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Son of Samurai's Blog

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How to Measure Voltage Drop Across Thermistors

Son of Samurai

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All of us techs are carrying around multimeters these days, and that means we have several different voltage measurement settings available to us at any given time. Usually these are DC voltage, AC voltage, and LoZ (low input impedance). Each of these functions has a specific use, and part of taking a voltage measurement is picking the correct setting for the job.

LoZ is by far the most common voltage function you should be using. In fact, there is almost never a time when you should do an AC voltage measurement with the standard VAC function of your meter. LoZ is a much more reliable measurement, for reasons I covered in this post.

But when it comes to measuring DC voltage across a load in the appliance, you need to always use the DC function on your meter. That may sound obvious, but it means that you have to have a functional understanding of the component that you're measuring and know what its power supply is by reading the schematic. Thermistors, for example, are always low voltage DC components, and therefore need to be measured with the DC function of your meter.

In fact, there's an interesting thing that happens if you try to use the LoZ function on a low-voltage DC circuit, like a thermistor circuit. Because of the relatively low impedance of the meter on this setting (about 3 kohms, as opposed to the 50 megaohms that the normal AC and DC functions have), current will flow through your meter -- way more current than the puny DC power supply is equipped to handle. This will result in loading down, causing your meter to read 0 volts even in a circuit with a good voltage supply, all because you're using the wrong setting for measuring a thermistor.

If you want to troubleshoot thermistors like a champ, you should check out these exclusive, premium-member-only webinar recordings right here at Appliantology.

 

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Son of Samurai

Posted

3 hours ago, Scottekarate said:

I use this meter

https://www.harborfreight.com/cm200a-200-a-fork-style-clamp-meter-64016.html

LoZ switches between ac and dc would you still recommend using the V function? It's switches automatically too.


If you’re measuring an AC circuit and you have a meter with a LoZ function, you should always use LoZ over the non-loading function. It’s the more reliable measurement.

If you’re measuring low voltage DC with that meter you mentioned, then you should use the normal V function, since you don’t want to load down the circuit.

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Samurai Appliance Repair Man

Posted

4 hours ago, Scottekarate said:

LoZ switches between ac and dc would you still recommend using the V function? It's switches automatically too.

The problem with using LoZ for low voltage DC measurements isn’t whether or not it auto ranges (AC to DC). The problem with using LoZ on low voltage DC circuits is that the input impedance of the meter on the LoZ setting is too low (“LoZ” stands for “low input impedance”).

A low input impedance means more amps can go through the meter than the low voltage DC power supply on the board can handle. (The first webinar recording that Sam posted explains all this in detail.) The low input impedance  causes the regulated power supply voltage to get “loaded down.” 

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Budget Appliance Repair

Posted (edited)

15 hours ago, Scottekarate said:

I use this meter

https://www.harborfreight.com/cm200a-200-a-fork-style-clamp-meter-64016.html

LoZ switches between ac and dc would you still recommend using the V function? It's switches automatically too.

@Scottekarate I use this same meter - It's seems to be a pretty good meter but I have found I really don't like the AMP fork design, (forks to fat to fit between tight wire bundles and can't let it hang from the wires like with a clamp), other than that it's a great meter.

The only other thing is I seem to always forget to push the function button to change to AC voltage check as it always starts in DC mode when set to voltage test.

Edited by Budget Appliance Repair

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Scottekarate

Posted

On 4/1/2020 at 7:25 AM, Budget Appliance Repair said:

@Scottekarate I use this same meter - It's seems to be a pretty good meter but I have found I really don't like the AMP fork design, (forks to fat to fit between tight wire bundles and can't let it hang from the wires like with a clamp), other than that it's a great meter.

The only other thing is I seem to always forget to push the function button to change to AC voltage check as it always starts in DC mode when set to voltage test.

Yeah, I never use that fork.  I stick with my trusty clamp on but I do like the meter.

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