# SPDT Switch Selection

1. Jul 6, 2015

### braren

Hi,

I'm working on a project where I need to use a couple of relays to send an input signal back to the controller. I'm using the 24V supply voltage from the controller to power a spdt switch, but I noticed that if I keep the switch powered for awhile, it gets hot to the touch.

The specs for the switch are...
Product Height 1
DC Coil Voltage 12
Coil Resistance 270
Max Current Rating 5
Max AC Voltage Rating 250
Max DC Voltage Rating 30
Average Bias Current-Max (IIB)(uA) .003

I know that some components will get a little hot when running under normal conditions, but I guess I'm just not sure if I'm overloading the switch. The specs say that the DC Coil Voltage is 12V, but the Max DC voltage is 30V. Does this mean I can put 30V across the coil and be fine? My next step is going to be measuring the current through the coil when operating, but what kind of calculations or otherwise should I do with that? As long as the current is under 5A, I should be good right?

Thanks for the help!

2. Jul 6, 2015

### wirenut

Are you referring to a relay?
If you are, then the coil can only be powered by 12 vdc. The 30 vdc/250 vac is for the switch contact. Your relay is probably getting hot due to overvoltage on the coil.
If you are going to use 24 vdc to power it, get a relay with a 24 vdc coil.

3. Jul 6, 2015

### Averagesupernova

The coil voltage is 12 volts DC and that is the end of it. You are doubling the specified voltage on the coil. The other voltage ratings are for the relay contacts. If you know ohms law you can tell what the coil current should be by the coil resistance spec and the voltage applied to the coil.

4. Jul 6, 2015

### braren

Sorry, yeah, I was referring to a relay not a switch, I have this bad habit of using the words interchangeably :P

The reason I was testing this relay with my system was because when I first wired everything up, I had a 24V relay that, although wired correctly, would not switch over when I applied a voltage to it. I don't know if this is because it was trying to pull more current than was available or what, but I changed some wiring in my circuit (jumpered the supply voltage for the digital Input and outputs together) and swapped the 24V relay back in, and now it's working!