polyfuse poping

[techsalvager]

I have a ms2 pcb v3

I managaed to break it somehow

I was testing and found I destroyed the 5volt regulator, no 5volts out, replaced and 5volts worked again, but not passed the polyfuse, replaced polyfuse and that blew again and is super hot to touch. Anyone know a likely reason? maybe another part thats not doing its job?

[racingmini_mtl]

Check D19.

Jean

[techsalvager]

Check D19.

Jean

Thanks, Hopefully I can repair this since I just bought one of your boards last night for seq fuel\spark.

[techsalvager]

D19 reads 890~ one way and 0 the other way. I cut one leg off the board to test.

[techsalvager]

D12 seems to read backwards compared to the rest on the power circuit, have to reverse my leads on my multimeters ohm setting to get a reading. That make sense?

edit: not sure run back to back checks, seems possible wonky or maybe I accidently touched something, have to test again later.

[techsalvager]

I replaced the 5 volt reg with a lm7805c from radioshack, does its metal back need to be isolated from the metal heatsink?

[techsalvager]

Does the polyfuse only disconnect the circuit over a certain temp?
I tested the orginal one and get contitunity either way.

[techsalvager]

Anyone with a pcb v3 measure at gnd and 5v in proto area, tell me if that should be open or closed circuit?

[elaw]

Okay, let me take a crack at a few of these...

First, are you using a meter with diode-check mode? A regular ohmmeter without that mode will usually work, but the readings are a bit harder to interpret. If you're using a regular ohmmeter, you should be on a 200-ohm to 2000-ohm range, not higher or lower. Also some ohmmeters have the lead polarity reversed which can confuse things.

Re D19, in general if *any* diode reads "0" in either direction, it's junk. So I'd recommend changing D19.

With most meters, D12 should read about the same as a regular diode in one direction, and infinite ohms (on an ohmmeter) / volts (in diode-check mode) in the other.

LM7805C is not the best choice for the regulator, but it does not need to be isolated. The 7805 will not work right at low input voltages, which might make you unable to start the car in cold weather. If you're in the USA, go to Digikey.com or Mouser.com and they should have the correct LM2937ET-5.0 regulator.

The polyfuse actually works by generating heat. Regular fuses do too as you probably know - when an overload occurs the wire inside just melts and opens the circuit and you get to replace the fuse. Polyfuses get hot too when an overload happens but they open the circuit chemically (sort of), and they reclose the circuit when they cool off. So they're reusable.

And on the +5-to-gnd in the proto area, on most meters it won't measure either 0 or infinite ohms, but somewhere in the middle. Infinite I wouldn't worry about, but if you measure zero or close to that, you might have a short somewhere (this includes D19).

[techsalvager]

I found the issue being jumper legs of c26 and c29 with some solder.