Select PTC fuse

I have this Arduino based circuit(Speeduino) running my car’s EFI, and a couple of days ago it blew the 32 and 16 mil traces. I’m thinking it blew because it was running on the car’s 12V and I had just plugged the laptop into the common 12V cigarette lighter. Both used to run on the same 12V, it’s only the datalogger in the controller program(TunerSudio) would freeze. I was testing why the traces blew on the bench using my DIY PS, but when I plugged my laptop into the same 240V as the PS the USB chip of the MEGA2560 went up in smoke. It kind-of looks like the USB protection in the laptop is shorted and letting it ground through the laptop now.

I now want to isolate everything from the high current MOSFETs so I’m leaving the 16mil cut, running a wire from that trace to the Arduino side of the 32mil trace, and putting a PTC fuse across the 32mil break. I want to select a PTC low enough that the MEGA won’t fry, but not too low that the car might stop unexpectly. When I plug the Speedy into a USB doctor meter is shows a sudden surge to 1.5-1.8A and quickly drops to 200ma, but across the 32mil broken trace I’m only getting 30ma. What would be a good size PTC?

This is the original sketch without the mods -323-v3.6-Ver2.fzz (419.1 KB)

I will be using a USB isolator from now on, and might even take the laptop apart to see what’s happening, but the extra protection shouldn’t hurt.

I don’t know much about ptc fuses and the data I saw in littlefuse’s site isn’t very illuminating, but they mentioned a slow-blow varient which should be what you want if they are really like fuse. A slow blow fuse will take a surge (like your 1.8A inrush) for a little while if the steady state settles reasonably quickly to the 200ma level (or 30ma level if thats whats running in the trace). I’d use something around twice the expected steady state current if it was a fuse. Hopefully someone here is familiar with specifying and using ptcs (as it isn’t me :slight_smile: ).


I’m not sure about PTC fuses either but I think they trip when hot and reset when cold.

I ordered some Ebay 0.56A, 0.9A, and 1.1A a couple of day ago, but I was thinking maybe they are too high. I think I will go with 200ma because I don’t want this thing dying just as you pull out into a line of heavy traffic.

I can not say I know a lot either but I would like to add you want a PPTC not a PTC thermistor as one is a fuse and the other is just a temperature dependent variable resistor. At room temperature the PPTC will has a resistance of x and will quickly increase as current causes it to heat up at which point it will reach a resistance high enough to effectively shut down the circuit similar to the way a fuse does. On the other hand a normal PTC will not reach a point where the resistance is high enough to shut off the circuit quick enough to protect the circuit.

With all that said I have recently been working on a circuit with a few Farad of super capacitors being charged by a LiPo cell where I needed to deal with Inrush current and my research lead me to NTC power thermistors like 16D-9 which limits the inrush current but overtime allows higher currents if a constant demand exists. I think this may be a better option for you since it only sounds like an inrush current issue rather than a constant current issue.

These PPTCs aren’t that good. For them to cut in a reasonable amount of time you have to have 3 times the rated current. Double the rated current and it’s maybe 15 sec. Iv’e got 100 and 200ma coming, so they should be ok for 0.5A.