Testing sensitive circuit

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schickb
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Posted by schickb: Mon. Mar 24, 2008 - 09:57 PM
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I have a CMOS IC that is 3.3v with an absolute max rating of 3.9v. Once its on my pcb I want to test the circuits to make sure none are shorted to ground. But I noticed that my multi-meter (Fluke 87V) uses 7.9V for continuity testing. The power is very low, but I'm wondering if just the over voltage could do damage.

I found a circuit that is shorted to ground, and then found myself wondering if I just blew something by testing it or if it was a reflow soldering problem. Sort of a Heisenberg uncertainty thing going on here...

-Brad

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ka7ehk
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Posted by ka7ehk: Mon. Mar 24, 2008 - 11:15 PM
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Brad -

There MIGHT be a problem, but most multimeters are VERY current limited. If the points you check have any protection diodes (to Vcc) there is probably no problem.

Actually, if you look at the spec, it probably says, somewhere, something like Vcc+0.4V. Unpowered, that Vcc will be (approximately) zero. So, you will run into that limit first, if there is one.

Jim

 

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schickb
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Posted by schickb: Thu. Mar 27, 2008 - 11:32 PM
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Thanks Jim. I found the problem, and it was a soldering issue. Although now that I know the multi-meter is 7.9V I'm hesitant to use it on the unpowered 3v3 circuit. Guess I'll just use software to test the boards and a multi-meter after if something isn't working.

-Brad

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barnacle
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Posted by barnacle: Fri. Mar 28, 2008 - 07:37 AM
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There should be no issues using the meter on an unpowered circuit for connectivity testing; you're not referencing to the IC's ground or rail; just across a copper wire - or at worst, across a diode junction.

The meter is current limited to a very low value.

And of course, in voltage measurement mode, which you're more likely to use if the board is powered, you're talking megaohms per volt input impedance to the meter.

They're *designed* to be used like this...

Neil Barnes
www.nailed-barnacle.co.uk

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jgmdesign
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Posted by jgmdesign: Fri. Mar 28, 2008 - 11:40 AM
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Another idea, albeit kinda funky is to put a 10kohm resistor in series with your probe. First connect the resistor between the probes and get the "shorted" value, then probe the circuit with the resistor as the probe point. if you read the new 'zero' you have a short, anything else, subtract 10k from the reading and you are golden. The resistor will drop the current to a few mills and won't annoy your devices.

Jim

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