Help Calculating Switch Rating

[manic]

My brain does not want to switch on today.

If a switch is rated for 0.1 amps at 80 VDC how many amps is it rated to switch at 12VDC?

[Homestar]

.1 amps at 12 volts if my brain is working... Current rating of the switch won't change, it has a designed maximum current rating, and a maximum voltage rating. You can run it at a lower voltage, but you can't exceed its current rating. Total load is 8 watts, that's a different story. 8 watts at 12 volts is .67 amp, but that will kill the switch.

Also, is the switch designed for AC or DC? A maximum AC current rating will be a lot higher than a maximum DC current rating for a given switch.

[bee utey]

It depends on what kind of load the switch is rated for. Inductive loads (eg brush motors) will cause a significant arc, burning the contacts at higher operating voltages. Plain resistive loads will arc less so there will be less difference in current handling capacity from high to low voltages. Basically there's the current the switch will hold long term without melting, and the current at which the contacts rapidly evaporate when used to break a circuit frequently.

I'd find out more about the switch before proceeding with its re-use in a 12V system.

[Homestar]

I'd find out more about the switch before proceeding with its re-use in a 12V system.

Agreed, and if it is only rated at .1 amp, it can't be much of a switch...

[manic]

It is the DB3 model microswitch - 0.1A @ 240VAC (so correct, not much of a switch )
*link to datasheet in post below

But also has a VDC rating which is 0.1A @ 80VDC

I need to switch a solenoid that pulls peak 0.8amps at 12v. Trying to figure out if the switch is good for that or if I need a relay.

So we think a drop to 12v makes no difference to amount of amps it can handle?

[Homestar]

I wouldn't use that one for what you want, it won't last 5 minutes. Does it need to be a micro switch like that - what application is it being used in? There are plenty of micro switches that will drive that coil without issue, but not that one...

[manic]

whoops this is the correct datasheet
http://datasheet.elcodis.com/pdf/19/...0/db2ca1aa.pdf

[manic]

I wouldn't use that one for what you want, it won't last 5 minutes. Does it need to be a micro switch like that - what application is it being used in? There are plenty of micro switches that will drive that coil without issue, but not that one...

See this post:
http://www.aulro.com/afvb/projects-t...ml#post1871275

I figured I could use it to trigger a relay but then wondered if it could handle it without one... I suppose not.

So its not good for any more than 0.1Amps at 12V then?

[turkeybrain]

Power = current^2*resistance

Voltage is irrelevant.

If you draw 0.8A through the switch, it will have to dissipate 64 times more energy than it's rated for. I'd be very, very surprised if it survived.

-Martin

[bee utey]

Well just look at your data sheet and see how many cycles that switch is designed for, 100,000 cycles. This switch may well be able to handle larger currents with a much lower lifespan. I would suggest you use a diode across the solenoid to damp arcing or spend a few bucks on a bigger microswitch.

You could also use a NPN power transistor between the microswitch and the solenoid. Arrange the microswitch with a 1K resistor in series across the 12V supply, have the base of the transistor go high when the switch opens and drive the relay from the collector to the emitter. Check with an electronic shop for a suitable transistor and circuit. Always remember the surge suppressing capacitor or diode or you can quickly burn out any switching element out of its comfort zone.

Edit: Also Turkeybrain's calculation only applies to static DC loads. Arcing is what will destroy a switch, especially with an inductive load like a solenoid.