Best Way To Discharge Capacitors When Repairing - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: August 03, 2025, 01:42:46 pm
content: This topic has been discussed, but wanted to get a better understanding regarding safety of making sure capacitors are discharged when repairing a unit.Back when I worked on CRT style monitors (a very short career as flat screens were being introduced), most circuits had bleeder resistors across the main capacitors. At the time I didn't know much and thought a unit that was unplugged was safe to handle without anyone telling me otherwise. Due to this combination, I dove into the repair without concern.At another job a certain model monitor would have cracked PCBs; ironically cutting off the connection to the bleeder resistor(s). This is where I began learning unplugged electronics doesn't mean it's safe. The common practice was to take a screwdriver and slide it around the PCB until we heard a zap.Moving forward, my buddy told me his amplifier "exploded" the other day (I'm assuming a capacitor exploded) and I'll probably be working it. I'm guessing it has really large capacitors because it weighs 65lbs.Now that I'm more educated, I know the screwdriver approach isn't good including it can place voltage from the capacitor onto other components that will be damaged.Manually measuring the capacitors can be risky as more fiddling with the PCB can result in a shock.What are some of the best ways to discharge a capacitor and/or confirming they are discharged? Maybe I can build something so I can have it for now and future needs.

author: ledtester
author_rank: Super Contributor
timestamp: August 03, 2025, 02:29:50 pm
content: This guy built his own...The BEST Capacitor Discharge Tool I ever had! - Mend It Markhttps://youtu.be/pNkEchLQWDUSchematic attached. Skip to about 1:40 for a discussion of the circuit.A second design from Aliexpress uses a FET for the load and discharge rate is limited to ~ 100mA -- tested by IMSAI Guy:#1764 Capacitor Discharge Pen -- IMSAI Guyhttps://youtu.be/yvG1yGgTN6o

author: madires
author_rank: Super Contributor
timestamp: August 03, 2025, 02:33:06 pm
content: Capacitor Discharger de Luxe with dual polarity level indicator:Works pretty well. I already had a small project box with a discharge power resistor and two sets of 4 mm jacks, one for probe leads to discharge the cap, and the second one for connecting a DMM to monitor the voltage. So I only had to add the circuitry with the indicator LEDs.

author: wraper
author_rank: Unknown
timestamp: August 03, 2025, 02:47:31 pm
content: Quote from: ledtester on August 03, 2025, 02:29:50 pmTo me circuit looks not only overengineered for what it is but also quite stupid in its operation. It cannot even fully discharge capacitors without pressing a button. If you're throwing a bunch of parts at the problem, at least you could make a decent tool without quirks.

author: SteveThackery
author_rank: Super Contributor
timestamp: August 03, 2025, 05:21:43 pm
content: A piece of wire with a 10 ohm resistor in it and a multimeter-style probe on each end. Connect straight across any capacitor you are concerned about.If I was working on a vaccum tube circuit I'd probably bump up the resistor to 330 ohms or so.

author: floobydust
author_rank: Super Contributor
timestamp: August 03, 2025, 07:09:45 pm
content: Safety is #1 with a cap discharge tool, keep it caveman simple. Only resistors used to discharge. Wirewound resistors are OK with (5W,10W) good for 5x overload for short period to 5 seconds. For 25W-100W, 10x overload to 1 second and then derate.I recommend using quality 25-50W resistors. The cheap chinese ones are sand and epoxy filled with big air bubbles common. Their isolation voltage is also scary because well, it's assembled crooked! It's best to use quality brands.RX24 50W some have a dielectric voltage rating of 2,000VRMS for 1 minute, this is with either lead to case. Arcol Ohmite HS50 is 2,500VRMS 1 minute 3,500VACpk, element 1,250V max. Stackpole KAL50 I think is 3kV. Under 1A peak discharge current I thought is reasonable.My idea to add an LED and piezo buzzer that indicate if there is over 5V present. Used a big PTC and zener, so hitting it with 1,000V is OK. If all that fails, the big resistors are independent and still do their job.There are around three voltage/energy categories I designed to. It's a bit of math for the resistor energy, peak power and time needed. You can't cover it all with one resistance value.Solid-state audio power amps say dual 50-85V rail and 20,000uF cap. I use around 100Ω and 25W resistor. That's 0.85A peak current, 72W and 10 second 5tau.A danger is discharging only one rail, you can get backfeed from the other rail- which damages the amplifier. So it's best to discharge both rails together, i.e. use a DPDT switch and have reverse diodes across the rails.SMPS primary and tube amps I use around 900Ω 50W.HV tube gear, commercial microwave ovens is another world, a special design.Mr. Carlson's cap discharge tool and that is quite heavy duty. Four of 50W resistors, on his Patreon channel. He was concerned about the resistor's isolation voltage failing, an arc from a resistor to the housing/metal box, so he mounted them to a metal (heatsink) plate and then on standoffs in a plastic box.

