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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 LemmyBr785@discuss.tchncs.de to c/askelectronics@discuss.tchncs.de

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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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[–] jjagaimo@lemmy.one 0 points 1 year ago (1 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

[–] LemmyBr785@discuss.tchncs.de 0 points 1 year ago (1 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.

[–] jjagaimo@lemmy.one 0 points 1 year ago (1 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

[–] rstein@discuss.tchncs.de 0 points 1 year ago (1 children)

And the bulb is a strange resistor, it has a really high current when starting up cold. This can fry everything behind it.

[–] ConductiveInsulation@feddit.de 0 points 1 year ago

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.

[–] ConductiveInsulation@feddit.de 0 points 1 year ago

I've successfully used incandescent Bulbs in the past. They have barely any resistance when cold, which is the reason why they usually blow up when you turn them on.

For their size, usually I use more than the power the device will use but less than the wires can handle for a while. In your case id get a lamp with a E10 socket, for example 3.5V 0,7W.

Alternatively you can use a PSU with current limiting features.