12VCompressor for a Brake Car

[cbrew]

I have a question for you all, is there a lower pressure 12 volt pressure switch available? My brakes are going to be air pressure applied to release them and I'm using 1-1/4" cylinders so I don't think I will need more than 60-75 PSI. All the switches seem to be 95-110 or similar.
So that unloader valve on amazon, that is a purely mechanical valve that dumps the pressure off the compressor when it stops running? I know my little pancake compressor reaches pressure and shuts off and there's a "hiss" so it must have one but I don't know how it works.

the square D switch i have will go down to 60 psi on with about 85 off.
just because they are preset to 95 to 110 psi does not mean they can not be adjusted. its just a nut on top of the spring, at least the model i have.
I did run mine that low for a short time but the air cylinder on my booster truck required a higher pressure to move the gear so i had to crank it back up. the brake system is supplied through a regulator set between 40 to 60 psi depending on how many cars have brakes and the layout grades.

[rkcarguy]

I have a question for you all, is there a lower pressure 12 volt pressure switch available? My brakes are going to be air pressure applied to release them and I'm using 1-1/4" cylinders so I don't think I will need more than 60-75 PSI. All the switches seem to be 95-110 or similar.
So that unloader valve on amazon, that is a purely mechanical valve that dumps the pressure off the compressor when it stops running? I know my little pancake compressor reaches pressure and shuts off and there's a "hiss" so it must have one but I don't know how it works.

the square D switch i have will go down to 60 psi on with about 85 off.
just because they are preset to 95 to 110 psi does not mean they can not be adjusted. its just a nut on top of the spring, at least the model i have.
I did run mine that low for a short time but the air cylinder on my booster truck required a higher pressure to move the gear so i had to crank it back up. the brake system is supplied through a regulator set between 40 to 60 psi depending on how many cars have brakes and the layout grades.

Good to know, I was wanting to have a lower on-off pressure to be easier on the compressor. I plan on having a regulator as well to use the minimum air pressure I need to lift the brakes "off".
Any chance of a part # on that square D switch?

[cbrew]

I have a question for you all, is there a lower pressure 12 volt pressure switch available? My brakes are going to be air pressure applied to release them and I'm using 1-1/4" cylinders so I don't think I will need more than 60-75 PSI. All the switches seem to be 95-110 or similar.
So that unloader valve on amazon, that is a purely mechanical valve that dumps the pressure off the compressor when it stops running? I know my little pancake compressor reaches pressure and shuts off and there's a "hiss" so it must have one but I don't know how it works.

the square D switch i have will go down to 60 psi on with about 85 off.
just because they are preset to 95 to 110 psi does not mean they can not be adjusted. its just a nut on top of the spring, at least the model i have.
I did run mine that low for a short time but the air cylinder on my booster truck required a higher pressure to move the gear so i had to crank it back up. the brake system is supplied through a regulator set between 40 to 60 psi depending on how many cars have brakes and the layout grades.

Good to know, I was wanting to have a lower on-off pressure to be easier on the compressor. I plan on having a regulator as well to use the minimum air pressure I need to lift the brakes "off".
Any chance of a part # on that square D switch?

I will have to see if i can find a sticker on the, its been in service for about 10 years. I will be in the shop tomorrow, i will try to remember to open up the tender

[SPSteam2491]

I went back through my purchase order records and found the pressure switch that I am using. https://www.grainger.com/product/SQUARE ... =P2IDP2PCP It actually have a pressure differential of 20 psi, not 15 psi as I stated earlier.

I was also able to locate the same pressure switch with the unloading valve https://www.grainger.com/product/SQUARE ... s=altItems. I am going to get one of these valves on order and do some experimenting.

My thought is to test four different systems (systems listed below) and determine the starting amps, peak amps, and running amps on the compressor. I will use the same compressor for all the tests so that the data will be comparable. I will post all the data and finding when complete. I will also use a couple set point for the baseline tank pressure (40, 60, 80 psi) to also compare the starting, peak , and running amps for systems B-D.

System A: Baseline test; no load. Compressor pumping air to atmosphere.
System B: Compressor attached directly to load.
System C: Compressor attached to timer relay system.
System D: Compressor attached to unloading valve.

[Pipescs]

Ordered Square D Switch

[rkcarguy]

Thank you for the link for the pressure switch with unloading valve. I see what they do, they have a check valve of sorts, so that when the contacts break it vents the line between the valve and the compressor head.

[Fender]

Unloading the pressure should dramatically reduce the starting current on the compressor motor; however, this switch is rated at 10 amps at 575 vac. I assume this will be ok on dc?

[SPSteam2491]

I use an automotive relay to power the compressor so the pressure switch only sees the current from the signal line on the relay. The switch likely would handle the starting amps from the compressor but I figure a $5 relay is cheaper to replace than the $30 pressure switch.

[cbrew]

i have found with mine, the contact size is much larger on the square D switch when on a 40 amp auto relay.
I have over 10 years service on mine and the contacts are still clean.

[rkcarguy]

The contacts are going to be far more beefy on that switch than in a relay, that's rated at about 5700 watts or about 450 amps @ 12 volts. That could be across all the contacts though, where 12 volts will only use one set of them.

[cbrew]

i did run both hot and ground through my, using both sets of contacts. but doubt that matters

[cbrew]

you can hear the compressor doing its thing in this video