# odd numbers when measuring current using talon srx

I’m pulling the output voltage and current down from the talons and intentionally stalling a 775pro.

at 2 volts I get about 5 amps. pretty close to the 0.4 ohms I’ve seen mentioned around here for this motor.
at 5 volts I get about 20 amps ಠ_ಠ .
at 7 volts I’m near tripping the 40a breaker.

Remember that all of this is happening at a stall, so no backEMF.
I noticed that the stall current on the spec sheet was something like 134a at 12v, coming out to just below 0.1 ohms.

Is there something that I’m missing about how motors work?

I would be interested in seeing how directly measuring the voltage across the motor terminals during these tests compares to the commanded value. Even if the motor controller is perfect, there should be some deviation. Even though the talon output is actually 12V pulses, you should get a good constant reading because the frequency is so high (15kHz).

The 0.4 ohm figure seems very high to me. I found one post where it is mentioned, but I strongly suspect that the 0.1 ohms that you calculated from the stall current spec is more accurate.

Even on a cold motor, you shouldn’t expect to be able to get a current as high as the stall spec with an FRC setup because of the other resistances in the circuit. Let’s say the motor has a resistance of 0.09 ohms, and everything else in the circuit (battery, wires, connections) has a resistance of 0.05 ohms (I do not know what a typical value for this figure would be, but the battery alone is ~0.01 ohms). With a 12V source voltage, there would be a substantial voltage drop across the battery and wiring, limiting the current to approximately 85 ohms, rather than the expected 134 ohms. In robot performance terms, this means that the torque output of your motor is limited by the electrical characteristics of the circuit - all the more reason to make sure all the connections on a robot are properly crimped and/or soldered!

I’m not sure why you’re getting such low current at 2V (perhaps it has to do with the motor controller?), but hopefully measuring the voltage across the motor will shed some light on it.

You don’t seem to be missing anything about how DC motors work, after all, for what you’re doing, it’s basically just a wire.

/\$0.02

interesting. I took a look at voltage over the terminals and what do you know, it seems that the can talon is overestimating by about 10% (I’ve only run a couple of tests so not entirely sure what that number really is)

doesnt seem to explain the low current at low voltages, but interesting information. maybe I can’t rely on those outputs as much as I thought.