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I asked a question where I was trying to figure out how my truck controls the speed of its blower motor.

This answer provided the following diagram showing that it uses a set of resistors in series to control the voltage level.

enter image description here

This is what I assumed was going on, but when I was testing it I could not detect the voltage drop at the motor. Instead I always got the full voltage of the battery, no matter how many resistors were added to the circuit.

I now know the reason this happened is because I had the volt meter wired in series with the circuit, instead of in parallel.

My question is why did I have to wire the volt meter in parallel? I know the volt meter doesn't let current pass through (when measuring voltage), but wouldn't the resistors still restrict the force of electrons from the battery? Or do resistors only "resist" when actual current is flowing through them?

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  • \$\begingroup\$ I = V / R ...... \$\endgroup\$
    – D.A.S.
    Commented May 13, 2022 at 18:52

2 Answers 2

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Ohm's law says the the voltage drop across a resistor is equal to the resistance of the resistor times the current through the resistor. $$V=I*R$$ So as your experiment proved, no current, no voltage drop.

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  • \$\begingroup\$ And also, small current -> small voltage drop. Because there will be a tiny amount of leakage through the multimeter. But only a tiny amount, so the voltage drop over the resistors turns out to be very small, likely below what the meter can even measure. \$\endgroup\$
    – Hearth
    Commented May 13, 2022 at 18:26
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I = (Vbat-Vbemf) / R (including motor DCR, switch R and blower shunt equivalent resistance) ...... but when the motor rotates it also generates back emf voltage, Vemf, so the current reduces when it is up to speed. The coil also has a DC resistance DCR which limits stall and start current.

But in the off-state car wind can move the blower to generate a small voltage proportional to RPM, so the resistor draws enough current to force it to stall, but cannot start in the Off position. (Presumably to save bearing life)

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