author: bostonman
author_rank: Super Contributor
timestamp: August 03, 2025, 11:58:46 pm
content: I assumed lower value resistors and high wattage to ground, but this is all certainly more helpful information. Regardless if I look at this amplifier or not, going forward, I need to practice discharging capacitors.Usually looking at capacitors, large size equals lots of voltage, small equals much lower voltage. Has anyone ever seen a capacitor that can have lethal voltage stored in it (several hundred volts perhaps) and be physically small?My concerns are:> Not recognizing which other capacitor or capacitors may be holding a charge that can be lethal. Usually I assume the capacitor that's several inches in height and width is the dangerous one and the rest are safe.> Does any other way exist to discharge a capacitor without getting so close to it?I know the screwdriver idea is not good, but, just using it as an example. I like this method because I have very little chance at getting zapped since my hand is far from the PCB and it's just swinging the screwdriver around hitting every point on the PCB. Trying to connect or touch wires to leads has much higher risks. Is the only real way to connect (or touch) to the capacitor leads?Also, assuming the capacitor is disconnected from the circuit (i.e. no load), what is the longest anyone has see a capacitor hold a charge (I'm talking in general, any value/voltage).I remember about five-years ago obtaining a free flat screen. I opened it, and, if I remember correctly, it had short large value green capacitors. The TV was unplugged for a few days and maybe I used the screwdriver trick (I wouldn't do it now though) and got a spark causing me to realize even after two or three days they caps can still be dangerous. I want to say they were only about 1" in height but were 150uF and 250V (I threw them in a box to keep and added them to my inventory list - this is why I know the value and voltage - but not sure which box they are in to see their size); hence why I'm asking about whether a capacitor needs to be large in size to have lethal voltage stored on it.

author: ledtester
author_rank: Super Contributor
timestamp: August 04, 2025, 03:30:15 am
content: Dave's own video on the subject:EEVblog 1655 - How to Discharge Capacitors SAFELY using a Multimeter!https://youtu.be/pXZtk68B3zYBe sure to watch the video to learn about the features of the multimeter that this method relies on.QuoteSize can be deceiving. See this video starting at 0:38...Capacitor Safety - How to Discharge Capacitors Safelyhttps://youtu.be/eQsfuS0VjJE?t=38

author: floobydust
author_rank: Super Contributor
timestamp: August 04, 2025, 03:45:42 am
content: I think it's easiest to simply take a minute to measure the larger capacitor's voltage with a multimeter. Alligator clip to ground and a quick probe, one-handed here and there. I'll recognize the power supply section and go from big to small capacitors.Of course you can miss one. I had a 0.01uF ceramic disk cap, thought it too small to bite but turned out it was at several kV.Crack! and that killed the multimeterThe screwdriver method, well it can cause damage. Shorting things, better not touch anything but the across cap's leads/pins. Molten metal spraying not great for your eyes or a PC board.Attaching a ground wire to a screwdriver, well it depends on where you ground it! Discharging CRT's, you never ground to the circuit board, you ground to the CRT frame/aquadag. At the very least include a resistor in the ground wire to lessen the current.A long time ago in the repair shop, as a prank one tech would charge up can-type electrolytic caps up to 450V (Heathkit cap checker leakage test goes to 600V!), yell CATCH! and then toss the cap at somebody.The danger is catching one with two hands and a hand-to-hand HVDC shock is not good for a person's heart.I just stepped out of the way let the cap roll on the ground, but a few techs got zapped. I don't forget that stunt, totally stupid.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: August 04, 2025, 08:09:31 pm
content: And don't forget, Bostonman, it may have been mentioned already, or in the videos, but discharged caps can recover quite a bit of their charge.

