Those of you at our last few workdays will have heard me moaning about transients from our new solenoids causing the PyroCardium USB controller to trip off. The back story is that the 24V DC solenoids we had been using before turn out not to have seals compatible with propane. So I’d like to switch to some 110V AC solenoids I have which have propane compatible nitrile seals. I got 16 of these solenoids for free from my dad - they’d be about $800 new.

The problems began when I replaced the 24V solenoids with the 110V AC ones. As soon as I tried controlling them from the computer, the software crashed immediately. When you turn off a solenoid, the energy stored in the coil gives rise to a rapid increase in voltage across the inlets. This is called inductive kick. I have a varistor in place to dissipate this kick, but it doesn’t appear to be enough - my theory is that this transient voltage spike is getting coupled into the USB board and giving rise to errors. This theory is supported by the fact that adding a capacitor in parallel with the varistor, which should also help dissipate the transient, reduced the crash frequency (though it still crashes after about 5 or 10 minutes of cycling).

To fully isolate the USB board from the rest of the system, I’ve ordered some optoisolators. Optoisolators chips that electrically isolate one circuit from another by coupling them with light. They have an LED driven by the input coupled to a phototransitor that forms the output. I’ll have them on Saturday, and with them I’ll be able to fully isolate the USB board from the high power switching circuitry. Hopefully that will fix our problems.

This entry was posted on Wednesday, March 26th, 2008 at 9:38 pm and is filed under PyroCardium, Workshop.