EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: MLXXXp on March 19, 2014, 06:33:13 pm
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I recently bought a BK Precision 2709B multimeter. I was surprised to find that the voltage output polarity, for the ohms and continuity functions, is reversed. The black common connector is positive with respect to the red connector.
However, the polarity is "correct" for the diode and capacitor test functions. Red is positive and black is negative.
Every other multimeter that I've owned or used has been red positive and black negative for all ohms, diode and continuity tests.
Are reversed polarity outputs fairly common or rare for multimeters?
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It's not common, but there are a few that are like that. (reversed polarity in resistance measuring mode)
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The idea with this is that if you check a resistor in parallel with a diode, the diode won't be biased into significant conduction, changing the reading.
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The idea with this is that if you check a resistor in parallel with a diode, the diode won't be biased into significant conduction, changing the reading.
On the ohms ranges, a good multimeter will have an open circuit output voltage low enough that it won't turn on a semiconductor junction. It won't matter which way the leads are to measure a resistor in parallel with a diode. This is true with the BK 2709B, except on the 0-660 ohms range, where the open circuit voltage is about 1.2V. For in circuit measurement, I always check both ways anyway, just in case, and don't always assume that there isn't something else in parallel that's messing up the reading.
I just looked at the manual for the BK 2709B again and it actually does specify negative voltages for the ohms ranges. I hadn't noticed the minus sign when I initially read the manual.
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Negative polarity in the ohms range was typical for old analog multimeters. It was just easier doing the range switching. The meter designers didn't have to switch the COM terminal around, but could have it permanently wired to the movement.
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I've realised that this reverse polarity ohms thing has an advantage:
When testing diodes, you can check the voltage drop using the diode test range and then just switch the meter to the ohms range to check for high impedance in the reverse direction, without having to reverse the leads. :-+