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How to properly size a serial bulb for protection when troubleshooting a circuit? (discuss.tchncs.de)

submitted 1 year ago by [email protected] to c/[email protected]

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Hello. I'm a hobbist on electronics repairs, and I did some repairs on stuff powered from the wall socket before, using a serial bulb to protect the circuit in case there's a short. Now I want to troubleshoot a board that works at 3V and very low current (like no more than 100mA). So, how can I properly size the bulb I need for this circuit before powering it? Or is there a better way to protect the circuit than a serial bulb? Thanks in advance.

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[–] [email protected] 5 points 1 year ago (7 children)

For this application you should be using a bench power supply with current limiting, not a "serial bulb" (I assume you mean a fuse, which is designed to break at a low current, however these are most typically rated for several amps, not typically in the mA range). You can set the voltage and a current limit. If the current goes beyond the limit, then the power supply will drop the voltage to keep the current below the limit or latch off. You can get a fairly cheap one for about $50-60 off of eBay, which won't be the best but is sufficient for hobby use

[–] [email protected] 1 points 1 year ago (6 children)

Hey, thanks for your reply. By serial bulb I mean a incandescent lamp in series with the circuit. I was looking for a cheap and diy option, but I'll take a look on a bench power supply. I still need to get me a decent one anyways.

[–] [email protected] 1 points 1 year ago (4 children)

Ah. It's not going to be possible to size it because the bulb is then acting as a resistor essentially. Unless you know what the equivalent resistance of the circuit you're testing is, and it draws a fixed current, you aren't going to be able to cap the current; Adding a resistor (or bulb) is just going to drop the input voltage and you will probably end up having other issues

[–] [email protected] 1 points 1 year ago (1 children)

Ah got it. Then a bench power supply is the way to go. Thanks again!

[–] mattaw 1 points 1 year ago* (last edited 1 year ago)

Figure 22 has a really cheap voltage and current controlled bench supply using two lm317s from an existing DC supply like a laptop charger etc:

https://www.onsemi.com/pdf/datasheet/lm317-d.pdf

Pre built ones with heatsinks may be available really cheap on eBay too.

If you need a lot of DC power there are some kits on eBay etc. that can convert a PC PSU to a bench supply.

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