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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 (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

[–] [email protected] 1 points 1 year ago (2 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 (2 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.

[–] [email protected] 1 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.

[–] [email protected] 2 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.

[–] Krististrasza 1 points 1 year ago

That's for mains-powered devices. It will do nothing at 3V.

[–] [email protected] 2 points 1 year ago

You can make your own current-limited power supply, probably from bits and pieces you already have. Let's say that you have a 5v dc power supply and a hand full of rectifier diodes and resistors (various values and sizes).

Put a series chain of forward biased rectifier diodes and resistor(s) across your 6v supply. Choose enough diodes to give you a 3v output. Now choose a combination of series/parallel resistors to give you a 2v drop with a current of, say 100mA. You need 20 ohms - so that could be 5 x 100 ohm resistors in parallel.

The most current that can put out is the full 5v across 20 ohms - but at that point the output voltage will be near zero.

Bench supplies, well reasonable ones, allow you to set a current limit as well as an output voltage. At loads below that current limit - it operates as a constant voltage supply. At loads above - it operates as a "constant current" supply. You would set the output current limit to 100mA and that's the most that it will output.

Now the rectifier diodes plus resistor would allow the current to increase above 100mA, up to 250mA when the output voltage will be near zero (short circuited) - if you want better than that, then you can add a transistor and a few other components.

[–] [email protected] 1 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.