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Old 17th December 2010, 03:46 PM   #181
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Quote:
Originally Posted by piano3 View Post
I had a microphone phantom power circuit that in the haste of constructing to a deadline I forgot to put bleed resistors in; I found the caps still had 35V(out of about 50) after nearly two months. I think this must be an extreme case. They were described as computer grade electrolytics and were 6800 uF.
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.
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Old 17th April 2011, 07:54 PM   #182
r2k is offline r2k  Estonia
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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
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Old 19th May 2011, 07:06 PM   #183
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by r2k View Post
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.
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Last edited by M Gregg; 19th May 2011 at 07:30 PM.
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Old 19th May 2011, 07:22 PM   #184
tomchr is offline tomchr  United States
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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
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Old 19th May 2011, 07:42 PM   #185
M Gregg is offline M Gregg  United Kingdom
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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
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Old 27th May 2011, 12:01 AM   #186
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Default Don't press the exploder button

This was a personal experience

http://www.vintageelectronics.net/IMG_0130.jpg
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Old 7th June 2011, 03:17 PM   #187
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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
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Old 29th June 2011, 04:20 PM   #188
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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.
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Old 9th July 2011, 11:36 AM   #189
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Awesome thread

Thanks too everyone who contributed.
I have lots of preparations to take care of before I start tinkering with my little tube amp, aside from rolling some tubes that is

Harvey
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Old 19th July 2011, 10:44 PM   #190
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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.
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