.
The main filter capacitors in the low voltage power supply should have bleeder resistors to drain their charge relatively quickly - but resistors can fail. Don't depend on them. There is no discharge path for the high voltage stored on the capacitance of the CRT other than the CRT beam current and reverse leakage through the high voltage rectifiers - which is quite small. In the case of old TV sets using vacuum tube HV rectifiers, the leakage was essentially zero. They would hold their charge almost indefinitely.
(From: Edwin Winet (ewinet@softwareresearch.org).)
Some of us work in areas where capacitors are huge, unusual or sometimes both. Many people believe that only "big" capacitors can kill you, knock you across the room, blow a hole in you, or get your attention. Here are a couple of comments:
When a capacitor is safely discharged, do not stop there. Some capacitors, due to their ability to leak---are "dead" after being safely discharged with a "bleeder resistor" of the right value for the job. Using a resistor that is under-rated - wattage-wise - can result in the bleeder going open circuit DURING a discharge sequence LEAVING some energy! High voltage capacitors, or worse yet, high energy-high voltage capacitors require correct wattage AND correct resistance to be bled safely. Also, high microfarad low voltage capacitors can vaporize a screwdriver and spray metal in your eyes. (Adequate voltage margin is also essential for resistors used in high voltage circuits. --- Sam.)
Certain types of capacitors are made of VERY good materials, which can hold a charge for YEARS! Putting away charged capacitors of this type is an invitation to disaster!
Low inductance capacitors that are used in energy pulse circuitry, many times are of the oil-filled high energy/high voltage type. This type can give a MOST un-pleasant surprise AFTER it has been completely drained by a safe bleeding technique. After the capacitor has been bled, IMMEDIATELY short it, from terminal to terminal AND to the external metal can (if applicable)!!! These capacitors RE-charge from their internal fluid and can STILL deliver a lethal, as they "recover" a certain amount of energy! this type of capacitor, or any capacitor of any high (enough) energy value MUST be LEFT shorted.
Be particularly leery of any capacitor with a broken off lead that is sitting in a drawer! Sometimes, these units break off during testing and don't get thrown out - but remain charged - to kill or shock years later.
Lastly, the word "electrocution" is used in many high voltage device writings. That's bad, because it was only intended for the "electric chair", short for electro + execution.
Capacitor Discharge Technique
The technique I recommend is to use a high wattage resistor of about 5 to 50 ohms/V of the working voltage of the capacitor. This isn't critical - a bit more or less will be fine but will affect the time it takes to fully discharge the capacitor. The use of a current limiting resistor will prevent the arc-welding associated with screwdriver discharge but will have a short enough time constant so that the capacitor will drop to a low voltage in at most a few seconds (dependent of course on the RC time constant and its original voltage).Then check with a voltmeter to be double sure. Better yet, monitor while discharging (monitoring is not needed for the CRT - discharge is nearly instantaneous even with multi-M ohm resistor).
Obviously, make sure that you are well insulated!
- For the main capacitors in a switching power supply, TV, or monitor, which might be 400 uF at 350 V, a 2 K ohm 25 W resistor would be suitable. RC=.8 second. 5RC=4 seconds. A lower wattage resistor (compared to that calculated from V^^2 / R) can be used since the total energy stored in the capacitor is not that great.
- For the CRT, use a high wattage (not for power but to hold off the high voltage which could jump across a tiny 1/4 watt job) resistor of a 1 to 10 M ohms discharged to the chassis ground connected to the outside of the CRT - NOT SIGNAL GROUND ON THE MAIN BOARD as you may damage sensitive circuitry. The time constant is very short - a ms or so. However, repeat a few times to be sure. (Using a shorting clip lead may not be a bad idea as well while working on the equipment - there have been too many stories of painful experiences from charge developing for whatever reasons ready to bite when the HV lead is reconnected.) Note that if you are touching the little board on the neck of the CRT, you may want to discharge the HV even if you are not disconnecting the fat red wire - the focus and screen (G2) voltages on that board are derived from the CRT HV.
