Checking that capacitors are discharged. - UK Vintage Radio Repair and Restoration Discussion Forum

Collaro · 14th Jul 2017, 10:07 am

What do experienced members of the forums do to ensure all capacitors are inert before going in with both hands?

JacKam_ · 14th Jul 2017, 10:40 am

Depends on capacity and nominal voltage, so you know what charge it can store. If it is above 0.0002 [C] AND nominal voltage is > 50 [v] I usually short-circuit all the terminals with a piece if wire.

russell_w_b · 14th Jul 2017, 10:40 am

Use an earthing wand or a Brinkley stick (discharge resistor then short-cct).

russell_w_b · 14th Jul 2017, 10:43 am

Be aware also of dielectric absorption where the charge is regained, and if you're going to be in there some time, leave the capacitor terminal connections strapped together.

Argus25 · 14th Jul 2017, 1:21 pm

I avoid direct short circuiting the capacitor terminals for high capacity and high voltage capacitors. Sparks fly and the terminals get damaged with the analogy of spot weld marks. It also might be possible to damage the capacitor, in some cases, with the high peak current. Its best to have some current limiting so a 47R or 100R 5W resistor is fine for practically any capacitor, connected to two crocodile clips. And as suggested by Russell w b, it is best to leave them shorted if you can, as the terminal voltage can climb up after a brief short due to dielectric absorption.

In a lot of vintage gear, bleeder resistors were added across the main filter caps, so if you wait a few minutes they discharge. But these magically disappeared in a lot of modern switch-mode psu's that can be quite hazardous if not discharged before going in to work on them.

G6Tanuki · 14th Jul 2017, 1:51 pm

Bleeder resistors have an annoying habit of quietly going open-circuit with age.

I've used in the past a homemade 'death stick' - a length of clear plastic tube with a terminal one end, a resistor-chain and a NE-2 type neon, with a trailing croc-clip lead to connect to earth. Poke all likely high-voltage points before getting any closer!

russell_w_b · 14th Jul 2017, 1:52 pm

We never had a problem with directly shorting out capacitors, and I'm talking up to 50uF at 22kV. It was expected they'd be discharged at switch-off. In fact, a direct earth was applied as the interlocked enclosure was entered. The wand was a necessary back-up. It does seem a tad severe on the capacitors, having said that, but the BBC always used direct earthing on their transmitter smoothing enclosures. The RAF RADAR lot use Brinkley sticks and apply the resistor first.

For electrolytics, I would avoid direct discharge and use a one or two incandescent lamps as a discharge resistor (of course, you need to check they work first or you're no better off), so it can be seen to be discharged. Then short it out. I've seen tools damaged by the energy stored in SMPSU mains caps and it isn't pleasant: most certainly a danger to eyes.

My Beethoven exploder has a 5.6k resistor applied to the 8uF 1600V cap if the key is withdrawn with a charge applied.

A quick check with a multimeter is always worthwhile too, before 'diving in'.

kalee20 · 14th Jul 2017, 2:05 pm

I like circuits that discharge their capacitors when powered 'off'. Most do, but there are exceptions.

Valve radios sometimes have a bleeder chain to feed the screen grids of IF and Frequency changer valves, and this does the job fine - but sometimes there is just a dropper resistor, and although when switching off, the valves discharge the HT electrolytics, when the valves have cooled the aforementioned dielectric soakage can cause the voltage to climb again. Battery valve radios can be horrors because the valves only stay hot enough to draw current for half a second!

As to how to discharge, I use a low-value resistor (depending on voltage). Direct short-circuits are good fun, but can cause damage, and zap-marks are unsightly. So I avoid it.

I have seen high-voltage (well, 1600V) capacitors supplied with shorting links for storage, and this is good practice.

G4_Pete · 14th Jul 2017, 2:27 pm

Hi, For simple radio reservoir/filter capacitors I always have one of these hung up in easy reach near my work bench. It performs a controlled discharge with indication of the voltage plus you can remove it and re-apply after a pause to check if there is any significant re-charge. However whatever you use you must test it and trust it, in industry our procedures were to check the checker prior to measurement and after measurement on an approved volt source.
Pete

McMurdo · 14th Jul 2017, 3:19 pm

in with the avo first and then a 100W lightbulb on the end of 2 probes or my drummond test lamp. Then check again with a meter.

I dont do much EHT stuff but ionisers, antistatic generators, fence energisers and insectocutors just get the screwdriver treatment.

