You may have run across dielectric absorption. Some caps, electrolytics in particular, can have a memory effect so they bounce back after a discharge. That's why keeping a resistor across the terminals is good practice even after you've bled a capacitor.
Dielectric absorption - Wikipedia, the free encyclopedia
Always assume a gun is loaded, always assume a capacitor is charged.
hmmzz... so far most of my experience is in low voltage area. but recently i took up a tube amp project to gain some of that arcane knowledge and some exp with higher voltage stuff. now the main problem i see is that im working in ESD safe lab. ESD safe floor, ESD safe table, ESD safe chair, ESD safe shoes, ESD safe jaket. i dont use the wrist strap because everything else is already ESD safe so there can be no ESD buildup anyway. now pretty much everything is connected to groung via 1Mohm. the greatest danger i immediately see is the ESD jacket and its long wire meshed sleeves, so thats first to go. but am i still just waiting to be electrocuted? max working voltage for my current project is 400V in the beginning 'll be running it at less than 200V
Please note,
You do not want to be connected to Gnd or earth! The way to be safe is to reduce shock current to Earth. The ESD idea is great for semi conductors to remove the static build up on "you". By connecting yourself to Gnd even through an ESD device is asking for trouble.
If you think about voltage drop across a resistor and you are the resistor with 400V across you what is the voltage drop across your Heart!
The voltage is the pressure to overcome the resistance of your skin so the current can flow. It likes the marrow and salt in your body to transmit current to Gnd. You need to isolate yourself completely from return paths e.g. Ground. that’s why you keep your hands off the chassis when testing.
ESD is the opposite of HT safe.
Just for fun.
Last edited:
A proper ESD safe setup grounds the user through a 1~10 Mohm resistor. It's electrostatic dissipative rather than conductive. This limits the current through the user should the user touch a hot wire while being strapped in properly.
As far as I know, the table-top ESD mats are dissipative rather than conductive as well.
I'd be more concerned with the exposed metal on the wrist strap than the connection to the bench.
~Tom
As far as I know, the table-top ESD mats are dissipative rather than conductive as well.
I'd be more concerned with the exposed metal on the wrist strap than the connection to the bench.
~Tom
Tom, I understand your thoughts on the high resistance.
However any connection to Gnd that could potentially carry current under fault conditions especially across your body is dangerous.
I have known people just connect themselves to the chassis of a computer with a cable to equalise the potential as ESD safe.
Moderators please remove this link if regarded excessive!
Just for interest!
Electric shock - Wikipedia, the free encyclopedia
Regards
M. Gregg
However any connection to Gnd that could potentially carry current under fault conditions especially across your body is dangerous.
I have known people just connect themselves to the chassis of a computer with a cable to equalise the potential as ESD safe.
Moderators please remove this link if regarded excessive!
Just for interest!
Electric shock - Wikipedia, the free encyclopedia
Regards
M. Gregg
Last edited:
Don't press the exploder button
This was a personal experience
http://www.vintageelectronics.net/IMG_0130.jpg
This was a personal experience
http://www.vintageelectronics.net/IMG_0130.jpg
Just to reiterate, an isolation transformer doesn't protect you from electric shocks, just from shocks between L and E. L and N is still just as dangerous.
also if you are inside a valve amp, then the voltages inside are more dangerous than the ones on the supply, being both higher, and being DC, and an isolation transformer won't make a blind bit of difference.
think of it like a seat belt in a car. you are safer if you wear it, but you still try not to crash.
and an earth leakage breaker feeding an isolation transformer is an excercise in contradictory cancellation
also if you are inside a valve amp, then the voltages inside are more dangerous than the ones on the supply, being both higher, and being DC, and an isolation transformer won't make a blind bit of difference.
think of it like a seat belt in a car. you are safer if you wear it, but you still try not to crash.
and an earth leakage breaker feeding an isolation transformer is an excercise in contradictory cancellation
The old TV high voltage adage of the early 1950's sticks.... keep the other hand in your pocket whilst carrying out between point to point HV measurements. Use a probe with a spring clip....Once done, the golden rule sticks as one is very vulnerable with both hands......and not all probes are clean.The old rule may feel inconvenient but is worth the trouble.
It also goes in practise, despite all the theory being correct... that not all circuits run right; the ones that rebel violently like PFC and sim work, I wear ear defenders and safety glasses as flying hard plastic semis destructed from alot of joules can inflict grenade injury and hearing percussion. As ISOTOP brick exploding is quite dangerous.
