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How to properly size a serial bulb for protection when troubleshooting a circuit?
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this post was submitted on 18 Jun 2023
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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
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.
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
And the bulb is a strange resistor, it has a really high current when starting up cold. This can fry everything behind it.
It has the high current because it's cold, it only needs a short time to heat up and light up and the majority of circuits can handle very short overcurrent really well because the connections need to heat up before they break. Using a lightbulb for current limiting works pretty well.
Ah got it. Then a bench power supply is the way to go. Thanks again!
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.
That's for mains-powered devices. It will do nothing at 3V.