Electronic Circuits and Projects Forum

You mean that you want to switch 19 AMPS at 12Vdc? Seems like a lot of current, but a logic level FET with a really low RDS(on) would do the job. You'd need a heatsink for that much current.

What the part number of the relay?

Originally Posted by erinbanwell

im not sure how to calculate what ratings i need for the logic level FET. Do i need an RDS equal to the power of my pin?

For low speed switching (Less than 1Khz):
You need a ID (drain current) spec greater than your load. Choose a FET with an ID rating of at least 2x the load current. The lower the RDS(on), the less power that will be wasted in the FET when it is on. ie: An RDS(on) of 0.05Ω at Vgs of 4.5V would cause 0.125 Watts to be wasted as heat for a draw of 1.58Amps. But if you used a FET with an RDS(on) of 1.0Ω at Vgs of 4.5V you would generate 2.5 Watts of heat for a draw of 1.58Amps which would mean that you'd need a heatsink.
Also spec the Vds to be at least twice the supply voltage for safety.
Usually what I do when purchacing parts is look of one that can be used for future projects even if it means grossly over specing the part. That way you can take advantage of volume pricing. Sometimes a higher spec part is cheaper due to volume production. I bought a bunch of IRLZ44's because they were the cheapest logic level FET from the supplier at the time. Tell us the supplier you will be using and we can recommend an all round general purpose FET for you.

Don't forget the freewheeling diode to clamp the inductive spike of the coil when the FET turns off. Spec it's voltage rating the same as the FET. You can usually spec the diode current rating lower due to the transient nature of the inductive spike. Just don't exceed the surge current rating of the diode.