The last big electrolytic I discharged was rather by accident, I was checking it for voltage and mone of my probes brushed a metal upstand on a power resistor; something like 10,000uF at 50V and I lost the end of my multimeter probe, the bang made me deaf for a few minutes in one ear. It's not big and it's not clever.

Collaro · 15th Jul 2017, 1:56 pm

Quote:

Originally Posted by russell_w_b

Be aware also of dielectric absorption where the charge is regained, and if you're going to be in there some time, leave the capacitor terminal connections strapped together.

Thank you for all the replies. Useful info as ever!
Dielectric absorption never occurred to me. I wonder if anyone has experienced a shock because of this here on the forum?

AC/HL · 15th Jul 2017, 4:32 pm

This is the internal circuit of a simple probe, similar in construction to the one in post 9. The zeners are 5 watt 1N5333B.

TonyDuell · 15th Jul 2017, 6:19 pm

Quote:

Originally Posted by Collaro

Dielectric absorption never occurred to me. I wonder if anyone has experienced a shock because of this here on the forum?

My late father said he once got a nasty shock from a home-made Leiden jar after he dischaged as a result of it recharging due to dielectric absortion. He was always very careful when discharging a CRT as a result.

russell_w_b · 15th Jul 2017, 8:10 pm

I took a shock off a HV cable I'd pressure tested at 4.6kV d.c. and had not left the earth on for long enough afterwards. It was enough to make me swear, but nothing worse than that.

JacKam_ · 16th Jul 2017, 12:57 am

It is a charge, not voltage that kills. My father used to check CRT HV by touching TC of flyback rectifier (like EY86) with a tip of a screwdriver kept in bare hands. If the sparks were 1 cm long that would mean there is about 15 kV, so OK. ( He is 85 and doing well )

J

hannahs radios · 17th Jul 2017, 6:50 pm

When I worked in the test department of a capacitor manufacturer we used to always check with a meter first then apply 4 lightbulbs in series 4x 100 watt bulbs in series discharged everything although I also had a 50 volt 60watt bulb for LV caps we always reckoned that when the bulbs went out the caps were discharged enough

Collaro · 18th Jul 2017, 6:53 pm

Wow! Interesting

When would that have been, can I be so bold to ask?

FrankB · 19th Jul 2017, 2:21 am

I made a shorting stick with a very dry broken drumstick and a ground wire for reg. caps.
Saved me a couple of times, esp. on microwave oven caps.
Also HV caps can take a charge if wind blows across the terminals! I use thin copper wire to short across them in the junk box. Just enuf to keep any charge from accumulating.
CRT anodes are famous for regaining their charge after they have been discharged too.
(Don't ask).

Craig Sawyers · 19th Jul 2017, 6:03 am

Quote:

Originally Posted by russell_w_b

Be aware also of dielectric absorption where the charge is regained, and if you're going to be in there some time, leave the capacitor terminal connections strapped together.

I found that out the hard way in a very close brush with death. As part of my time at University I designed and built a pulsed CO2 laser. That had a high voltage multiple ceramic "hockey puck" capacitor energy store wired in parallel between two aluminium plates and charged to 20kV. So I turn everything off to dismantle part of it, and short the capacitor bank via a resistor to discharge it. After what I thought was a reasonable time (measuring the capacitor voltage with a meter) I took the resistor short off. A number of minutes later a took hold of the bank and it put my arm most of the way behind my head.

Dielectric absorption found out the hard way.

Craig

turretslug · 19th Jul 2017, 11:02 am

Sort of similar tale re. dielectric absorbtion, I slightly glibly shorted a removed TV CRT ultor dimple to 'dag with a handy test-lead and put it on the side in the physics lab. A few minutes later, I picked it up again, one palm resting on the 'dag, the other palm opposite on the non-'dagged patch, but thumb brushed over the ultor.... Boy, my eyeballs felt like they did the sort of thing you see in Tom and Jerry. Very silly bit of complacency, and lesson firmly embedded. (It only fell a couple of inches onto the wooden bench....)

14th Jul 2017, 10:07 am	#1
Collaro Retired Dormant Member Join Date: Jul 2017 Location: Great Malvern, Worcestershire, UK. Posts: 72	Checking that capacitors are discharged. What do experienced members of the forums do to ensure all capacitors are inert before going in with both hands?

