Capacitor Discharge Tool - for small, low voltage caps?

[kentone]

Hi Everyone,
I've just read through the existing threads regarding the safest way to discharge capacitors. We have a bunch of electrical techs and engineers working on low voltage products (like printed circuit boards, low voltage power supplies, etc.). When they open up these units (after unplugging them from line power) they need a way to safely and immediately discharge the remaining stored energy in those capacitors. We're talking 5 V up to maybe 110 V. I understand there are several methods for discharging that have been posted here and elsewhere:

• Using a multimeter and lightbulb to drain capacitance
• Hooking up a wire up to a resistor (10ohms, 100 ohms, etc.) and touching that to any capacitor leads to drain them
• Tapping a screwdriver across capacitor leads to short them out
• Waiting 5 or 10 minutes for the draining of energy to occur naturally

Time is of the essence, as sometimes a customer is standing over them waiting for their repair to be done. Unfortunately, the techs will not ask their customers wait an extra 10 minutes for these to discharge. I don't have the power to change this part (corporate culture driven), so I need to come up with an efficient and safe alternative. The idea of having to make their own resistors or using the lightbulb method will also not fly with them.

The screwdriver method appears to be sanctioned by some qualified persons I've spoken to (and read) given the low voltage nature of their work. However I'm not ready to recommend that, given that a screwdriver was intended to screw and unscrew screws, not handle voltage. (Just like it's not supposed to be used as a lever, pry bar, or hammer). I've been looking (without any success) for a tool that was designed to safely discharge (short) capacitors, similar to how a screwdriver would work but designed and intended solely for that purpose. I'm aware there are large, high voltage discharge rods out there for large, utility type caps that are associated with transformers, however they wouldn't work on this scale. We need something that will fit on their work bench and in the palm of their hand.

Please offer any experience or advice on this topic. If I'm SOL, offer that feedback too.

 

[chris kennedy]

I'm bumping this.

However I'm not ready to recommend that, given that a screwdriver was intended to screw and unscrew screws, not handle voltage. (Just like it's not supposed to be used as a lever, pry bar, or hammer). I've been looking (without any success) for a tool that was designed to safely discharge (short) capacitors, similar to how a screwdriver would work but designed and intended solely for that purpose.

Welcome to the Forum, I appreciate the above statements and the quest for the right tool for the job.

 

[hurk27]

Back when I did more electronic work, I took a common 12 auto tester, removed the lamp, and found a round wire wound 10 OHM 2 watt resistor that fit inside it, out of the handle it had a lead with an clip on it to connect to one side of the cap, or chassis ground If applicable, and the point of the test end made a good contact point to touch to the cap, worked good for me, and I always kept it on my bench.

 

[dbuckley]

I do something similar to that, though not so well constructed.

Once discharged, I put a short across the caps, because they will self-recharge a bit, and you can get a surprise if not careful and the conditions are right.

 

[kwired]

solenoid type voltage testers will discharge capacitors.

 

[zog]

Or maybe a Capacitor discharge tool, they make these specifically for this purpose.

http://www.cementexusa.com/pdf/CCDT.pdf

 

[renosteinke]

ZOG, that's a very nice bit of kit, and if I ever see a capacitor that is big enough, I'll consider it.

Fact is, I can't see even getting that gizmo into any of our power cabinets- let alone using it on the thumbnail-size circuit board mounted capacitors I think the OP is talking about.

Our gear has circuiting ("freewheel diode," etc.) specifically place there for the very purpose of draining off the stored charge. Still, the equipment manufacturer advises a 3-second wait before you open the cabinet (or, if the cabinet is open, before working on the specific component).

Someone's in a hurry? Too bad. Dead-shorting the capacitor tends to cause other parts to let their smoke out - and I'm sure the customer doesn't like to see that happen. It sounds like the 'culture' needs to invest in some tech training.

 

[petersonra]

I never heard of anyone using anything other then a screwdriver or similar metal object.

It's perhaps not the safest way to discharge something that might have a high voltage on it, but for most lower voltage stuff it seems like a reasonable thing to do.