author: Seekonk
author_rank: Super Contributor
timestamp: August 04, 2025, 08:17:02 pm
content: I think an old test probe with five 10 ohm 1W resistors in series stuck in heat shrink and then an allegator clip would be suitable. And like everything else I'd never find it on the test bench when I need it.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 04, 2025, 08:51:07 pm
content: My ultimate capacitor discharge tool is 2W resistor 10-270 R value which I can find on my desk. I use old soviet МЛТ-2 resistors (I have a lot of them) which have size of nowadays 5W resistors.

author: geralds
author_rank: Contributor
timestamp: August 04, 2025, 09:00:38 pm
content: Quote from: ledtester on August 03, 2025, 02:29:50 pmThanks, Mend-It-Mark's Cap Discharger has (also) been on my "projects-to-do" list for some time... Do you have a recommendation for the relais to be used?What is the max V you would use this discharger for?I was also thinking of adding two banana sockets to connect a DMM to monitor the remaining voltage.

author: bostonman
author_rank: Super Contributor
timestamp: August 05, 2025, 02:22:41 am
content: The videos were informative. Although it appears safe to use the multimeter in resistance mode to discharge capacitors, I'd be reluctant to risk my meter.In any case, it seems a PTC might be a better choice to discharge capacitors since a resistor can overheat.Guess I'm still uncertain how others determine which capacitors are high voltage since size is deceiving. If you open a piece of equipment and can't see the capacitors and/or top side of the board, what is the best approach? Also, if a capacitor gets discharged such as a 400V one, what is done to prevent it from self charging to a lethal voltage?

author: twospoons
author_rank: Frequent Contributor
timestamp: August 05, 2025, 03:31:32 am
content: When I was building electric fence energisers we used a piece of plastic pipe as a handle, with a couple of prongs on one end and a 1k 10W resistor inside. Also had a buzzer in it so you could hear when discharge was complete. Used for discharging 100uF 600V bus caps.

author: Shock
author_rank: Super Contributor
timestamp: August 05, 2025, 04:21:38 am
content: As flooby said, measure first. I'd go as far as saying measure, discharge, measure. Don't mess with microwaves and CRTs etc until you understand how to ensure they are bench safe. Never have the DUT connected to mains and discharge it's such an easy mistake forgetting and if you used a straight short on the mains good luck.Learn ohms law and capacitor discharge curves. This allows you to calculate a suitable load resistor to dissipate them slowly and safely. It's a case of how long is a piece of string.Depending on usage case, an old multimeter lead chopped in half with a couple of insulated probes and a beefy resistor half way can be quite flexible. Make it unique then you know its a discharge probe in your hands. You can get croc clips that slip onto probes if you have a lot to discharge and a negative or common rail. Otherwise always measure across the capacitor and not expect everything's referenced to earth/ground/ common somewhere.

author: bostonman
author_rank: Super Contributor
timestamp: August 06, 2025, 02:17:35 am
content: QuoteThese are some great tips. Microwave ovens I wouldn't mess with (although I've tinkered with the display portion of my microwave due to the VFD being dim - and didn't figure out how to repair it). 90% of all electronics I've worked on that have a power supply are ones that I didn't need to worry about charged capacitors.So after discharging, let's say a capacitor on the main that's 400V, does a resistor need to be soldered across in order to prevent it from self charging?As for all the advice, it seems the advice is anything from low ohm resistors to Ray's discharge tool with four 1K resistors in series including using PTCs, leaving me a bit uncertain which direction to go.Looking at my inventory list (need to confirm if I didn't already use them), I have ones similar to the attached photo. I have QTY 5 15ohms/50W, and QTY 1 20ohms/100W. I'm thinking something like small PVC pip, metal tip attached to the resistor(s) inside, sealed, and a wire coming out for ground.

author: madires
author_rank: Super Contributor
timestamp: August 06, 2025, 11:50:36 am
content: The maximum capacitor voltage, the max discharge current and one's impatience determine the value of the discharge resistor. Dielectric absorption can happen, but is rarely a problem. If unsure measure the cap's voltage after discharging and waiting a moment.