14th Jul 2017, 10:40 am	#2
JacKam_ Pentode Join Date: Jul 2015 Location: Newbury, Berkshire, UK. Posts: 243	Re: Checking that capacitors are discharged. Depends on capacity and nominal voltage, so you know what charge it can store. If it is above 0.0002 [C] AND nominal voltage is > 50 [v] I usually short-circuit all the terminals with a piece if wire.

14th Jul 2017, 10:40 am	#3
russell_w_b Dekatron Join Date: Jul 2007 Location: Penrith, Cumbria, UK. Posts: 3,687	Re: Checking that capacitors are discharged. Use an earthing wand or a Brinkley stick (discharge resistor then short-cct). __________________ Regds, Russell W. B. G4YLI.

14th Jul 2017, 10:43 am	#4
russell_w_b Dekatron Join Date: Jul 2007 Location: Penrith, Cumbria, UK. Posts: 3,687	Re: Checking that capacitors are discharged. Be aware also of dielectric absorption where the charge is regained, and if you're going to be in there some time, leave the capacitor terminal connections strapped together. __________________ Regds, Russell W. B. G4YLI.

14th Jul 2017, 1:21 pm	#5
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	Re: Checking that capacitors are discharged. I avoid direct short circuiting the capacitor terminals for high capacity and high voltage capacitors. Sparks fly and the terminals get damaged with the analogy of spot weld marks. It also might be possible to damage the capacitor, in some cases, with the high peak current. Its best to have some current limiting so a 47R or 100R 5W resistor is fine for practically any capacitor, connected to two crocodile clips. And as suggested by Russell w b, it is best to leave them shorted if you can, as the terminal voltage can climb up after a brief short due to dielectric absorption. In a lot of vintage gear, bleeder resistors were added across the main filter caps, so if you wait a few minutes they discharge. But these magically disappeared in a lot of modern switch-mode psu's that can be quite hazardous if not discharged before going in to work on them.

14th Jul 2017, 1:51 pm	#6
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: Checking that capacitors are discharged. Bleeder resistors have an annoying habit of quietly going open-circuit with age. I've used in the past a homemade 'death stick' - a length of clear plastic tube with a terminal one end, a resistor-chain and a NE-2 type neon, with a trailing croc-clip lead to connect to earth. Poke all likely high-voltage points before getting any closer!

14th Jul 2017, 1:52 pm	#7
russell_w_b Dekatron Join Date: Jul 2007 Location: Penrith, Cumbria, UK. Posts: 3,687	Re: Checking that capacitors are discharged. We never had a problem with directly shorting out capacitors, and I'm talking up to 50uF at 22kV. It was expected they'd be discharged at switch-off. In fact, a direct earth was applied as the interlocked enclosure was entered. The wand was a necessary back-up. It does seem a tad severe on the capacitors, having said that, but the BBC always used direct earthing on their transmitter smoothing enclosures. The RAF RADAR lot use Brinkley sticks and apply the resistor first. For electrolytics, I would avoid direct discharge and use a one or two incandescent lamps as a discharge resistor (of course, you need to check they work first or you're no better off), so it can be seen to be discharged. Then short it out. I've seen tools damaged by the energy stored in SMPSU mains caps and it isn't pleasant: most certainly a danger to eyes. My Beethoven exploder has a 5.6k resistor applied to the 8uF 1600V cap if the key is withdrawn with a charge applied. A quick check with a multimeter is always worthwhile too, before 'diving in'. __________________ Regds, Russell W. B. G4YLI.

14th Jul 2017, 2:05 pm	#8
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: Checking that capacitors are discharged. I like circuits that discharge their capacitors when powered 'off'. Most do, but there are exceptions. Valve radios sometimes have a bleeder chain to feed the screen grids of IF and Frequency changer valves, and this does the job fine - but sometimes there is just a dropper resistor, and although when switching off, the valves discharge the HT electrolytics, when the valves have cooled the aforementioned dielectric soakage can cause the voltage to climb again. Battery valve radios can be horrors because the valves only stay hot enough to draw current for half a second! As to how to discharge, I use a low-value resistor (depending on voltage). Direct short-circuits are good fun, but can cause damage, and zap-marks are unsightly. So I avoid it. I have seen high-voltage (well, 1600V) capacitors supplied with shorting links for storage, and this is good practice.

