The compressor pressure switch provides some key functions on the air compressor.
One purpose of the compressor pressure switch is to shut the compressor off when the air compressed into the tank has reached the correct air pressure.
Another function is to turn the air compressor back on on when the air pressure level in the compressor tank has dropped to the point where more air is needed.
One more key function of the compressor pressure switch is that it is the first defense to over pressurization of the tank and air lines on the air compressor.
Pressure switch appearance
On many of the smaller DIY type air compressors, there will be only a few manufacturers of pressure switches. Two common makes are shown in the photo below.
Compressor Pressure Switch for Smaller Compressors
There are a lot of air compressors sold today that we would classify as mini-air compressors. These are air compressors that can be operated by a 12 VDC power supply from a vehicle accessory port. Some of these mini-air compressors also work on 120 VAC. They all use small, fractional HP, electric motors to power the small compressor pumps. They do not use the air pressure switch shown above, but rather, theirs may be a tube shaped switch as shown in the next image, one from a DeWalt mini-compressor.
Compressor Pressure Switch Operation
A pressure switch is an ON/OFF device that reacts to pressure. They are used in many industries and in many media. We’re concerned here only about the air compressor pressure switch.
The pressure switch will have a device inside that reacts to pressure applied against it. In the Condor and Lefoo switches shown above, that internal device is often a diaphragm of some sort, one of which is shown in the photo.
When air pressure in the air compressor tank creates enough back pressure, that back-pressure deforms the switch actuator (diaphragm or other sort) inside the pressure switch and causes it to move. On the high pressure side, that actuator deformation typically opens contact inside the switch. With the contacts now being open, power stops flowing to the compressor motor circuit, and the compressor stops.
When the air pressure inside the compressor tank and lines drops, the compressor pressure switch internal operator begins to deform or move in the opposite direction. This movement is typically caused by an internal spring. When that deformation or diaphragm movement is far enough the contacts inside the pressure switch are made (come together) and power flows across them. The power flows to the motor circuit on the air compressor, the motor starts, and air starts pumping into the tank.
As the back pressure inside the tank increases, once again, the compressor switch internal actuator begins to deform or move the other way, eventually the contacts break, and the power stops.
The cycle repeats as long as compressed air is being used, or until external power to the air compressor is stopped.
Adjusting the compressor pressure switch
There may come a time when you have to adjust the compressor pressure switch. Typically that will only be when you have replaced the original switch that came with the air compressor, and the new switch cut in and cut out settings are different, and have to be adjusted to the levels of the replaced pressure switch.
As far as we know, you cannot field adjust the pressure settings of the tubular pressure switch similar to those shown in the photo of the DeWalt pressure switch. If your compressor uses this type of pressure switch and the switch fails, you are going to have to replace the failed switch with another of the same size and type. That will typically mean quite a search, as many of the brands of compressors on the market are mass marketed through big box stores which have a dismal record of stocking parts for the air compressors they sell.
Please follow this link for more on adjusting the compressor pressure switch.
Comments, advice or questions are welcome using the form below.
I have a Campbell Hausfeld HL540200. Sometimes the LEFOO LF10-4H pressure control has to be tapped/hit to make the motor run. And sometimes the motor’s breaker trips. Any ideas?
I’m thinking the no-start is caused by the hole under the pressure control’s diaphragm plugging up,
The tripped breaker – a either a faulty capacitor or sticking check valve.
Hi, I have an emglo wheelbarrow electric compressor. I replaced the pressure switch with a new lefoo 95-125 psi pressure switch. the switch cuts off at 125psi but then starts turning the motor on and off in rapid succession and will not stop until unplugged. I’m afraid i wired or piped the air wrong to it or something. Odd as it runs like a champ up to pressure but then when it tries to stop, the pressure switch keeps turning on and off every two seconds
Can I use a different pressure switch for my 21 gallon harbor freight compressor in stead of the Lefoo one that it comes with should work right if I get one that fits?
What causes the pressure switch to fail and not relay power to the motor side? My compressor stopped running and I found out using a multimeter that the power was not running to the motor and had failed. I purchased a replacement pressure switch (at $80!) and installed it. The compressor started right up. I thought my problem was taken care of.
Now 6 months later the compressor is again not working. I went through the same procedure: drained and cleaned the tank, checked the capacitors, checked the oil, checked the air filter, checked the check valve, checked the lines to see if anything was clogged, and checked the pressure switch. Again the pressure switch has failed and another one needs to be purchased at another $80 cost!
What is causing these failures? I have the compressor plugged directly into the wall socket using a 20A line. Any suggestions?
Where do I hook up My quarter inch line From pressure switch go to the tank?
The pressure switch contacts fuse together at startup due to sparks do you think I might have a bad breaker in the electric panel?
One thing that would most helpful is a diagram showing the internal mechanism of a pressure switch. I have a home compressor that has a Chinese made pressure switch…really just an off/on type with a red button on top. However it needed some work recently and while doing this a small spring came out from the bottom. Trying to figure out where that goes and how to get it all back together has proved very difficult despite being able to see what it does.
I just purchased a Lefoo LF10-4H pressure switch. I have everything wired correctly, but it won’t auto fill. There is this little black button in the middle of all the wiring connections, that if I press it , the compressor kicks on. What am I missing?
Compresser runs but will not shut off at a 120 psi, I have manualy pushed the shut off down and it has locked and when I pull the air relase valve the compresser comes back on itself at a 115 psi, but it will not click the compresser off, theres no name on the compresser i dont know what age it is but i have some pictures of the cut off valve, i dont know if it is wired up wrong or if something is faulty, any help would be much appericated.
I have an older 60g/, 5hp, Campbell Hausfield, compressor. I have needed to replace the pump, so now I have a “V”, cylinder Omega pump, to replace the older vertical cylinder pump.The New Omega, has a definite orifice for the main line. But unlike the original, that had a fitting that went to the manifold for the pressure switch, I can find no such fitting on the new pump. Can I tap into the line somewhere to the switch is enabled?
I just picked up an old Campbell hausfeld compressor. I’m trying to replace the pressure switch, and I’ve noticed there’s no unloader valve on the switch. Similarly there’s no line or port coming off the two fill connection. Anyone familiar with this type of set up? This machine is from the mid 70s, but it runs great and is quiet, so I’d like to try and keep it working. The current switch is a Furnas 150 pound cut out.69GD1076.
Thanks in advance for any info.
MH