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HVAC School - For Techs, By Techs

Dec 17, 2019

In this short podcast, Bryan briefly explains why we use a voltmeter to measure “voltage drop” across loads and switches. He also covers some of the differences between passing and consuming power.

Many of us are naturals at using voltmeters already. Voltmeters have two leads, and those exist to measure the difference or potential between them. Voltage is a reference to what is going on between the leads; whenever resistance exists, we have a voltage drop. Resistance can sometimes be designed or undesigned.

When we think about power passing and consuming, we should note that "consuming" refers to turning energy from a usable form to an unusable one. Stored energy becomes potential energy when it needs to do work. Power consuming results in work; a coil in a contactor or a filament in a lightbulb is a load (the load has resistance). On the other hand, power-passing components do not have resistance, and the charges merely move. We must keep the intended resistance in mind whenever we measure the voltage of energized components; resistance will impact the voltage drop.

If you have a high-limit furnace safety, you will want to measure the voltage drop across the limit. There should NOT be a voltage drop across it because it is a power-passing component; there should be no resistance. Of course, you must determine if there is an energy potential present in the first place. Conversely, you SHOULD see a voltage drop when measuring the potential across a heater or fan motor.

Overall, wires and switches are power passing components that should not have voltage drops across them. Heaters, compressors, and fan motors are all loads that "consume" power.


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