Sidecar PWM output current

[EVanWyk]

I was just directed to this question from FRC175:

I have been trying to determine the maximum current output for each of the Sidecar PWM terminals. I have found the following sidecar data in the Manual:

1. 6 volt, 3A Buck power supply and,
2. 15 mA output source

I am not sure if either of these are giving me the correct info I need to operate at least two Hitec 322HD servos on the same PWM output without causing overload. The specs I got from Hitec are:

a no load current of 180 mA,
idle current of 7.4 mA,
stall current of 800 mA.

I want to be sure the sidecar PWM terminal can handle more than 1.6 amps.(1600 -2000 mA)

I recommend against this practice generally, though it is feasible specifically.

All of the other 3XN connectors in the system are rated for 1A per pin. However, the PWM outputs are special - they use a KK Friction lock that is rated at 4A continuous. Upstream of this is a shunt jumper, which is typically rated for 1A continuous. However, there are jumpers that are rated for more - Molex's 0900591020 can handle 3A continuous. This would leave the burden on your wiring.

If you use an appropriate jumper AND your wiring is done appropriately, this is a safe practice. However, I would place this practice in the "Don't do this unless you need to" category: Why bother with the headache and the I2R losses?



One reason I am concerned is that servos do not play nicely with each other. If you have two servos drive the same mechanical device with the same signal, they will likely fight each other and be forever stalled. This is because servos typically control position, rather than force. They will inevitably disagree with the exact position and start a stalled grudge match as they attempt to convince each other of the appropriate way to interpret the command.

If you add a bit of series elasticity to the coupling you can reduce the severity of the fighting. This converts them to force controllers (ish). Two force controllers working in tandem aren't so bad - a mismatch results in one being lazy, rather than a fight.

If your servos are not mechanically coupled, I'm not worried. For example, using two servos for two separate shifters: Not a problem.

[Russ Beavis]

To further elaborate on Eric's advice, the legality of attaching 2 servos to the same PWM Output is somewhat questionable. In my interpretation of the rules, multiple servos per PWM Output is permitted whereas attaching multiple motors to a single Spike/Victor/Jaguar is clearly illegal.

Whenever you question such interpretations or rules, you really need to post a question to the official Q&A forum.

Russ