It also goes in practise, despite all the theory being correct... that not all circuits run right; the ones that rebel violently like PFC and sim work, I wear ear defenders and safety glasses as flying hard plastic semis destructed from alot of joules can inflict grenade injury and hearing percussion. As ISOTOP brick exploding is quite dangerous.
I bypass all large electrolytic caps with a 470k 2 watt resister (bleeder) this discharges the cap when the amp is turned off . It is amps that kill , I once
saw the imprint of a wrench that had fallen accross the main bus in a telephone
exchange , 60 volts dc enough amps to turn the wrench into a fast blow fuse.
saw the imprint of a wrench that had fallen accross the main bus in a telephone
exchange , 60 volts dc enough amps to turn the wrench into a fast blow fuse.
.Worthwhile mentioning: I get many queries about Power factor circuits as tube amp B+ supplies as the regulation performance they provide is outstanding: I've been using the technology now nearly for 40 yrs...If one is new to the physics, study it first as the chance to botch and experiment with the H.V is over. CV Joules are the issue here and a short circuit is potentially dangerous with sand and hard plastic splinters flying and there is not much escape. I work upwards 600V B+ at the kW level......but even at lower voltages, a mistake leading to the big bang is always possible.Adage....Please use safety glasses and ear protectors.
Photo: 200+200W tube power amp currently in build: PFC box as part of the power supplies of the B+ 300-600V rated at 1kW.
richy
Attachments
When is an isolating transformer not...?
...When it's bought at a builder's yard or the like.
Might be worth mentioning that what site-tool suppliers sell as an 'isolating transformer' doesn't provide isolation from earth. It's a transformer, but with an earth connected to the centre tap of the secondary. Thus, both sides of the supply are live, but at half AC voltage, and in antiphase. Thus effectively, an autotransformer. The idea is to reduce the voltage to earth on any conductor, for outdoor/wet use of tools.
For the benchtester this transformer doesn't provide any protection against shocks to-earth, plus it creates the hazard that a short between a direct supply and the transformer's antiphase output might involve 50% higher voltage than the supply.
You can in most cases modify such transformers to provide a safer benchtesting supply... but beware the misnomer.
IMHO the tool suppliers should be forced to stop using this name for something which doesn't, in fact, isolate.
You can also, of course, use an RCD for increased protection, although it's important to realize that RCDs only protect against shocks to earth (and it takes quite a bad shock to trip them, too)
HTH.
...When it's bought at a builder's yard or the like.
Might be worth mentioning that what site-tool suppliers sell as an 'isolating transformer' doesn't provide isolation from earth. It's a transformer, but with an earth connected to the centre tap of the secondary. Thus, both sides of the supply are live, but at half AC voltage, and in antiphase. Thus effectively, an autotransformer. The idea is to reduce the voltage to earth on any conductor, for outdoor/wet use of tools.
For the benchtester this transformer doesn't provide any protection against shocks to-earth, plus it creates the hazard that a short between a direct supply and the transformer's antiphase output might involve 50% higher voltage than the supply.
You can in most cases modify such transformers to provide a safer benchtesting supply... but beware the misnomer.
IMHO the tool suppliers should be forced to stop using this name for something which doesn't, in fact, isolate.
You can also, of course, use an RCD for increased protection, although it's important to realize that RCDs only protect against shocks to earth (and it takes quite a bad shock to trip them, too)
HTH.
A friend and I were listening to his 100+ watt stereo when !POP! Flames and smoke comes pouring out the back. It was a capacitor. Lucky it went when it did because we had planned on going out to eat after the song that we were listening to was over. He normally left his stereo on all the time. Had that been the case he would have came home to well, to no home.
Gary
Gary
Probably an RF filter cap.
Some of the vintage gear (tube or early transistor) had a paper or polyester RF-suppressor capacitor across the mains switch, and these things were notorious for going on fire. If restoring vintage gear it's always best to look for these and remove them (they are nonessential) or replace them with modern 'Class Y' equivalents. Otherwise they are a fire waiting to happen.
OTOH when electrolytics blow (if that's what it was) they release a LOT of smoke, and you really don't want to breathe it in as it contains toxic chemicals. The other main hazard here is eye damage if it goes up under your nose. It's rare for electrolytics to cause fire though.
Anyway, don't despair as if it was mains-filter cap, then the repair may consist of simply removing it and any of its evil brethren across the primary side of the transformer. .
