# Questions about short circuit protection circuit

#### Green Bean

Joined Mar 31, 2017
126
My friend's breadboard power supply bit the dust the other day, so I wanted to whip up something quick for him. I have a 5V DC wall wart, but I figured I should probably make some sort of short protection. I found this circuit:

http://www.daycounter.com/Circuits/Short-Circuit-Protection/Short-Circuit-Protection.phtml

My questions are:

1. Will 10 ohms be ok for R1. I don't have a 0.5 ohm resistor.
2. Will C3914 transistors be ok for this application? They look like power transistors but I just want to be sure.
3. Since I'm using a DC supply, shouldn't VP go to Vin (+5V)?

#### Green Bean

Joined Mar 31, 2017
126
That's a pretty crappy circuit, it's not really intended for what you want.
The internet is full of crappy circuits.

Add one of these in series.
https://www.digikey.com/product-detail/en/littelfuse-inc/RUSBF120/RUSBF120HF-ND/5029817

You don't mention the current level, I just chose 1.2 Amps for the hell of it.
Oh yeah, it's 1 Amp. I thought about a fuse, but I don't have any and would like to just make something really quick. Is there any good ways to make a DIY fuse? Like, what if I just used a really thin piece of aluminum foil?

#### AlbertHall

Joined Jun 4, 2014
9,999
1. Will 10 ohms be ok for R1. I don't have a 0.5 ohm resistor.
That resistor sets the current limit. The current it will limit at is about 0.7V/R1 (0.7V being the base emitter voltage of Q2). With 0.5Ω the limit would be 1.4A. With 10Ω the limit would be 70mA!

Also you will lose up to 0.7V through this circuit (depending on the current) so with 5V input you might only get 4.3V out.

The wall wart is probably switch mode and so will most likely have a current limit built in but, for peace of mind a fuse would be no bad thing.