Re: Switch Power Rating

No, you have it backwards. 125 V AC has a peak value of 177 V. But, as others have said, the problem with DC is the lack of a zero crossing as with AC. But, for 24 V DC, you can get aircraft DC switches that are rated to break DC, even with inductive loads. They will be more expensive.

Jon

All,

Okay just to clarify. This is being used as a main power switch to a system. The system is either on or off. I am not rapidly using this to switch off and on.

This switch is double pole double throw and I would like to get 10A at 24V through both poles of the switch. 10A on one pole going to a resistive load and 10A going to a system with fans, and stepper motors.

I don't have the DC ratings from the manufacturer and it seems hard to get because I am buying from a distributor who doesn't seem to know.

Here is a link to the switch:

Part Number: MR-21N-3BB-R2

-Mike

You should be looking at places that deal with low voltage, like marine supplies, tractor trailer supplies etc..

Jamie

All,

I am looking for a switch that is rated for at least 24V.

I found a switch on Mouser:

The Mouser page says that:

Contact Rating: 10 A at 250 V, 16 A at 250 V

Is it rated at 10A or 16A?

From the datasheet:

Everything is listed as AC voltage. I wonder how they are getting the DC values if it is not provided in the datasheet. Am I missing it?

Thanks.

-Mike

You've been given good answers: use a switch rated for the load your device presents. If you use something not rated for your load, it's a crap shoot as to whether it will work, or how long it will work. You also invite disaster by using a switch that might not be able to break the circuit, should you need to shut it off.

Ed

It's amazing how much arcing you can get at only 24 volts with a 10 amp inductive load. For a hands on experiment: Try hooking up a 24v 10amp motor by just brushing heavy single strand clean copper wire ends together and watching the sparks when you make and break the contact with the copper ends. Watch the arcing burn away tiny bits of copper each time, and then imagine what the switch contacts have to contend with. Even tiny inductive loads commonly "induce" a counter EMF "kick" that is often shorted to ground with a diode. If you did that with the kick back from a spinning motor it could possibly force the motor to suddenly act as a magnetic brake, with the diode shorting a big surge. Perhaps a massive diode is also used to pass forward bias but resist or isolate most of the counter EMF? But even if all of the counter EMF "kick" was eliminated with isolation and snubbing diodes, just the forward current alone could still make charred little sparky bits out of the end of the wires in such an experiment or the switch contacts you are concerned about. Sometimes the contact ends of wires used in such an experiment will even partially weld together right before your eyes.

Well, let's see...

They specify an initial contact resistance of 20m ohms. Two poles, yes?

Does that sound okay to you?

In the proposed 42 V (3x14 V) automobile electric system, the load switching was handled by FETs, not relays exactly for this reason.

As a historical side note, the Thomson-Houston 1880's street light system consisted of a 6 A series circuit with up to 30 arc lamps driven by a 1500 Vdc generator. Thus, there was about 50-55 V across each carbon arc.

So you really have to pay attention putting out the arc, e.g. by blowing out the arc with air flow, magnetic field, inserting isolating material or running the current through a FET and then the relay (since security issues might require a mechanical contact).

If you are browsing Mouser why dont you just pick a 24V DC/ 10A switch like you want. They stock a few and others are non-stock but can be ordered. I have ordered qty 10 for non-stock items from both Mouser and Digikey. Hit they buy button and follow the sequence. You may need to call them and check on the lead time.