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Comment by barbegal | Hacker News Reader

IEC 60335 and IEC 62368 state that the voltage across the capacitors must be below 34 V in 1 second after the input power is removed. Modern electronics are remarkably safe and I have never heard of anyone being shocked by power supply capacitors in the last decade. Of course cheap electronics sometimes aren't standards compliant and bleed resistors can conceivably fail hence the advice to wait a bit longer if you want real piece of mind and don't mind waiting.

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I had a friend in high school who found it funny to hand unsuspecting people a fully charged capacitor he pulled from a camera flash. This was 30 years ago, before "modern electronics", so the charge lasted longer than a few seconds. Why this never started a fight I don't know.

> before "modern electronics", so the charge lasted longer than a few seconds.

If you take any capacitor and charge it, it'll take quite some time to discharge on its own.

The discharge is usually controlled by a resistor in parallel with the capacitor on the PCB. Removing the capacitor from the board disconnects it from the discharge resistor. So not much "modern electronics" going on here :-P

I'm working on a project where I may need to open and repair an old device that probably contains some high-voltage capacitors. Would it be relatively safe to do this if I was wearing rubber gloves + shoes, and used a multimeter to check for voltages, and a large resistor to discharge any capacitors? What kind of bleed resistor is typically used, e.g. for old TVs?

Just short the positive side to ground to discharge them.

Note: do NOT do this with thin wires and large capacitors / arrays of capacitors, or you may get a hot copper shower.

Preferably connect it to ground through a resistor.

This might be a nice feature of a multimeter: discharge mode.

Putting the leads over a capacitor, the multimeter will show the voltage and discharges the capacitor with a constant safe current. Beeps can be used to indicate when the capacitor is safe to handle or not.

It kind of already has a discharge mode, just short the leads of the capacitor with one of the probes.

That is not how you properly discharge a capacitor, though. Especially big ones.

If it's 250V or less then that's probably overkill. Rubber gloves alone will protect against a few hundred volts. Bleed resistors depend on the capacitor size but are usually in the high kilaohms.

> a multimeter to check for voltages, and a large resistor to discharge any capacitors

That's all that I use. Don't hold the resistor in your hand though, depending on the size of the capacitor they can get quite hot. I usually use some pliers to hold the resistor.

Computer was unplugged:

"Shawnee teenager is electrocuted while working on a computer":

https://www.kctv5.com/story/19767067/teen-electrocuted-while...

Same story as above as link not available:

https://www.dailymail.co.uk/news/article-2215200/Teenager-Sh...

And a warning:

"I just received an electric shock from an unplugged computer"

https://www.reddit.com/r/computers/comments/53cio2/i_just_re...

What do bleed resistors drain towards if the ground wire has been unplugged by unplugging the monitor?

They don't bleed to ground. Even if the power-chord is disconnected, there still is a circuit between the resistor and the capacitor, and this is where the current runs.

It's just a resistor over the capacitor that slowly draws current from it. Little enough that it doesn't matter in normal operation, but enough to reasonably quickly discharge the capacitor to a safe level when power is removed.

A charged capacitor has a surplus of electrons on one side, and a shortage on the other. A bleed resistor allows current to flow from one side to the other, balancing the charges on both sides so there is no relative voltage. Q=CV

Thank you! I understand this now.

Great explanation, very helpful! Thank you!

You should never discharge towards the ground wire. The resistor is simply across the two poles of the capacitor.

How does this work in practice... do compliant capacitors have to be designed to 'leak' stored charge across an internal resistor or something like that?

[edit] Ah I see this is called a bleed resistor, never mind

They usually just have a normal resistor in parallel with the capacitor. In my experience it's also kind of nice to have minimal load at the output, which makes it a bit easier for the voltage controller.

https://en.wikipedia.org/wiki/Bleeder_resistor

Probably this is only about exposed terminals? Like touching the power plug will not shock you, but poking at the internals could.

IEC states what should happen. I strongly suggest making sure that the capacitors are discharged before working on any circuit.

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