I feel I should know this but I don't - Page 1
Forum section: electronics / beginners

author: Discotech
author_rank: Regular Contributor
timestamp: July 20, 2018, 10:01:34 pm
content: Say I want to reverse engineer a circuit and figure out what everything is, if I start probing around with my DMM trying to find continuity, voltages, resistance etc while the circuit is powered on do I run the risk of shorting the circuit and killing components ?I'm trying to figure out a 10 pin header, I know 1 pin is for pwm signal, and I know there's a 5v and ground in the pins but I do not know which pins are the 5v and ground as I can't find a schematicI just don't want to poke around with the risk of breaking things and it's not something I've actually needed to know until now

author: eKretz
author_rank: Frequent Contributor
timestamp: July 20, 2018, 10:16:14 pm
content: If you aren't careful. Shorting the wrong pins together by bridging them with a single probe (which is easy with the tiny ones) while they're under power can do damage. Just correctly probing pins with a multimeter is unlikely to hurt anything.

author: Discotech
author_rank: Regular Contributor
timestamp: July 20, 2018, 10:26:00 pm
content: Yep I knew bridging with a single probe would be bad I just wasn't sure as I've never actually used a DMM on a circuit I didn't already understand so the thought has never occurred until now

author: IanB
author_rank: Super Contributor
timestamp: July 20, 2018, 10:29:54 pm
content: Quote from: Discotech on July 20, 2018, 10:01:34 pmQuote from: eKretz on July 20, 2018, 10:16:14 pmAs long as you are trying to measure voltages. Probing for resistance or continuity is not so safe...

author: eKretz
author_rank: Frequent Contributor
timestamp: July 20, 2018, 11:16:40 pm
content: Yeah, forgot to mention that. Really don't need to be doing that with power applied to the circuit anyways for the most part. Mainly checking under power is usually for voltage or signal.

author: frozenfrogz
author_rank: Frequent Contributor
timestamp: July 20, 2018, 11:20:49 pm
content: In regard to reverse engineering a PCB I would advise to put the board on a flatbed scanner and do a high resolution scan of both sides.Afterwards you can re-draw the traces and create an overlay of both sides. This only works for double layer boards of course..

author: JS
author_rank: Frequent Contributor
timestamp: July 21, 2018, 03:57:32 am
content: Even with the low voltages a DMM would provide you could put a sensitive junction in everse bias and while unlikely to burn it, low noise transistors can loose the low noise characteristic when reverse biased. Other than that is pretty safe, like in no low noise circuitsJS

author: tooki
author_rank: Super Contributor
timestamp: July 21, 2018, 09:13:04 am
content: Quote from: eKretz on July 20, 2018, 10:16:14 pm^^^ this. I’ve damaged stuff while probing around just to understand the circuit, with a slipped probe that shorted two pins.Don’t be me, be more careful!!

author: Discotech
author_rank: Regular Contributor
timestamp: July 21, 2018, 10:32:57 pm
content: Thanks for the advice, I've managed to find a schematic for the board I was going to probe so I no longer need to probebut at least I know in future

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 21, 2018, 10:46:10 pm
content: With full knowledge of the CONTEXT, there are pretty reliable ways of figuring out which is the circuit ground/common pin, and working out from there to find the power bus(es), which are input and output terminals, etc. There are many videos on YouTube showing reverse-engineering techniques, from figuring out the basics (Ground, Power, inputs and outputs), through discovering internal serial terminal nodes, and offloading firmware code and reverse-engineering and hacking the code.