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I'm a beginner/intermediate in electric engineering and had to discharge a few electrolytic capacitors last week in a hobby starter kit (not too high voltage) and now I have to discharge some more caps - but this time they are radial instead of axial - meaning that both leads come out of one end and they're mounted vertially on the pcb board.

How can I access the leads to discharge them? Do I have to get under the pcb board (which has been mounted above another pcb board so I'd have to unscrew a lot of things) - or can I hook one end of a wire (with alligator clips) to the ground, hook a resistor on the other end and touch the capacitor somewhere near its positive side or does the wire absolutely have to come in contact with the positive lead underneath the cap?

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  • \$\begingroup\$ Are you sure you need to discharge them? If they're charged to something like 5V there's not really any risk leaving them charged, but if they're charged to 100V you definitely need to discharge them. \$\endgroup\$ – Hearth Jun 12 at 15:50
  • \$\begingroup\$ I have some at that voltage and others a bit higher but I want know these things if in the far future I'll discharge bigger caps. Just wondering if the only touching point to discharge is on the positive lead it self? Or maybe if you could touch the side of the cap somewhere to discharge (with a jumper cable with one end to the ground and a resistor of course). \$\endgroup\$ – Jon Th Jun 12 at 15:53
  • \$\begingroup\$ You need to touch the lead. Usually you can manage this even with the can variety. But if it's really tight against the board and you can't reach the bottom. Maybe just wait? \$\endgroup\$ – hekete Jun 12 at 16:18
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How can I access the leads to discharge them?

An easier thing would be to find the traces that the leads are connected to, which are probably on the top and bottom of the board. This may require taking the unit apart once. Another thing that can be done is ohm out the leads and see if there is a more accessible trace/conductor. Then short the capacitor with a large resistor, and be careful because voltages above 60V are considered hazardous.

Most devices are designed to safely discharge after 30 seconds (especially UL Or ETL listed devices) but kits are not subject to those rules.

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You need to touch the lead. It doesn't need to be the bottom of the board.

Usually it's easiest to just turn off power and wait, although if you're working with lethal voltages, or with enough capacitance to cause high current* you want to check before you touch.

* My brother still bears a scar on his wrist that involves a road-side repair, a plain old car battery, a wrench, and a wristwatch with a metal strap.

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