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New to building circuits, I’m trying to develop an understanding as to when a bleeder resistor should be connected across a capacitor for safety reasons. I’ve seen several references talking about how quickly the bleeder resistor should reduce voltage (usually 5T), under what threshold the bleeder should reduce voltage (either 50V or 60V, used for certification purposes), and how to size the resistor. However, in my mind it wouldn’t be voltage alone that determines when a capacitor is "safe enough" – it would be related at least partly to the amount of energy/Joules remaining within it. I certainly wouldn’t want a capacitor charged with “only” 12V but with the stored energy of a car battery being discharged when a metal object I’m holding touches it. Is there a convenient rule of thumb people use to determine the threshold when one should consider adding in a bleeder resistor?

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    \$\begingroup\$ IMO, for HV. I wouldn't place a resistor for 12VDC. \$\endgroup\$ – Marko Buršič May 21 '16 at 12:36
  • \$\begingroup\$ @MarkoBuršič So if not 12V, when does LV cross over into HV? Is the 50/60V that seems to be used for consumer electronic certification a reasonable threshold to start considering it? Although I wonder if, say, a 4700uF capacitor charged up to say 40V wouldn't still be something to bleed off... \$\endgroup\$ – JimMSDN May 21 '16 at 12:41
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They used to be used on huge computer power supply capacitors (the size of a pop/soda/beer can and up).

Just roughly I would not worry too much about it for normal capacitors until the stored energy gets well up into the joules (1J = 1W-s) and the fault current is high (at least tens of amperes).

Much smaller amounts of energy can cause electrical damage, and we don't generally worry about that, so the main concern is heating and welding (and possibly damaging the capacitor itself).

There is a similar concern with batteries- especially those which can supply enough power to cause internal or external heating that results in welding, dangerous temperatures or explosion of the battery.

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  • \$\begingroup\$ What number of Joules would you consider to be getting "up there"? A handful? Perhaps more than, say, 10? \$\endgroup\$ – JimMSDN May 21 '16 at 14:01
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    \$\begingroup\$ I'm just trying to be mindful - when growing up I worked in a car dealership and once was asked to remove a battery from a car. I accidentally turned my socket wrench too quickly and crossed the terminals, leading me to have an appreciation for "low voltage" power sources at an early age...8-) \$\endgroup\$ – JimMSDN May 21 '16 at 14:06
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    \$\begingroup\$ More than about 3-5 J. People have severely injured fingers from getting rings across 12V when working on vehicles. \$\endgroup\$ – Spehro Pefhany May 21 '16 at 14:27
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How will a bleed resistor across a power supply help?

The only circumstance I have come across is on the AC side of a power supply that uses internal X and Y capacitors to improve EMC. You could pull out the plug and if you then swapped hands, the plug prongs could simultaneously come into contact with the other hand and you could get a nasty little "bite". This assumes that those capacitors were charged up to a significant peak voltage when disconnected from the AC.

So, I've seen 100k to 1Mohm bleed resistors used in these circumstances.

But how does a bleed resistor help in other circumstances? You would have to have access to both terminals but, if you have access to both terminals and power is applied the bleed resistor is ineffective because the power is on and that can provide the same sort of energy into someone's hand.

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  • \$\begingroup\$ What about storage of the caps? Do you want your pet or child to get zapped when they find it on the floor? \$\endgroup\$ – user148298 May 18 '19 at 20:34
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In HVAC trade, we teach that a 20k ohm resistor, 2 watt is needed to make a capacitor safe. Our Back EMF is as high as 440 volts from an induction motor.

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