Compressor not shutting off at 115 psi

[Jon Stratis]

Originally Posted by jalmos

Thanks for the quick response.

The compressor doesn't shut off and gets to a pressure of 100 psi.

We have the compressor wired into a spike replay, which goes back to the power distribution board. The pwm wires do to the DSC.

The pressure relief valve is wired to the DSC using a small red and white wire, only two not three. Would there be a ground missing? Please let me know if you need more info. Thanks so much!

jalmos

You should be using the black and white wires on the PWM cable, not the red one. It's important to have the pressure switch wired up to the DSC with Ground and Signal. The digital inputs on the DSC all have "pull-up resistors" - this means that when there is an open connection, they'll read (in the code) as though the signal was connected to ground. So when your pressure switch closes, you want it to connect ground to signal to pull it down.

Once you get that fixed, check to make sure the code is working. Do this by removing the wires from the pressure switch and manually touch them together. when touched together, the compressor should turn on. When you pull them apart the compressor should turn off. If this happens, then you know everything is set up correctly both in the code and electrically!

Once you know all of that is working correctly, hook the pressure switch back up and address your issue - you can't generate more than 100 PSI, so the pressure switch never trips! As others have suggested, the blow-off valve is likely the issue. When you run the pressure up to 100 PSI, put your finger over the hole - you'll be able to feel the air pushing out if it's tripping early. To set it properly, do the following:
1. Look at the valve. you should see a big nut on the end (where the air comes out), a thin nut in the middle, and another nut on the other end that you used to tighten the valve to the connector.
2. grab the big nut with one wrench and the thin middle nut with a second wrench. Give them a twist to loosen the middle nut, and spin it down a bit to give you room to work with.
3. Turn on the compressor and let it get up to the max it can. Slowly twist the big nut (by hand, it should move pretty easy) to increase/decrease the pressure. Assuming you got the code/wiring figured out already, you should see the compressor turn off around 115 PSI!
4. Once the compressor turns off at 115 PSI, grab a screwdriver or a wrench and lay it across the two prongs on the pressure switch to turn the compressor back on. Turn the blow-off valve until it starts letting air out at 120 PSI.
5. Lock down the blow-off valve. Hold the wrench on the big not completely still, don't twist it at all! tighten the thin middle nut against the big nut to ensure that neither one will move on its own.


As a side note, at the MN State Championship I had a team that appeared to have a bad pressure switch - from all indications it wasn't turning off the compressor until they hit 140 PSI. We tried swapping it with two other pressure switches, but neither of them seemed to work (we turned off the robot after passing 125 PSI for each of them). We finally realized the issue was with the pressure gauge, not the switch! Once we swapped the gauge out, we saw that the original pressure switch was working perfectly correctly.