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.