Depending on just what it was I might just use an appropriate sized resistor and clip it to the cap leads with insulated alligator clips.

Chances of getting a serious shock seem limited in most cases when dealing with the average capacitor circuit, but there are a few cases where you could get hurt.

I am not sure I would want to make a such a big deal out of it that the techs look silly because that would discourage them from taking more appropriate precautions when needed.

BTW, I see nothing wrong with using a screwdriver as a lightweight pry bar, or the screwdriver handle as a lightweight hammer. I think we get way to obsessive about this kind of thing now and then. A tool is a tool. A person qualified to use it is going to know what an appropriate use of the tool is. if he is not qualified to make that judgement, he should not be doing the work in the first place.

 

[zog]

ZOG, that's a very nice bit of kit, and if I ever see a capacitor that is big enough, I'll consider it.

Fact is, I can't see even getting that gizmo into any of our power cabinets- let alone using it on the thumbnail-size circuit board mounted capacitors I think the OP is talking about.

There are also smaller ones, the point is you want to have 2 paths, one with a limiting resistor so you don't damage the circuitry, and a second for completing the discharge that bypasses the resistor.

 

[hockeyoligist2]

I have a problem with "sometimes a customer is standing over them waiting for their repair to be done.

Working on anything with someone looking over my shoulder is one of my pet peeves. They could be injured, they could cause you to be injured by distracting you. I have stepped back many times and asked people to leave the area for safety's sake.

 

[nakulak]

I have a problem with "sometimes a customer is standing over them waiting for their repair to be done.

I keep a couple of "party poppers" in my toolbag for "overseers"

(not in explosion proof areas, though)

 

[SG-1]

You may be able to use this Fluke SV225 in you multi-meters. As a added bonus you can watch the voltage decay. The device places a 3K ohm load between the voltage probes.

 

[hurk27]

I keep a couple of "party poppers" in my toolbag for "overseers"

(not in explosion proof areas, though)

When I worked in maintenance benching boards we had this green maintenance labor kid who was always noising around my bench even after being told not to touch, because of some of the test equipment was quit expensive, so one day I figured I teach him a lesson got some aluminum wool and pull a little apart and too a old battery from a back up system but still held a charge, I think is was 24 volts, anyway, I ran two wires from the battery to the bench one connecting to the wool, the other to an old burnt out servo board, connecting it to the edge ground reference, I carefully placed it on top of the wool making sure it didn't touch it in the wrong place.

me and a couple other guys went across from the maintenance crib and stayed out of site, sure enough here he came strolling along, in he went, but then our supervisor wasn't too far behind him, but by the time the super caught up to him it was too late he moved that board and poof, he didn't know what hit him, and as soon as he turned around there was the super, who also jumped at the flash, never saw a guy get so chewed out before in my life. but before it got to far we told the super what I did, he was cool and laughed about it, but still wrote up the labor for not obeying orders.
He never went into the maintenance crib again without permission

 

[SG-1]

I keep a couple of "party poppers" in my toolbag for "overseers"

(not in explosion proof areas, though)

Had a storeroom attendent that liked to come onto the Test Floor without permission to deliver material. One evening as he was strolling by some energized control circuits a 900 micro-F cap blew, right in front of him. It was installed upsidedown & had been "cooking" for several hours.

He never crossed that chain again. Even if we told him it was safe, he would just stand there with the box at arms length.

 

[kentone]

There are also smaller ones, the point is you want to have 2 paths, one with a limiting resistor so you don't damage the circuitry, and a second for completing the discharge that bypasses the resistor.

Thanks for the email and link. Yes indeed, I have been looking for the smaller ones which you speak of. The link showed the smallest of these to be a 24" pole with a 6' grounding lead. I've sent an email to the company to inquire about a much smaller, benchtop version of this.

By chance have you yourself seen any smaller capacitor discharge tools in person or online?

Thanks again for your help.

 

[kentone]

Fluke SV225

Fluke SV225

You may be able to use this Fluke SV225 in you multi-meters. As a added bonus you can watch the voltage decay. The device places a 3K ohm load between the voltage probes.

Thanks SG-1 for the excellent idea. I will look into it.