- For the high voltage capacitor in a microwave oven, use a 100 K ohm 25 W (or larger resistor with a clip lead to the metal chassis. The reason to use a large (high wattage) resistor is again not so much power dissipation as voltage holdoff. You don't want the HV zapping across the terminals of the resistor.
Clip the ground wire to an unpainted spot on the chassis. Use the discharge probe on each side of the capacitor in turn for a second or two. Since the time constant RC is about 0.1 second, this should drain the charge quickly and safely.
Then, confirm with a WELL INSULATED screwdriver across the capacitor terminals. If there is a big spark, you will know that somehow, your original attempt was less than entirely successful. At least there will be no danger.
DO NOT use a DMM for this unless you have a proper high voltage probe. If your discharging did not work, you may blow everything - including yourself.
Reasons to use a resistor and not a screwdriver to discharge capacitors:
- It will not destroy screwdrivers and capacitor terminals.
- It will not damage the capacitor (due to the current pulse).
- It will reduce your spouse's stress level in not having to hear those scary snaps and crackles.
Capacitor Discharge Tool
A suitable discharge tool for each of these applications can be made as quite easily. The capacitor discharge indicator circuit described below can be built into this tool to provide a visual display of polarity and charge (not really needed for CRTs as the discharge time constant is virtually instantaneous even with a muli-M ohm resistor).- Solder one end of the appropriate size resistor (for your application) along with the indicator circuit (if desired) to a well insulated clip lead about 2-3 feet long. For safety reasons, these connections must be properly soldered - not just wrapped.
- Solder the other end of the resistor (and discharge circuit) to a well insulated contact point such as a 2 inch length of bare #14 copper wire mounted on the end of a 2 foot piece of PVC or Plexiglas rod which will act as an extension handle.
- Secure everything to the insulating rod with some plastic electrical tape.
This discharge tool will keep you safely clear of the danger area.
Capacitor Discharge Indicator Circuit
Here is a suggested circuit which will discharge the high value main filter capacitors in TVs, video monitors, switchmode power supplies, microwave oven capacitors, and other similar devices quickly and safely. This circuit can be built into the discharge tool described above (Note: different value resistors are needed for LV, HV, and EHV applications.)A visual indication of charge and polarity is provided from maximum input down to a few volts.
The total discharge time is approximately:
- LV (TV and monitor power supplies, SMPSs, electronic flash units) - up to 1000 uF, 400 V. Discharge time of 1 second per 100 uF of capacitance (5RC with R = 2 K ohms).
- HV (microwave oven HV capacitors) - up to 5,000 V, 2 uF. Discharge time of 0.5 second per 1 uF of capacitance (5RC with R = 100 K ohms)
- EHV (CRT second anodes) - up to 50,000 V, 2 nF. Discharge time of 0.01 second per 1 nF of capacitance (5RC with R = 1 M ohm). Note: discharge time is so short that flash of LED may not be noticed.
(Probe) <-------+ In 1 | / R1 \ 2 K 25 W (LV) Unmarked diodes are 1N400X (where X is 1-7) / 100 K 25 W (HV) or other general purpose silicon rectifiers. \ 1 M 10 W (EHV) | Resistors must be rated for maximum expected +---------+--------+ voltage. For non-continuous use, the wattage __|__ __|__ | can be lower. _\_/_ D1 _/_\_ D5 / | | \ R2 __|__ __|__ / 100 ohms _\_/_ D2 _/_\_ D6 | | | +-----------+ __|__ __|__ __|__ __|__ Any general purpose LED type _\_/_ D3 _/_\_ D7 _\_/_ LED1 _/_\_ LED2 without an internal resistor. | | | + | - __|__ __|__ +-----------+ Use different colors to _\_/_ D4 _/_\_ D8 | indicate polarity if desired. In 2 | | | >-------+---------+--------+ (GND Clip) The two sets of 4 diodes (D1 to D8) will maintain a nearly constant voltage drop of about 2.8-3 V across the LED+resistor as long as the input is greater than around 20 V. Note: this means that the brightness of the LED is NOT an indication of the value of the voltage on the capacitor until it drops below about 20 volts. The brightness will then decrease until it cuts off totally at around 3 volts. WARNING: Always confirm discharge with a voltmeter before touching any high voltage capacitors!
