How to avoid tripping cortex PTCs

My robot has had significant problems tripping the cortex PTCs. My solution was to make scale all off the motor inputs by .75 (e.g. telling the drive to go 127 actually sent 95), which let the robot work for almost a full match, but was very very slow. I will probably rewire, but I’m not sure to what. The y-cabling is flexible, but ports 1 and 10 are unusable. Also, after I rewire, what should I do in the event I continue tripping the PTCs?

My current wiring config:
2 - 2 of the left drive wheels y-cabled together.
3 - left drive wheel
4 - both lift motors, y-cabled
5 - claw motors, y -cabled
6 - right drive wheel
7 - right lift motor
8 - right lift motor
9 - 2 of the right drive wheels, y-cabled

Ideally you’d like to have the claw motors split between two sides and have the right lift motors on Y-cables if you really can’t/don’t want to use ports 1 and 10. We don’t have one and really have trouble with burning out for some reason (knock on wood), but most people use a power expander to add an extra PTC that shares the load. Personally, I would make sure it doesn’t burn out without a power expander, and then add a power expander as redundancy so you stand no chance whatsoever of burning out.

we were having similar issues so we put the claw motors on a power expander. This didn’t competely solve out problems, but it made it a lot better.

Which ones are tripping ? Drive, lift, everything?

I’m assuming that the problem is cortex PTCs. Those are what generally give trouble unless there is a huge problem somewhere else. So if it’s cortex PTCs, it’s a combination of everything, but there may be a specific culprit of course.

If it’s the cortex PTCs then add a power expander. If it’s motor PTCs (more likely in my experience) then you have to figure out how to drop the amount of current used and keep all the motors in their sweet spot. My old smart motor library can help with the drive sub-system but will not help much with a lift, re-evaluate gear ratios, remove sources of friction, keep motors running at 70rpm or higher when sending full power.

It’s incredible that it has been almost 5 years since the rule change that allowed 10 393 motors. When that happened there was a lot of concern about current draw on the cortex, now we are allowed 12 ! It took a while to find this thread (I had to go back to the old forum in the wayback machine) but it’s worth reviewing the calculations we did back then.

We concluded that 10 motors all running together at their optimum point (ie. not tripping their internal PTC) would be at the theoretical limit of the cortex and power expander. I used a 1.35A current in those calculations, so now with 12 motors we will be at 12 x 1.35 = 16.2A We get away with this as it’s unusual for all motors to be used together, however, with this years designs that may be happening, drive backwards whilst lifting stars and gripping them with a claw. This is just an observation and something to think about.

It is the cortex PTCs. We would see half the drive die, the claw die, and the lift be weak. The way I saw it tripping most often was turning then driving backward while holding the claw shut (~20 power) and lifting the lift in the early version of the autonomous. I’ll rewire and see how then helps, then use the smart motor library and/or add a power expander. We don’t have one, but I should be able to order one before next competition.

you are putting a lot of strain on the ptc through ports1-5 and it sounds like that is the one that is tripping. try moving the claw or lift y cable to a port between 6-10.

Did you ever figure it out? My robot’s drive stalls after 40 ish seconds. We have tried it alone on the cortex, alone on the power expander, and with y-splitters.

I haven’t gotten my power expander yet. I did rewire, but since last Saturday we’ve been working on some minor hardware fixes/changes, so I haven’t gotten a chance for any extensive testing yet.

Also, it depends on your drive ratio, motor count, etc., but more likely than not you’re tripping the internal motor PTCs, not the cortex ones. Try making your robot lighter or regearing your drive at a less aggressive ratio.

It’s not like the problem of burning out drives always has the same causes. What’s the weight of your robot? What’s the gear ratio and motor count?

This is another thing to consider for the mythical cortex V3… more amperage support.

I’m not sure if I’d like that simply because a new cortex would now be very vital to high levels of success… that’s expensive for big teams.

Our robot was also having stall problems because the PTCs’ were tripping. We realized that the cortex we were using was many years old, and with an older cortex, the PTCs’ get less reliable and trip at a smaller voltage than they are supposed to trip at. Upon discovering this, we ordered a fresh cortex and that cortex worked fine.

@JPearman specifically tested a PTC device for 500 trigger cycles, (more than normal use is ever likely to cause) and didn’t find evidence of it.

See this post:

Note that in the first post of that thread (scroll up if you want to read it) @jpearman said data seemed to show the PTC devices became ** less ** sensitive with age and use. (This is the opposite of your belief.) However, after testing the PTC for 500 cycles, he found no change.