Capable Digital Multi Meter

[NewKleer]

at uni a few years ago, one of the electronics guys had a "really expensive" multimeter which measured ohms very accurately - ie accurate enough to find short circuits in circuit boards etc.

now the only multimeters ive ever owned have no where near any kind of accuracy close to 0 ohms...ie shorting the leads gives u 0.5ohms and definitely no precision.

most of the need for the multimeter would be detecting points of short circuit, secondarily all the common stuff, and optionally a frequency counter/measurer. any suggestions?

[heller44]

You'd be looking at something like a low ohm meter, or microhmeter for that sort of measurement. Most of those insert a current through the part to be measured and reads the volt drop across that part, not really the thing for finding shorts on boards.

A fluke 87v or 189 has a relative function, to zero out the lead resistance.

I've been testing multitapped transformer windings with an 87v recently, resistance between these winding sets was in the range of .1 ohm difference to the next tap. Seemed accurate and repeatable.

Another way to chase shorts in circuit boards is to inject a frequency into the track, then use something very similar to an inductive pickup (hall effect maybe) to chase the path and follow the signal. I've only seen one used once and have no idea what it was called, so can't help there.

ps. That consult cable works great. Pity that I have a pfc turning up and probably will be selling it soon. Ecutalk is bloody brilliant as well.

james.

Edited August 5, 2007 by heller44

[NewKleer]

the relative thing sounds good. though on small circuit boards, im not sure if the resistance on tracks that are like 1cm long would be in the range of 0.1ohms

an example of where id use it is ive got 5 of the tiny smd boards from the consult cables that dont work, almost certainly because of shorts somewhere in the smd soldering

working on an update for ecutalk atm

[T04GTR]

oh ok. i thought that was what the hold option was.

[khunjeng]

I'm curious, why would you think 0.5 Ohms on a short is not accurate? that would indicate you have a short...if its not it wold be infinity?

[NewKleer]

sorry not 0.5, i get 3.5ohms when short leads on bog standard cheapo

having one which can zero out the lead resistance would be a good start

[discopotato03]

I ended up with the Meterman 38XR and will get the RS232 cable and disk in the near future . A fellow I know who used to work with industrial scales has a Fluke 225 which I think is no longer available . If he ever gets sick of it ...

Cheers A .

[heller44]

3.5 ohm is a real bad reading for leads.. I'd be looking for another set of leads, then another meter definately.

An example of the resistance for a short length of copper, 25 AWG wire (0.163mm2) has a median resistance of 32.37 ohms per 1000ft. 1ft resistance is 32.37mohms, 1cm is 1.062mohms. Pretty much near on squat. Copper tracks are a bit harder to work out because different manufacturers use differing thickness copper layers. But you get the idea.

Best bet will be just to work through the circuit, since you have the luxury of having a small board (not very complex) and the schematic for it. Even with a cheapie you should be fine. A judicious use of the ohms reading and the diode test fuction and you should be fine.

The min/max function picks the minimum maximum and usually average level measured whilst the min/max funcion is enabled, one issue is with most meters the scan rate is slow and very quick transients can be missed but for auto stuff it shouldn't be an issue, if it is then a scopemeter will need to be used. The hold fuction only holds the value that is on the screen at that time, so you need to be able to watch it and hope that it doesn't go higher, or lower if that is what you want, after you hit the button.

If that is a 125 fluke that would be good for auto stuff. no idea what a 225 is..

Hope that helps anyone. (I think i was bored.. )

james.

edit for screwed up my maths. damint.

Edited August 5, 2007 by heller44