author: bostonman
author_rank: Super Contributor
timestamp: August 13, 2025, 03:25:46 am
content: QuoteThe possible upcoming project is the repair of an amplifier, so this is a good warning. Are you saying touch both capacitors (one on each rail) simultaneously? This would mean making two capacitor discharge units and carefully trying to touch both capacitors without shaking.Although I haven't decided which one to build, the one that looks simple is the Capacitor Discharger de Luxe with dual polarity level indicator. I'm uncertain if this is better or worse than the Mend it Mark one.In the video, he suggests relatively small wattage 1k resistors. Assuming a peak voltage of 400V (probably over estimated, but going by some of the voltage values I've seen on capacitors), two 1k in series would mean 200V across each resistor leaving about 40W each resistor.Most likely a 10W or 20W resistor can handle that for a short burst, if it doesn't cool down enough before touching another capacitor, then it may blow due to heat. Why not go with 50W resistors?Also, in the video he used a metal can. My thought for safety would be a plastic case, or maybe a metal case with the metal connected to ground and placed inside a plastic case. I don't have experience with epoxy to select one that would fill the case to protect against the voltage arcing to the person holding it, but thought this would be a great solution providing it also dissipates heat from the resistors.

author: floobydust
author_rank: Super Contributor
timestamp: August 13, 2025, 08:26:15 pm
content: I'm saying with a bipolar power supply, I don't discharge one rail one at a time.In the jig I connect I have reverse-diodes across each rail to protect from backfeed which can cause the other rail to go opposite polarity.In other words, you measure say +40V and -40V across the two filter caps. You start to discharge the +40V rail with a resistor and it goes down to zero as expected. But when you disconnect that resistor, the other cap discharge path can cause the rail to go reverse polarity.If an audio power amplifier is damaged, open transistors, it can draw very little current and just sit there with charged caps. They are more vulnerable. In normal operation, the bias idle current discharges the filter caps. So, like I said always measure first the big caps, they may or may not have charge on them.For a cap discharge tool on bigger power amps, it's reverse-diodes and a DPDT switch connecting two discharge resistors, so you are draining both caps simultaneously. Three wires.It depends really on how expensive the amp is, how big the main caps are lol.Just a side note, I did blow up an old Dynaco ST-120 - any amplifier that uses output coupling caps- those must be discharged as well. They won't if no loudspeaker/load is connected, and even then it can take a very long time or they never actually discharge, no turn on thump. What I found is when power is turned off, the 72V rail falls to 0 but 36V is still in the output cap and because the transistors are off, the cap holds that charge for a long time.

author: bostonman
author_rank: Super Contributor
timestamp: August 14, 2025, 12:42:37 am
content: QuoteWould you recommend the Mend it Mark version? It has the reverse diodes, but not a DPDT.

author: floobydust
author_rank: Super Contributor
timestamp: August 14, 2025, 12:56:30 am
content: It only does one cap at a time and seems silly to put a voltmeter in it. An LED/buzzer does good enough to show voltage is present.

author: bostonman
author_rank: Super Contributor
timestamp: August 14, 2025, 01:18:04 am
content: QuoteDid I quote the wrong one or overlook the name?I'm talking about the one in the video with two LEDs, a string of diodes, and two 1k resistors. It doesn't have a voltmeter inside. It also has a button to bypass one of the two resistors for quicker discharge after the initial discharge.QuoteYou mean one wire to ground, one wire to positive of one cap, and the other wire to the negative to the other filter cap, all simultaneously?As stated, this is good feedback and great to know in order to avoid damaging expensive amps, but certainly lacks the safety I was hoping to have with a well protected discharge tool.

author: floobydust
author_rank: Super Contributor
timestamp: August 14, 2025, 01:39:53 am
content: I'm using the mend it schematic in ledtester Reply #1, with the relay, voltmeter and extra stuff. Ain't nobody got time to watch entire youtube vids for a schematic.Yes I suggest a three-wire discharger for audio power amps with really big filter caps. It's just a dual-rail discharger kind of thing.

author: bostonman
author_rank: Super Contributor
timestamp: August 14, 2025, 01:55:55 am
content: I apologize; thought you watched the videos in the link and completely agree that videos can be time consuming (I have a friend that sends videos in text for the majority of his replies which is very annoying).Attached is the schematic for the tool he used and I was referencing, but, as you pointed out, it only discharges one capacitor at a time.Whether or not I am given this amp to attempt repairing, this is all valuable information. Never would I have thought about discharging both caps simultaneously.

