Thread: Open or closed relay?

Oh,, and a second question for extra points---

Does a draincock on an air tank HAVE to be on the bottom?
Yes I understand thats where water pools but will the pressure release force it out?
I dont have room under the tank when installed for a normal draincock...

Originally Posted by Pedro_The_Swift

An electrical question to start the day...
I have just bolted in a 24V air compressor controlled by a air pressure sensor and have tried unsuccessfully to wire it up. I asked the Auto Lecky for a 24v 20A 5 pin relay to suit this application and got a OEX ACX1942RBL which is sort of correct... except its a normally open relay. If I am switching the compressor on by literally applying large alligator clamps to the batt terminals,, dont I need a normally CLOSED relay? and then the pressure sensor will OPEN the relay ( basically switch outputs to the "other" 87 terminal)

Compressor pulls a max 12A.

Anyone know where I can buy something suitable?
All advice gratefully

Hi Pedro,

What you have is correct.

Power goes to pin 30, compressor to either pin 87, and your low current switching power across pins 85 and 86.

When you apply volts across 85/86 the relay closes, or switches "on", connecting the power to the compressor.

The compressor drain really needs to be at the bottom, that said, my ARB compressor doesn't have a drain, and it doesn't seem to get water in it.
I do use it exclusively for pumping up tyres so it doesn't cycle very much.

Tony

Screenshot_20241020_100252_Chrome.jpg

As above, your relay is correct- 'normally open' means that the compressor will be off unless is is switched on. Your pressure switch goes in the switching circuit, a drop in pressure closes the contacts and switches the relay 'ON', running the compressor until the pressure builds up and opens the switch contacts, de-energising the relay coil and allowing the compressor circuit to return to it's OPEN state which switches the compressor off.
I wonder if perhaps you are misunderstanding the terminology? OPEN and CLOSED are not like opening a door to let the electricity through- it is opening the switch contacts so the circuit is broken, i.e. OPEN is OFF.

The air won't force the condensate out of a valve that is above the water line, the drain cock won't do anything if it's not at the bottom.

What are you using the compressor for?
Is there room for a 90⁰ elbow on the tank? If so run a (100psi) hose from the elbow to a drain cock, below the level of the tank.

It may be difficult to accomplish in practice, but if you absolutely can't arrange a drain opening on the bottom of the tank, a small copper pipe from the drain cock into the bottom of the tank (inside the tank) would work. There may be fittings available to enable this.

Originally Posted by V8Ian

What are you using the compressor for?
Is there room for a 90⁰ elbow on the tank? If so run a (100psi) hose from the elbow to a drain cock, below the level of the tank.

Bus/RRS tyre pressure adjustment, will see what style 90 I can buy in Gympie...

Originally Posted by TonyC

snip--

Power goes to pin 30, compressor to either pin 87, and your low current switching power across pins 85 and 86.

When you apply volts across 85/86 the relay closes, or switches "on", connecting the power to the compressor.

Tony

Screenshot_20241020_100252_Chrome.jpg

People are saying 85 should be earthed..

Originally Posted by Pedro_The_Swift

People are saying 85 should be earthed..

That's how the switching power goes across it. Power from the switch to 86, then to ground completes the switching circuit. Ignition +ve via pressure switch to 86, 85 to ground. Alternatively, Ign +ve to 86, then from 85 to pressure switch to ground. Either way, the pressure switch opens the circuit when the pressure reaches maximum, and the ignition and pressure switches only carry the low switching current.