The Trainer #49: Testing the positive side of an electrical circuit

In our November 2015 edition of “The Trainer,” we investigated why it was possible to read voltage on your Digital Multimeter when one lead was on the negative battery post and the other lead was on a body ground. If that idea still throws you, take a moment to watch the video at http://www.MotorAge.com/nov15trainer.

Now, if you watched the video already you hopefully had a few “light bulb” moments as the concept of voltage drop on the groundside of an electrical circuit was demonstrated. Here’s the short version.

All voltage will be used to overcome all sources of resistance in the circuit. Normally, the load is the one real source of resistance and therefore will use the most. If I put one meter lead on the battery ground terminal and the other lead on the ground side of the load I’m testing (with the circuit closed and operating), I should read nearly 0.0 volts if all is well. If I read more than that, the amount of voltage I’m reading on my meter is telling me there is an additional source of resistance between my second meter lead and the first lead at the battery.

But what happens on the positive side of the circuit? If I leave the one lead at the negative battery post and move the second to the positive side of the load, I’ll read close to system voltage but not quite. I have to take a second measurement directly at the battery and then subtract the two to see how much voltage drop exists on this side of the circuit. That can be confusing, especially if the source voltage is varying while you’re attempting to test.

In this January 2016 edition of The Trainer, I’ll show you what I mean and show you how moving just one lead will remove the confusion. Understand this, and you’ll be tracking down electrical gremlins with the best of them!

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