author: 4lfre1re
author_rank: Newbie
timestamp: August 14, 2025, 10:20:40 am
content: This is my tool, works perfect...Cheers... ;-)

author: floobydust
author_rank: Super Contributor
timestamp: August 14, 2025, 07:30:04 pm
content: Quote from: bostonman on August 14, 2025, 01:55:55 amThat circuit looks fine, if the diodes are beefy enough.You can discharge only one (rail) cap at a time but with a bipolar power supply I have seen a rail go opposite polarity.Another example of what I am talking about - typical audio power amp with the usual two fuses, one for each rail. When one fuse blows, you get backfeed and reverse polarity into the circuit and it damages semi's. Say the +ve rail fuse blows, the -ve rail can backfeed into it in the circuit, causing reverse polarity on it.Decent amps include reverse diodes across their rails so this cannot happen. But not all EE's think of this scenario.Same when discharging the caps, you don't want current flow from one rail into the other, when that rail is 0V.I'll sketch what I am talking about with the switch etc. It's only for big scary amplifiers and to prevent wrecking them even more while working on them.

author: bostonman
author_rank: Super Contributor
timestamp: August 15, 2025, 02:41:27 am
content: Negative voltages always confuse me; especially combined with a positive rail.It seems the diodes protect against any negative feedback, but I'm mentally limited on seeing the pathway(s) for negative voltage going into the positive and vice versa.

author: floobydust
author_rank: Super Contributor
timestamp: August 15, 2025, 07:27:02 pm
content: Another way to look at this is considering the filter capacitors to be batteries. You essentially have two batteries in series, midpoint goes to circuit common.Manually discharge one battery dead, and any current from the other will push the dead battery negative. This is only for a short time if you are discharging one cap, then the other approach. But how much backfeed current flows, depends on the amplifier's design. I've seen some semiconductors will get damaged by this. Crappy Peavey.

author: bostonman
author_rank: Super Contributor
timestamp: September 15, 2025, 07:18:47 pm
content: Earlier it was mentioned that discharging one capacitor at a time can cause the other to feed back thus damaging the other rail.What about the opposite, what if I want to bypass the AC and power the circuit off of a DC supply for testing? How do you bring up both positive and negative supplies equally to avoid biasing one rail too much? This assumes something small like a 1 V difference is important.I have a thread regarding a few questions about the amp I'm repairing, so I'm not trying to double back, but thought the question was appropriate for this thread because discharging simultaneously was mentioned.

author: bostonman
author_rank: Super Contributor
timestamp: September 18, 2025, 01:27:26 pm
content: Just wondering if I phrased my question incorrectly or it wasn't worth replying.I'd like to possibly apply power to this board, but remain concerned about bringing up one rail too fast and dumping voltage into the other rail.

author: madires
author_rank: Super Contributor
timestamp: September 19, 2025, 01:42:24 pm
content: Possibly because there's no short answer. From a design point of view it's clear that one rail can fail or its voltage might rise slower at power-on (and fall faster at power-off) because it's loaded by additional circuitry. A good design would take this into account and add safeguards if needed. However, there are also bad designs. Typcially I connect first a dummy load when working on an audio amp to check for any surprises. So far I didn't have to discharge or power up both rails simultaneously.

author: bostonman
author_rank: Super Contributor
timestamp: September 20, 2025, 05:00:26 pm
content: QuoteAhhh okay. Your explanation made me realize it wasn't a great question to ask nor was it worded well.Guess maybe I was thinking more on the lines of a general answer such as: since it was suggested/recommended to discharge both rails simultaneously, what has been the general rule others have followed if/when they power the amp off DC? i.e. are both knobs turned simultaneously, slowly, and kept within X volts of each other, etc....I could just set the external power supply to something small, such as 5V, power it off, make the connections, and then turn on the power supply. That would certainly (and hopefully) allow for doing my part in bringing the rails up together.

author: Shock
author_rank: Super Contributor
timestamp: October 04, 2025, 07:43:01 am
content: There are dual tracking bench supplies and programmable supplies, variacs and you can make soft start circuits if you need. Same for loads.For certain voltages you can bodge together a supply or buy more expensive high voltage bench supply.Needing a resistor/shut or not depends on the capacitor, it's also a safety concern if you forget it's there. If you see a bunch of caps also consider seemingly innocent voltage doubler or jacobs ladder style supplies.