# Question about PTCs and current limits of Cortex, Motors, etc.

I recently read this fascinating study done by jpearman:

https://vexforum.com/t/ptc-performance-measurements/20872/1

However, I am still confused about how the Cortex and motors, etc. might respond to stall conditions on motors.

For example, I’ve read that the stall current of a 393 motor is 4.8 amps, but that the Cortex’s limit for motor ports 1-4 is only 4 amps. I know there is a time factor involved when it comes to the PTCs heating up and tripping out, but I also know that my kids stall their motors all the time and I do not see the motors or the Cortex trip out very often.

It seems to me that if two 393 motors stalled on ports 1-4, then the current draw would be 9.6 amps, and that would trip the Cortex within 6 seconds or so. But I don’t think I have seen that happen. Are they just lucky?

Obviously there is something about this that I’m overlooking. Perhaps the battery sags so much on stall that the Power=VI dissipation is not so high??? My ignorance on this issue makes me nervous because my kids want to add another two motors, some pneumatic solenoids, and a power expander to boot, and I would hate to see things crash and burn because they are making too many demands on their system.

It’s complicated.

Motors will tend to trip before the cortex.

The 4.8A stall current is not a constant, for example, look at post #1 in this thread.

In scope capture 3 it shows how a single motor responds when stalled, you have under 2 seconds before the motor PTC trips and during that time the current is dropping as the PTC resistance increases. Once the PTC is hot, stall current will be lower than the 4.8A theoretical maximum, I could generally only see that amount of current once during the first test of the day.

Also look at this thread, it has more background.

https://vexforum.com/t/estimating-motor-current-part-3/22231/1

Believe it or not, I’m relieved to hear that.

Your detailed analysis of all things Vex simultaneously blows my mind and serves to calm my anxieties. It’s comforting to see somebody has peered into all these little black boxes and revealed some of their inner workings. Who the heck are you, anyway?

I will need to study these links you provided. Wow. Thanks so much!

Be happy to know that a power expander will actually help with this rather than hurt. It as well as adding another battery to the system adds another breaker that the motors can be shared between. Very many teams are able to run 10 393 motors and a boat load of pnuematics within having severe electric issues if that helps ease the anxiety about it.

Side note: It is great to see more mentors really getting involved in the technical aspect of things. Keep up the good work.

Yes, that does provide some encouragement and that’s the route we’re headed for right now. Thank you!