14th Jul 2017, 2:27 pm	#9
G4_Pete Hexode Join Date: Feb 2014 Location: Southampton, Hampshire, UK. Posts: 419	Re: Checking that capacitors are discharged. Hi, For simple radio reservoir/filter capacitors I always have one of these hung up in easy reach near my work bench. It performs a controlled discharge with indication of the voltage plus you can remove it and re-apply after a pause to check if there is any significant re-charge. However whatever you use you must test it and trust it, in industry our procedures were to check the checker prior to measurement and after measurement on an approved volt source. Pete Attached Thumbnails Last edited by G4_Pete; 14th Jul 2017 at 2:31 pm. Reason: typo

14th Jul 2017, 3:19 pm	#10
McMurdo Dekatron Join Date: Jun 2009 Location: Staffordshire Moorlands, UK. Posts: 5,271	Re: Checking that capacitors are discharged. in with the avo first and then a 100W lightbulb on the end of 2 probes or my drummond test lamp. Then check again with a meter. I dont do much EHT stuff but ionisers, antistatic generators, fence energisers and insectocutors just get the screwdriver treatment. The last big electrolytic I discharged was rather by accident, I was checking it for voltage and mone of my probes brushed a metal upstand on a power resistor; something like 10,000uF at 50V and I lost the end of my multimeter probe, the bang made me deaf for a few minutes in one ear. It's not big and it's not clever. __________________ Kevin

15th Jul 2017, 4:32 pm	#12
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: Checking that capacitors are discharged. This is the internal circuit of a simple probe, similar in construction to the one in post 9. The zeners are 5 watt 1N5333B. Attached Thumbnails

15th Jul 2017, 8:10 pm	#14
russell_w_b Dekatron Join Date: Jul 2007 Location: Penrith, Cumbria, UK. Posts: 3,687	Re: Checking that capacitors are discharged. I took a shock off a HV cable I'd pressure tested at 4.6kV d.c. and had not left the earth on for long enough afterwards. It was enough to make me swear, but nothing worse than that. __________________ Regds, Russell W. B. G4YLI.

16th Jul 2017, 12:57 am	#15
JacKam_ Pentode Join Date: Jul 2015 Location: Newbury, Berkshire, UK. Posts: 243	Re: Checking that capacitors are discharged. It is a charge, not voltage that kills. My father used to check CRT HV by touching TC of flyback rectifier (like EY86) with a tip of a screwdriver kept in bare hands. If the sparks were 1 cm long that would mean there is about 15 kV, so OK. ( He is 85 and doing well ) J

17th Jul 2017, 6:50 pm	#16
hannahs radios Hexode Join Date: Jan 2012 Location: Weymouth, Dorset, UK. Posts: 422	Re: Checking that capacitors are discharged. When I worked in the test department of a capacitor manufacturer we used to always check with a meter first then apply 4 lightbulbs in series 4x 100 watt bulbs in series discharged everything although I also had a 50 volt 60watt bulb for LV caps we always reckoned that when the bulbs went out the caps were discharged enough

18th Jul 2017, 6:53 pm	#17
Collaro Retired Dormant Member Join Date: Jul 2017 Location: Great Malvern, Worcestershire, UK. Posts: 72	Re: Checking that capacitors are discharged. Wow! Interesting When would that have been, can I be so bold to ask?

19th Jul 2017, 2:21 am	#18
FrankB Heptode Join Date: Oct 2005 Location: Olympia, Washington, USA. Posts: 664	Re: Checking that capacitors are discharged. I made a shorting stick with a very dry broken drumstick and a ground wire for reg. caps. Saved me a couple of times, esp. on microwave oven caps. Also HV caps can take a charge if wind blows across the terminals! I use thin copper wire to short across them in the junk box. Just enuf to keep any charge from accumulating. CRT anodes are famous for regaining their charge after they have been discharged too. (Don't ask).

19th Jul 2017, 11:02 am	#20
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,395	Re: Checking that capacitors are discharged. Sort of similar tale re. dielectric absorbtion, I slightly glibly shorted a removed TV CRT ultor dimple to 'dag with a handy test-lead and put it on the side in the physics lab. A few minutes later, I picked it up again, one palm resting on the 'dag, the other palm opposite on the non-'dagged patch, but thumb brushed over the ultor.... Boy, my eyeballs felt like they did the sort of thing you see in Tom and Jerry. Very silly bit of complacency, and lesson firmly embedded. (It only fell a couple of inches onto the wooden bench....)