Some of the vintage gear (tube or early transistor) had a paper or polyester RF-suppressor capacitor across the mains switch, and these things were notorious for going on fire. If restoring vintage gear it's always best to look for these and remove them (they are nonessential) or replace them with modern 'Class Y' equivalents. Otherwise they are a fire waiting to happen.
OTOH when electrolytics blow (if that's what it was) they release a LOT of smoke, and you really don't want to breathe it in as it contains toxic chemicals. The other main hazard here is eye damage if it goes up under your nose. It's rare for electrolytics to cause fire though.
Anyway, don't despair as if it was mains-filter cap, then the repair may consist of simply removing it and any of its evil brethren across the primary side of the transformer. .
Last edited:
Thanks for the reply and the information but that was 20 some years ago. He unplugged the thing and we went out to eat. Then went to the stereo store and he bought another one ( JVC if I remember right and a pair of cerwin vega's 15" bass 3 way speakers which he still has, Now that I do remember ) The JVC blew a channel but I don't recall the details other than nothing blew up, no flames, and no smoke. lol.
Gary
Gary
Not really audio related, but definately HIGH VOLTAGE related. I'm used to building Amps with large tube's and very high Voltage. With this stuff there is no second chance so my rules are extreme. The Voltages in this are 4850 Volts at 1 Amp (Plates of the 4PR1000B's) Filament of the tubes, Start current (cold filaments) 100 Amps+, run current, 42 Amps at 7.5 Volts. Input AC 240 Volts 40 Amp Mains.
So.....Power disconnected before any service, this includes the mains. All High Voltage is bled off with a Large bleeder resistor at the end of a insulated pole,(4850 Volts.) After they are bled off, the Main filter caps are shorted with a jumper, then aligator clip shorted while servicing.
When dealing with high Voltage, always be the safest you can be. Whether it's 450V on the plates of some 6L6's or 150V on a 12AU7's plates....or in this case 4850V on the plates.
Here is a picture of the power supply (for those interested) The 2 big square blocks with the ribbed glass insulators, on each side at the rear are the filter caps.
Picture resized and attached by moderation.
So.....Power disconnected before any service, this includes the mains. All High Voltage is bled off with a Large bleeder resistor at the end of a insulated pole,(4850 Volts.) After they are bled off, the Main filter caps are shorted with a jumper, then aligator clip shorted while servicing.
When dealing with high Voltage, always be the safest you can be. Whether it's 450V on the plates of some 6L6's or 150V on a 12AU7's plates....or in this case 4850V on the plates.
Here is a picture of the power supply (for those interested) The 2 big square blocks with the ribbed glass insulators, on each side at the rear are the filter caps.
Picture resized and attached by moderation.Attachments
Not really audio related, but definately HIGH VOLTAGE related. I'm used to building Amps with large tube's and very high Voltage. With this stuff there is no second chance so my rules are extreme. The Voltages in this are 4850 Volts at 1 Amp (Plates of the 4PR1000B's) Filament of the tubes, Start current (cold filaments) 100 Amps+, run current, 42 Amps at 7.5 Volts. Input AC 240 Volts 40 Amp Mains.
So.....Power disconnected before any service, this includes the mains. All High Voltage is bled off with a Large bleeder resistor at the end of a insulated pole,(4850 Volts.) After they are bled off, the Main filter caps are shorted with a jumper, then aligator clip shorted while servicing.
When dealing with high Voltage, always be the safest you can be. Whether it's 450V on the plates of some 6L6's or 150V on a 12AU7's plates....or in this case 4850V on the plates.
Here is a picture of the power supply (for those interested) The 2 big square blocks with the ribbed glass insulators, on each side at the rear are the filter caps.
What is the application of equipment?
Regards
M. Gregg
A good sized resistor for a few time constants. Example: a 10,000 uF cap with 50 volts on it. Power is V^2/R, so a 250 ohm resistor would dissipate 10 watts at 50 volts. 10,000 uF is 0.01 Farad, so the time constant is 0.01 * 250 ohms = 2.5 seconds. Keep the resistor on the capacitor for 10 seconds or so to be safe, then short out the cap with clip leads because electrolytics have a nasty habit of regenerating voltage even if you think they're discharged.
Dielectric absorption - Wikipedia, the free encyclopedia
Do NOT discharge the cap with a screwdriver - it'll make a fat spark, at least mar the tool, and not do the cap any good at all. Most often the lifetime won't be affected, but it's that one chance out of a hundred which goes BANG when you power it back on.