The Stovebolt Site Forums The Shop Area The Electrical Bay A question about the common 12v relay

I had to replace the fan relay last night on my street rod. It's one of the little black squares made by "Who shot John?". Measured the current of the new fan (16.4amps) to make sure it didn't exceed the relays rating. The old relay says it is rated for 20/30 amps @ 12v. The new one is rated at 40/50 amps @12v. OK, the question...which one is it. Is it's max current 40 amps or 50 amps? Thanks in advance.

Beltfed,

I have no accurate answer, only a semi-uneducated guess. Maybe the first figure is a "make" rating and the second figure is the "break" rating. The dynamics of the load would determine if the relay were appropriate.

When a contact is opened while passing heavy DC current, there is a large arc created. Maybe the larger value takes this fact into consideration.

If the ampacity of the relay is based on the surface area of the contacts, which I think it is, then it seems like it would be an arbitrary value, so I'm with you, why two numbers.

Have you thought about putting a MOV in parallel with the contacts on your fan relay. They quench any arc and prolong the life of the relay by many times.

What do I know, I'm just a dumb, retired Millwright.


Stuart

Ok atomarc, what is a MOV?

"Metal Oxide Varistor" a type of Seried Resistor

http://en.wikipedia.org/wiki/Varistor

The higher rating is a "surge" amperage, what the relay can take as an initial flow as it closes. The lower rating is a "continuous" current flow, what the relay can handle as a constant load without overheating. For instance, the "pull-in" current of a 12 volt Delco starter solenoid on most applications can be 50 amps or slightly more, while the "hold-in" winding only flows about 10 amps. A starter button used to energize that solenoid must be able to carry the momentary surge current without burning contacts, even though the average flow is much less. Solenoids work the same way, initial surge against continuous current.
Jerry

Beltfed,

I did a little research and found Wiring Products dot something. Their description of the fan relays indicate two current ratings, the higher one is used if the relay is wired for NO service and the lower value is used when the relay is wired in a NC configuration.

With current flowing in the NC arrangement, upon opening, you would get the 'arc flash from heck', hence the lower rating.

It's as clear as mud to me.

http://www.wiringproducts.com/index...tml&gclid=CNunuoHC-5gCFRxNagodVX7DnA

Stuart

as been discussed here many times before
if in doubt use two as the start up current is higher than the run and because this runs a fan that you don't ever want not to have a power sourse for. (weather the fan runs or not depending on the core temp sender). the safe bet and and added cost of $8 at best.
but i would think a 40 is plenty enough.