Power expander helps?:

I’m contimplating getting a power expander because I need more power. :smiley: the reason is because my two geared for speed drive 393 motors are cutting off because it is taking to much juice from the cortex, so the cortex is cutting off the two 393’s. Will the power expander help the 393 motors to not shut off?

How can you tell that it is the Cortex that is cutting off the power to the 393, and not that the 393 are self-limiting due to their own breakers?

Neither Cortex nor power expander will prevent the 393 from shutting down on their own. A power expander provides 4A nominally. Cortex ports 1…5 have 4, and ports 6…10 have 4A. If you are using the 393’s in Cortex ports 1 and 10, and not using much on ports 2…9, and the 393’s are still shutting down, then I predict that adding a power expander wont help this problem.

Maybe rather than needing more power,
you need to design around the power constraints of the equipment you have.

The power expander can help, just be aware that the power expander has a single 4Amp breaker (similar to that on ports 1-5/6-10 on Cortex). To mitigate issues such as this, I try to spread motors that will often be used at the same time onto separate breakers (ie. put drive motors on ports 4-7 instead of 2-5)

I understand that I might be asking too much of the motors, but they run perfectly for 1:15mn then all of a sudden shut down.

That would suggest that you are tripping the thermal breaker within the motors by overheating/overworking them. Try changing the gear ratio so that the motors are in less of a stall.

But the motors are not hot after use. Can they overheat without being hot to touch?

I find that when 393’s overheat, it is hard to tell by feeling, as they only heat up in a few (very small) places.

Ah… I see

i think investing in a power expander will be a good investment anyways
when we first bought the power expander, we never went back… (even if we wanted to shave off weight)

I just want to make sure everybody understands what is actually happening in this case. The reason the motor stops is that its resettable fuse (aka PTC) “trips” because too much current passes through it.

The PTC’s main function is as an over-current protector. It doesn’t put out much heat, and is only somewhat effected by surrounding heat. Most of what you are feeling is from the DC motor warming up, but that is switched off by the PTC before it can get too hot.

Also, it is worth pointing out that the 4A protection devices in the Vex Microcontrollers and Power Expander are also PTCs. So the protection built into the motors is the same type as is built into the microcontrollers.

Wouldn’t it be cool (pun intended) if the new backpack encoders had a couple pneumatic air ports so you could blow compression-chilled air through to help cool your motors during a match?

Make sense?

  • Dean

The PTC in the 393 motor appears to be the HR30-090 as described in this spec sheet. This hasn’t been confirmed by IFI, and it is possible they have used (or will use) other vendors or PTCs over time.

I was not able to locate a spec for the PTC in the 269 motor, and I haven’t yet looked up the ones in the microcontrollers or Power Expander.


  • Dean