SPST Relay

I was wondering if standard automotive SPST relays have any current limiting device build into them. In a standard scenario, I have a 12v lead feeding both inputs on the relay. on the trigger side I have a switch to ground, on the load side, a fan. When the trigger side is flipped, is there any current limiting in that circuit? should there be?

I ask because I seem to be getting tremendous amounts of current pull when I use these accessories.


-brian

There are two component parts to a mechanical Relay or Contactor. (1) The Actuator Coil, and (2) the Switch Contact.

The Coil will have a Voltage Rating (and an indicator if this is an AC or DC coil) and a Resistance Value. From that, simple Ohms Law determines the current draw. eg. 12vDC 360R means this will draw around 33mA give or take. Some coils (especially crappy automotive ones) have a very low Resistance value (example Matsu****a ISO family springs to mind), with coil values of 100R or less you could easily pull well over 120mA. Usually, the bigger the current the Relay has to switch on it's contacts, the harder the contacts need to come together and the more powerful the coil (for a more powerful magnetic field) to achieve this.

The Switch Contact is mechanical and to all intents and purposes would be considered zero ohms and would not normally count in any current calculations (unless you're drawing serious Amps), other than it is your responsibility not to exceed the maximum ratings for the contact. Bear in mind that the ratings are usually taken as a non-inductive value - this means that a 12A 240VAC contact is capable of switching a 12A RESISTIVE Load. If you now wish to switch an inductive load, you must derate accordingly.

If your load is an Automotive type Blower Fan, it could easily draw 10A!!!!! Automotive electrics are kinda crude as the designers tend to be fixated on the fact that they have 50A available to play with (the first and only thing they remember at the Detroit design school), so they design everything from your Car Audio thru to the interior Glove Box lamp to draw that same kind of current.

Woohoo... unwarranted automatic censorhip... I've just discovered you can't enter the name of a famous Japanese multinational without the forum putting asterisks in place!!! That wasn't me I assure you! Jelsoft Enterprises are gonna have trouble selling this software to Panasonics parent!

Thanks for the reply Melanie. Your speaking in mA, so obviously this isnt the source of my problem. I am experiencing major power consumption, enough to dim headlights, etc.
The circuitry I speak of is in an old mustang, so power comes at a cost. I suppose its time to just bite the bullet and get a new generation alternator upgrade.
I am slowly upgrading those high consumption interior, exterior lights for LED's. Great success so far.