Wiring auto shut-off on airbrush compressor

[Richard Rumancik]

I need some suggestions in setting up the control for a small compressor. I want to use this compressor for airbrushing. I have a diaphragm type compressor to which I have attached a pressure regulator. There is a fixed relief valve on the compressor so that if the outlet is blocked then the relief valve releases the air. This relief valve is very noisy, however, and really meant to protect the compressor. I tried to add an adjustable relief valve on the output (before the regulator) so it would expel air when the outlet was closed, but this was unsuccessful as well as it was also very noisy and caused resonance and howling within the relief valve.

I did a bit of research and found some commercial airbrush compressors have a switch that shuts off the compressor when you close the trigger (output blocked.) It turns back on when you depress trigger (demand air.)

This seemed like a good idea as it stops the compressor from running instead of just wasting air through the relief valve. I tried wiring a switch in the circuit that had a cut-in of 20 psi and cut-out of 40 psi. (There is some adjustment available.) However, this does not work either. It will cut-out fine at 40 psi but tries to re-start at 20 psi when there is still pressure in the line. The compressor can't start under 20 psi backpressure - it just hums. In fact, it won't start unless I get the output to fall below a few psi. A normal switch doesn't go so low (e.g. cut-in of 1 psi).

So what am I missing here? How do they do it on the commercial airbrush compressors?

[Joe Pelonio]

I'd guess that your problem is the output of that compressor is way more than that of a regular airbrush compressor, they normally won't go above
25-50 psi. I have a couple of the ones like yours, that were originally made for a spraypainting system. They were given to me free and I keep them as backups but would never use them long because of the noise. I'd suggest buying an inexpensive airbrush compressor like this one that's practically silent for under $100 and has the automatic shutoff:

http://www.airbrushcity.com/comp45/

I have one that I use for the laser and for airbrushing and it's been going strong 6-8 hours a day for over a year now. At my local art/craft stores they don't have anything under about $300.

[Richard Rumancik]

Joe - the compressor has a built-in relief valve of 39psi. Not sure if this is inherent in the compressor design or there because of what it was originally used for. But that is high enough for me. My compressor is pretty quiet, it came out of an oxygen concentrator which runs in a house 24/7. It looks like the dual-diaphragm heavier duty ones at the bottom of the link you gave (8751-T), except no tank.

Since I already have the compressor I was hoping to put it to use; if I need to buy a switch that would be cheaper than a compressor. I just don't know yet what I need . . .

Since the commercial ones shown seem to be able to pressurize a tank with a diaphragm compressor, it would seem that their pumps can start up under a back pressure, as they would have to pump against whatever pressure is in the tank. I suppose I could play with the value of the start capacitor on the motor to boost the starting torque but I would have preferred not to.

[James A. Wolfe]

Richard,
Most compressors won't start under load. Most have some device allowing the air to bleed off from the compressor head prior to starting up. A dump valve or unloader valve. If you put a tee and a check valve at the output port of the compressor and add a small flow regulator (or just a small valve in the side leg of the tee, you can accomplish what you need very inexpensively. See attached. You'll need to play with the adjustment to get it to dump slowly.

Jim