A compressor made with fridge technology!

A small compressor is quite useful in the home workshop. I use mine mainly for spray painting, but many other uses exist. I built my compressor from a sealed refrigerator motor/compressor, a discarded freon container, a homemade presostat, some junk box parts, and some aluminium stock, plastic hoses, and the like.

The first photo shows it a bit from above. It has a very small footprint, and is a little bit lower than a normal table, so that it can either be pushed under a table, or its surface can be used as a little table, such as for putting down the spray gun. This little surface has a border, so that little parts won't fall off.

From the front you can better see all the parts. Basically these are the fridge motor/compressor, the freon tank, filter, presostat, pressure gauge, switch, hoses, wires, and mechanical structure

The air enters the compressor through a small universal fuel filter, used as air filter here. After all, we don't want any dust in the compressor nor in teh exhaust air! From the output of the compressor unit, a hose runs up and feeds the tank from above. Most of the oil blown out by the compressor remains in the hose during work, and eventually seeps back into the compresor while it is not pressurized.  The inlet nipple is mounted in such a position on the tank that any small oil particles coming in will collect in the bottom of the tank, rather than being delivered to the outflow.
The oil circulation stubs of the compressor unit are not used, and remain plugged.

The tank hangs from the top aluminium plate. Its original valve and outlet serve for the same purpose as intended by the tank's manufacturer. The inlet nipple, presostat and pressure gauge were installed by drilling holes into the tank, and soldering the proper parts to it, using common plain tin/lead solder.
The power switch (white) is within easy reach under the top plate, but out of range for falling stuff or accidental activation.

The electrical circuit is simple: The power switch and presostat are wired in series with the motor and fed from the power cable, which is has three conductors to properly ground the frame of the machine, tank and compressor housing. 

The presostat is the only slightly more elaborate part. It uses a microswitch, mounted to a copper sheet structure. I used copper because I had it on hand and it solders easily, but zinc-plated or tin-plated steel would be fine too. The structure is soldered to the tank. A 10mm hole in the tank is covered by a piece of rubber coming from a bicycle inner tube, which acts as membrane. A thick perforated bolt and nut is used to push the rubber down onto the tank and seal it. Inside that bolt there is a thin bolt, its head resting on the rubber membrane, and its tip touching the lever of the microswitch. A crossbar is tensioned down with two homemade springs, and presses the thin bolt down via a nut whose position can be adjusted, to vary the spring tension. This is used to adjust the device to act on the desired pressure.
As the pressure in the tank rises, the rubber membrane bulges out, lifting the bolt, stretching the springs, and finally making the switch shut off the current. As the pressure goes down, the bolt will move a little down again, and the switch reconnects power to the motor.

The nipple for the pressure gauge was soldered over a little hole in the tank, and the gauge screwed in using a little teflon tape. I got this gauge for free, because it's labelled only in lb/sq.in., and nobody wants these obsolete things here anymore. The gauge rests in a protected position, but still easy enough to see.

I set the presostat to 2 bar, which is a tad less than half scale on this gauge. That pressure works well for spray painting with the gun shown, and also with a larger low pressure gun. I would not be comfortable using such a recycled freon container for much higher pressure. When new, it surely will be able to survive much higher pressure, but some water builds up inside, and probably these containers are not rust-proofed inside, so I expect it to rip open on the bottom some day. The lower the pressure, the less dangerous that will be.

In the event of presostat malfunction (so far has never happened), the hose should fly off the fridge compressor outled, which is not barbed. I tightened the clamp there only enough to keep the hose from flying off at normal working pressure. By the way, the hoses are universal cheap fuel lines. Nothing stronger is needed at this level of pressure.

The outlet valve is a bit awkward to reach in that place, but it is rarely used anyway. It's open most of the time.

If you copy this compressor, you have to be aware that freon containers are intended to be single-use only, not for high pressure air, and that refrigerator compressors use oil, so that tiny traces of oil could be present in the air delivered by this compressor. In many cases that's no problem, but in some it is. I haven't seen any problems in this regard, so either the tank is doing a great job of collecting oil, or the paints I use aren't sensitive to it (I use mostly lacquer on wood, also some enamel on wood and metal).

The amount of air delivered by this contraption is plenty to run the shown spray gun continuously, but a larger low pressure spray gun can only be used with about a 50% duty cycle.

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