Author Topic: Load Resistors for power output measurement  (Read 684 times)

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Offline OleenickTopic starter

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Load Resistors for power output measurement
« on: May 31, 2019, 10:17:24 am »
Hello, EEVblog Forum.

I was getting a bit confused about load resistors for power output measurement.
Specifically, I'd like to measure the power output of a DC motor at different speeds.
I know I can measure the voltage, should I use a load resistor? Why would I use one?
My thought process lead me to thinking that the load resistor would also act as a current shunt.

If I use a 10Ω resistor as a load I can measure the voltage across that.
Does this also mean that this load is also a current shunt which will give me the voltage with when divided by 10 is the current?

I could definitely be mistaken here, I just need someone to clear this up.
Thanks!
 

Online mikerj

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Re: Load Resistors for power output measurement
« Reply #1 on: May 31, 2019, 11:23:05 am »
You cannot measure the power output of a motor using a resistor, for that you need some way of measuring RPM and torque (i.e. a dynamometer).

A resistor can be used to convert a current into a voltage, so you can indirectly measure the current with a voltmeter.  By measuring the voltage across the motor and the current passing through it, you can calculate the power input to the motor, which is not the same thing as the power output (motors are not 100% efficient).
 
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