#### Wylbur

##### Newbie level 1

I have built a test circuit and my results (in the area of heat) are just not matching my calculations (2A draw doesn't raise the temperature enough for a heat probe to detect).

I have a spec sheet on a 40A 50VDC diode/rectifier (NTE5980) which is the diode I am working with.

What I am working on is a circuit for an automotive application where I must assume a max of 15VDC and the alternator has a max output of 63A (rated).

W=I**2xR tells me that I must dissipate 11200 watts if the circuit is drawing 40A.

This just failed the reasonableness test. I say that because, 15*40 (V*A) gives 600 watts. Something wrong with conservation of energy from where I sit.

So I have to have something wrong in the print of the spec sheet (the junction resistance is supposed to be 7 ohms). Or, more likely, I'm just missing something here.

I also seem to have a problem with the spec sheet because I can't seem to be able to determine the junction temperature (Tj) delta so I know how much heat I have to dissipate at max current.

And I say this because I have formulae for these things (all nicely built into a spread sheet), but I can't seem to determine how much heat I actually have to dissipate (even with a test circuit to see what heat I'm getting). So I can't compute the right size for a heatsink.

Could someone give me a pointer or two on what I'm missing or doing wrong?

Regards,

Steve.T