Hi, I have to measure the voltage, current and resistance across a DC motor. I am going to use a voltage divider circuit where the motor is the variable resistance. The thing is I don't know what should be the value of the fixed resistor. I am using an 5V external power source (iPhone charger made into a power source). Here is the motor data sheet. Please help since I already burned one resistor.
school project?
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I am going to use a voltage divider circuit where the motor is the variable resistance. The thing is I don't know what should be the value of the fixed resistor.
For the current measurement, I assume?
The specs give a stall current of 500mA (1/2 Amp) so you can use [u]Ohm's Law[/u] to calculate the approximate lowest motor resistance. (I'd use the "rated" 6V for that.)
You don't want to loose too much voltage across the resistor, but the voltage drop has to be high-enough to measure. So, choose a resistor that's no more than 10% of the (calculated) motor's lowest resistance. You can use a lower value resistor if you can get enough voltage & enough resolution.
(You can use the optional 1.1V ADC reference for more resolution at lower voltages.)
Please help since I already burned one resistor.
Power (Wattage, related to heat) is calculated as W = Voltage x Current. (For the resistor, that's the voltage across the resistor and the current through it.) Or, it can be calculated as I2R (which should be handy in this case) or V2/R. It's considered good practice to use a resistor rated for at least twice the dissipated power.
I would not do it that way. I would put a very low value resistor in series with it, apply a fixed voltage, and use the voltage across the resistor to measure the current. That way, the motor can run from a voltage source. In a voltage divider, the motor will sometimes be effectively running from a current source. The hysteresis of the static vs. dynamic friction (i.e. the stalled and running condition) will be magnified, and you will be able to measure only one or the other but not both. You would be able to start and stop the motor by hand in this case, and will get completely different numbers in each case.
Your "professor" probably doesn't expect any differentiation of stall current, so it will be bonus marks if you want to tackle it. It would be better if you investigate stall current separately from the V/I curves measured while the motor is normally running .
I agree with aarg, small high precision shunt resistor is the correct way to measure this. I would also switch to internal 1.1v ref for higher resolution.
