3 ms·

Or just short out the capacitor terminals with a metal object like the screwdriver and you're fine. But still, if you don't do that and you get chocked it will

Or just short out the capacitor terminals with a metal object like the screwdriver and you're fine. But still, if you don't do that and you get chocked it will just be a little painful jolt.

exmadscientist 5y ago
You know, you'd think that would work, wouldn't you? But dielectric soakage runs in reverse, too. So even if you short out the capacitor once, it can recover some voltage after days to months left open again. (The energy comes from the molecules in the dielectric relaxing, very slowly.) This isn't a problem if the designer has put in a bleeder resistor across the capacitor. But some designers are cheap, and don't want to pay the extra few cents to make their products non-lethal to technicians. Some are just stupid (yes, dumbfuck "senior engineer" coworker, I am thinking of you here, you colossal waste of oxygen). What you want to do is short it out, with a screwdriver or otherwise, then keep it shorted with a gator cable or even resistor. Then you'll be safe even if you set the project aside for a week. Or three months.
- h2odragon 5y ago
  Had a pallet of military surplus capacitors, packaged as 3 big (soda can sized) caps on a blade frame and wrapped in conductive foil, plastic, then heavy paper. From the 70s and they were fine when i got them in 1997. I wired up an unwisely huge bank out of them for some fun HV experiments. They were in a shed outside and could self charge to useful levels in the course of a windy day. Dont recall the actual numbers but it could vaporize dimes handily.
  - LargoLasskhyfv 5y ago
    Reading that, I wonder how much solar power you'd need to charge them to that level? And if that could be applied to complement solar/wind power?
    - h2odragon 5y ago
      I tried some halfass "comb" antennas but wasn't seriously attempting to harvest static electricity to useful purpose. There's some "joule thief" circuits available cheap and easy now that make me wonder if its possible.
- userbinator 5y ago
  Dielectric absorption is a hazard for HV capacitors that are charged to over 1kV in normal use, since they can recover to dangerous voltages. Mains filter caps will be lucky to recover a few dozen volts, which isn't going to do much (and inside a PSU, if the voltage is high enough it will try to run and quickly discharge them anyway.)
  - exmadscientist 5y ago
    And yet, if the voltage is low enough, it will linger for minutes to hours. Plenty of time to kill someone. Maybe I'm just salty about having had this argument too often recently. But I strongly believe that every mains voltage or higher system needs bleeders across its capacitors. If you don't want to pay for the resistor, fine... but leave the footprint for it. This goes 10x stronger if the thing is open-frame, sits on a bench, and is regularly being probed by engineers. Just put the goddamn resistors in there before someone gets hurt. Ahem. Apologies for the intermission.
dendriti 5y ago
I still don't think it's a good idea for the average tinkerer to take apart a power supply. Almost never worth it.
- xxs 5y ago
  It requires some background in electronics. Personally I use gloves when working on electronics and soldering. There is nothing sophisticated about power supplies. Pressure vessels is where it gets real.
formerly_proven 5y ago
I recommend against this. Most supplies have a bleed circuit, but if it doesn't or it doesn't work (at all or not fast enough), then shorting primary-side capacitors can make a really nice bang and also damage your "shortening implement". That bang can startle you and make you hit something. Been there done both. Not a great idea IMHO. Check for voltage - If not present / marginal, Then shorten, Else discharge through resistor and GoTo Check. A "capacitor discharge tool" automates this cycle.

Or just short out the capacitor terminals with a metal object like the screwdriver and you're fine. But still, if you don't do that and you get chocked it will just be a little painful jolt.

exmadscientist 5y ago

You know, you'd think that would work, wouldn't you? But dielectric soakage runs in reverse, too. So even if you short out the capacitor once, it can recover some voltage after days to months left open again. (The energy comes from the molecules in the dielectric relaxing, very slowly.) This isn't a problem if the designer has put in a bleeder resistor across the capacitor. But some designers are cheap, and don't want to pay the extra few cents to make their products non-lethal to technicians. Some are just stupid (yes, dumbfuck "senior engineer" coworker, I am thinking of you here, you colossal waste of oxygen). What you want to do is short it out, with a screwdriver or otherwise, then keep it shorted with a gator cable or even resistor. Then you'll be safe even if you set the project aside for a week. Or three months.

h2odragon 5y ago
Had a pallet of military surplus capacitors, packaged as 3 big (soda can sized) caps on a blade frame and wrapped in conductive foil, plastic, then heavy paper. From the 70s and they were fine when i got them in 1997. I wired up an unwisely huge bank out of them for some fun HV experiments. They were in a shed outside and could self charge to useful levels in the course of a windy day. Dont recall the actual numbers but it could vaporize dimes handily.
- LargoLasskhyfv 5y ago
  Reading that, I wonder how much solar power you'd need to charge them to that level? And if that could be applied to complement solar/wind power?
  - h2odragon 5y ago
    I tried some halfass "comb" antennas but wasn't seriously attempting to harvest static electricity to useful purpose. There's some "joule thief" circuits available cheap and easy now that make me wonder if its possible.
userbinator 5y ago
Dielectric absorption is a hazard for HV capacitors that are charged to over 1kV in normal use, since they can recover to dangerous voltages. Mains filter caps will be lucky to recover a few dozen volts, which isn't going to do much (and inside a PSU, if the voltage is high enough it will try to run and quickly discharge them anyway.)
- exmadscientist 5y ago
  And yet, if the voltage is low enough, it will linger for minutes to hours. Plenty of time to kill someone. Maybe I'm just salty about having had this argument too often recently. But I strongly believe that every mains voltage or higher system needs bleeders across its capacitors. If you don't want to pay for the resistor, fine... but leave the footprint for it. This goes 10x stronger if the thing is open-frame, sits on a bench, and is regularly being probed by engineers. Just put the goddamn resistors in there before someone gets hurt. Ahem. Apologies for the intermission.

dendriti 5y ago

I still don't think it's a good idea for the average tinkerer to take apart a power supply. Almost never worth it.

xxs 5y ago
It requires some background in electronics. Personally I use gloves when working on electronics and soldering. There is nothing sophisticated about power supplies. Pressure vessels is where it gets real.

formerly_proven 5y ago

I recommend against this. Most supplies have a bleed circuit, but if it doesn't or it doesn't work (at all or not fast enough), then shorting primary-side capacitors can make a really nice bang and also damage your "shortening implement". That bang can startle you and make you hit something. Been there done both. Not a great idea IMHO. Check for voltage - If not present / marginal, Then shorten, Else discharge through resistor and GoTo Check. A "capacitor discharge tool" automates this cycle.