Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Basic question about relays

Status
Not open for further replies.

Ingeniir

New Member
Hey all,

Is the following a valid path of logic:
I need to switch on a 25 watt lightbulb using mains power. I have an automotive relay whose SPST contacts are rated at 30A at 12VDC. The voltage running through this circuit would be 120V, which is 10 times the voltage rating of the relay, but the current would be less than 1A, which is well below the current rating of the relay. I know that components usually fail because they are passed too much power, which overheats the part and destroys it. Since this relay is rated for 30A at 12VDC, using P=IV, it can handle 360 watts of power, therefore using a 120VAC lightbulb is safe.

My guess is that I made some incorrect statements and conclusions, can someone clear this up for me?
 
You have to be clear that relay has two individual circuits. One is the coil circuit where the relay get energised and the other is the contact circuit, where you connect your load( in your case the light bulb.

For the coil you have to use the rated voltage, in your case is 12v dc and not 120v ac. and for the contact the voltage and current are the max rate where you can alter but for smaller and not bigger.

In short, You've mixed the thins up, The relay coil has to get 12v dc to be energized. The relay contacts will act as a switch to switch ON/OFF your 120v ac to your 25W bulb.
 
Last edited:
Sorry for the ambiguity. Everything in my question has to do with the contact circuit. The SPST contacts are rated at 30A at 12VDC, and I am wondering if its OK to pass 120VAC mains power through those SPST contacts.
 
The voltage running through this circuit would be 120V, which is 10 times the voltage rating of the relay
Exactly, you are exceeding the rating of the relay, it is not safe and is quite dangerous.

but the current would be less than 1A, which is well below the current rating of the relay.
That is fine, but current rating is not the only consideration.

I know that components usually fail because they are passed too much power, which overheats the part and destroys it.
That is a common failure mode, but not the only failure mode.

Since this relay is rated for 30A at 12VDC, using P=IV, it can handle 360 watts of power, therefore using a 120VAC lightbulb is safe.
Sorry, but your reasoning is wrong.

Do NOT use a 12v relay on 120v, this is a DANGEROUS situation.

JimB
 
Contacts have both an AC rating and a DC rating. They also have horsepower ratings. There are even contacts rated specifically for tungsten (lighting loads). You may find intermittant and continuous ratings as well. Contacts also have minimum current ratings. They can have switching and passing current ratings. They all have to be abided by.

These are some off the wall relays: https://www.electro-tech-online.com/custompdfs/2011/06/k64c_h25.pdf I've used similar ones from this manufacturer before.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top