stuff anyways.
LIGO,
Measuring milli-Ohms is tricky. Ideally you'd like 4-point-probe measuring... voltage "viewing" pins are separate from current forcing pins.
I'd do it with an AC current source, so you can gain-up the resulting voltage. ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
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The only thing bipartisan in this country is hypocrisy
:
stuff anyways.
or LIGO,
Get an adjustable power supply and set it to low volts, 1 amp current limit. Connect that to a pc board, to a trace or a shorted plane or something, then measure microvolt drops here and there. With a good DVM you can resolve micro-ohms.
It's interesting to calculate the resistance of traces and actually measure them. Most pcb houses skimp on plating, so when you specify "1 oz" copper, you seldom get it.
John
I figured that it's a somewhat open-loop process where they stick your board in a tank and set a timer (based on calculations of their chemistry and the size of your board) so it's probably no better than +/-20%... and apparently many customers aren't really relying on the spec anyway so only a few places bother measuring it for you.
I wonder what the uniformity of plating is anyway though? It seems that it would be subject to "shadowing" and concentration effects just like etching is, and hence require agitation to make it more uniform.
On a sunny day (Fri, 12 Mar 2010 09:33:25 -0800) it happened D from BC wrote in :
stuff anyways.
LIGO,
I have a controlled current souce. Stuff 1 A through the .1 resistor and measure the voltage drop?
On a sunny day (Fri, 12 Mar 2010 09:19:04 -0800) it happened John Larkin wrote in :
Use a LM335, faster, cheaper, better :-)
For what?
I judt answered that to somebody else, use a controlled current source to measure low resistance.
It is super simple to make little opamp tool that allows you to probe 2 points on a PCB, takes five minutes to solder together, and have some gain to connect it to your 5 Euro meter, you only need difference, no need for big accuracy in that case.
On a sunny day (Fri, 12 Mar 2010 17:56:07 +0100) it happened Fred Bartoli wrote in :
stuff anyways.
LIGO,
Well, I dunno, I have met people who first thing wanted to stuff the lab with lots of super expensive stuff. Sort of an ego trip. It impresses customers perhaps, but not me. What you do with it counts.
There is an old saying: A good musician can play wonderful music on a child's flute. The beginner can not get a decent note out of a Stradivarius. Always remember that, my quote, I came up with it (bangs chest, ego sound LOL).
It is so true.
On a sunny day (Fri, 12 Mar 2010 10:50:48 -0600) it happened John Fields wrote in :
Sorry can you explain that again in electronics English?
Huh..My power supply does have constant I mode.. I'll try that someday.
stuff anyways.
or LIGO,
That's how a four wire ohmeter works.
By use separate pairs to supply current and measure voltage, you mostly eliminate the effects of the resistance of the probe.
He means you still need an accurate tool to make use of a less accurate tool. Which raises the question, why have the less accurate tool?
btw... Boat anchor is a term I sometimes use for large heavy antiqued test equipment. Pre LCD scopes are boat anchors.
stuff anyways.
or LIGO,
-- And you've determined the accuracy of the current source and the resistor, how??? JF
-- If you can't understand it in plain English, what makes you think you'll be any less confused in "electronics English", whatever that may be? The point I was making was that your statement that 6.5 digit multimeters aren't needed is wrong, since without one you wouldn't be able to determine the accuracy of the cheap multimeter. JF
Be careful using power supplies that way. It's more intended as a current limit than as a constant current supply. For one thing, it's not very stable. A potentially bigger problem is the BFC across the output, which means you might get a very high current for a while, enough to destroy transistors etc. if the output cap is charged to a high enough voltage. The current will eventually settle down to the limit setting, but that could be small consolation if you've fried something.
By his logic, you wouldn't need a wife if your neigbor has one.
Spehro Pefhany a écrit :
:But I sure wouldn't use the $5 wife, even if I had the million dollars baby to compare against...
-- Thanks, Fred.
:
-- How sad, though, if you had a cheap one and found out how woefully inadequate she was by comparing her with your neighbor's classy one. ;) JF
Except that you never get too much copper... always too little.
John
John Fields wrote in news: snipped-for-privacy@4ax.com:
that fails to verify the voltage divider. the reference would only verify the low range.
-- Jim Yanik jyanik at localnet dot com
D from BC wrote in news:MPG.26041ac522f727379896f0@209.197.12.12:
the 6.5 digit lab meters usually have 4-wire ohms measurement; you get a more reliable reading,don't have to worry about lead resistance.
-- Jim Yanik jyanik at localnet dot com
D from BC wrote in news:MPG.26044b27299d843e9896f7@209.197.12.12:
Not true.
-- Jim Yanik jyanik at localnet dot com
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