For the specific case of the main filter caps of switchmode power supplies, TVs, and monitors, the following is quick and effective.
(From: Paul Grohe (grohe@galaxy.nsc.com).)
I've found that a 4 watt 'night light' bulb is better than a simple resistor as it gives an immediate visual indication of remaining charge - well down to below 10 V.
Once it stops glowing, the voltage is down to non-deadly levels. Then leave it connected for a little while longer, and finish it off with the `ole screwdriver.
They're cheap and readily available. You can make dozen 'test-lamps' out of an old 'C7' string of Christmas lights (`tis the season!).
Editor's note: where a voltage doubler (or 220 VAC input) is involved, use two such bulbs in series.
(From: Dave Talcott (75711.1537@compuserve.com).)
I built the capacitor discharge tool. I had all the parts to hand except for the series resistor, for which I used a 2 watt axial unit, since the power dissipation is not critical. I decided to package it in probe form for convenience. Except for the series resistor, which lives in a counterbore, everything is surface mounted and communicates through a LOT of cross-drilled holes. A piece of heat-shrink tubing holds everything in place. The only tricky part was making two small recesses to locate the LEDs. The probe tip is a short piece of solid copper wire salvaged from some Romex house wiring and ground to a point.
Voltage Checkers
Whereas a multimeter is intended to measure voltages (and other things), a checker is used mostly to just produce a quick indication of the presense of voltage, its polarity, and other basic parameters. One use is a quick, but reliable indication of the status of the charge on a BIG capacitor. An, example of a simple version of such a device is the "capacitor discharge indicator circuit" described above.(From: Ian Field (ionfieldmonitors@ic24.net).)
The version of the checker that I have, also contains a miniature 12 V battery for continuity checking - any resistance less than about 22K will produce some glow. It's handy for quick checks of semiconductor junctions - in general if it produces a slight glow it's leaky, but transistor B/E junctions have an inherent zener voltage, so there is usually some glow. Also schottky-barrier diodes give a reverse leakage glow - this does not mean they're faulty, check the Vf with the diode-check on a DMM before binning! Any zener diode above 10-11 V can be given a quick test for S/C, lower Vz will produce some glow - again check Vf before binning.
These checkers are getting hard to obtain, most of the component stockists here only carry vastly over complicated (and expensive) versions with built-in measurement computer and LCD - these wouldn't last 5 min's around flyback circuitry! Some Automotive accessory shops have a simpler version with no battery - always check that it's stated to be capable of measuring AC or DC at 4 to 380 V before parting with money! The internal circuit should contain the LED's, a 15 ohm resistor to limit the maximum surge current when the PTC is cold and the special PTC film-thermistor. The battery can be added with a button from a VCR front panel - but don't blame me if you kill yourself because you didn't insulate the added components properly! There is a more complicated non-battery version with 2 LED's close to the front of the handle to indicate polarity and a row of LED's along the length of the handle to indicate the voltage-range. This version contains 2 special PTC's and a discrete-transistor bargraph circuit - there might be room to add a battery inside the case. As for the special PTC this is the only place I've seen them - one possibility that might be worthy of looking into is the Siemens PTC SMPSU startup thermistor for TDA4600 control chips, this usually has a series resistor of at least 270 ohms and is more likely to turn-up in European TV set's, but I have seen it in early Matsushita IBM displays and a few others (possibly Tandon) the PTC thermistor is always blue and looks like a very-miniature copy of the Philips white-plastic PTC degauss thermistor.
Go there Read allot More...http://www.repairfaq.org/sam/captest.htm#ctcds
Don
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