I recently bought the 34:1 Metal Gearmotor 25Dx52L mm 12V with 48 CPR, and I was interested in measuring it’s resistance. So according to FAQs tab, you can estimate it to
be:
R = Supply Voltage (V) / Stall Current (A) = 12/2.1 = 5.71 Ohms. But when I placed my
meter across the red and black motor power pins, I got a value that exceeded 300 ohms.
Also I noticed, it says that the Free-Run current is 200mA, but I measured it to be about
70mA at 12V.
Could you explain why this is the case ? Does the encoder circuitry provide a path to
the motor’s input that adding some resistor components ?
We have three motors that you could be referring to (LP, MP, and HP), but from the specifications you are using, it sounds like you are talking about the medium power version:
The free-run current listed on the product page is a very conservative maximum, so it’s not surprising you are seeing something much lower in practice. We are in the process of better characterizing those motors, which is giving us more accurate numbers to publish, and we will be updating the information for them when we finish testing. I have asked for the #3240 to be bumped up in the list, so I can post some preliminary data here when we have it.
As for your resistance measurement, I think that you are seeing a number of different effects conspiring against you. One is that it is difficult to accurately measure low resistances, which it is why it is typical to instead take a four-point measurement of the current and its corresponding voltage drop. Another is that a motor is more complex than a simple resistor (most of the time you are measuring the resistance of one coil in parallel with the other two coil resistances in series), so the measured resistance across the terminals depends on the actual position of the rotor. If you turn the output shaft by hand, you should see the measured resistance change. Lastly, it is possible the carbon brushes just aren’t making optimal contact with the rotor at the moment because the motor is cold and stationary (and maybe the brushes haven’t been broken in yet?). Can you try putting a low voltage across the terminals while holding the output stalled, and then simultaneously measure the voltage and current?
