On board air manifold

[mickeyw]

Well it does in that the current rating at 12Vdc will not be the same (lower) than the rated value shown at 240Vac.

That fact isn't going to stop it switching 12v for this application. I'm controlling an air con compressor clutch which doesn't draw much power. If I was wanting to switch a higher load such as an electric compressor on and off, I'd add a relay to the circuit.

[Cynic-al]

I have one switching the ac compressor which pulls 5a at 12v and it's quite happy.

I'm a bit rusty so you can prove me wrong if you like but I thought the heat that would damage the contact is generated by the current rather than the voltage so if it's rated at 10 amps it's rated at 10 amps.

[CwazyWabbit]

There are usually different ratings for DC and AC.

[HoSS]

Yes its the current that causes the heating, but its far from that simple.

Firstly whilst DC current is flowing all of the time, AC is dropping to zero 100 times per second, so the average current is less than the maximum.

On top of this if the load is inductive the back-emf generated across a opening contact in a DC circuit will high and will start to burn the contact.

Its not uncommon for a DC current rating of a contact to be only 30-50% of the AC rating. ANd the DC voltage rating only 10% of AC voltage.

Yes it may work, but dont be surprised if it stops working in a few months or years. Ok we are not building safety critical systems here, but still i would use a relay of the correct rating for reliability.

[mickeyw]

HoSS I must bow to your greater knowledge of the science involved with switching equipment.

As I said in my earlier post, the switch doesn't care what the voltage is, within reason. I think this acknowledges that there are limitations to what it can handle, as there will be with any switch gear.

I do not wish to come across as being antagonistic, but seeing as we have now introduced science to this discussion, perhaps you wouldn't mind working out the differences in amps drawn for these applications? It would demonstrate to readers the reality of the impact of the different applications on this device. I appreciate what you are saying about AC and DC having a differing effect on the contacts. They're pretty hefty contacts by the way!

My 3HP compressor (240V) is fitted with a apparently identical switch. What are the amps involved for this task?

Cynic-al mentions that his air con clutch pulls 5A at 12V, I have never measured mine, but the size of the original wire to the clutch would suggest a fairly light load.

My OBA setup must have been in my truck getting on for 10 years and switches in and out fairly regularly. The switch has been happy so far. Equally, the compressor being used is not within its intended application either, and remarkably is still working well.

Al - out if interest, how long has your system been in service?

[CwazyWabbit]

A quick google suggests those switches are rated at 16A at 240V AC (some claimed 20A) so even with the conservative 30% for DC it would handle 5A at 12v happily.

[Cynic-al]

I accept AC & DC is different, but effectively once it's closed and assuming its making proper contact it's just a funny shaped wire right? and it's thicker than the wire that LR used so I can't imagine it being the weakest point. ok that doesn't take account of the drive to 'jump' as you make or break contact but we are at 1/20th of its designed ac voltage and 1/3 current with I'm sure less startup and breakdown current than a 2hp motor. Obviously I don't have any figures to back this up so it's all just finger in the air I accept it wouldn't do on an airbus but I'm happy enough for a system that I use now and again to inflate a tyre.

However to double back on myself, as it draws 5amps it makes sense to put a relay off the ignition so it doesn't sit there drawing 5 amps when the engine isn't turning the compressor, so it would make no real difference to the wiring if it was switching the clutch or this relay.