replacing mosfets in dc motor controller

[grant fair]

I would lke to replace the 3 mosfets in an dc motor control for a mini-mill. (One of the mosfets is fried). I was using IRF840 mosfets (Rds of about .8, current capacity of 8 amps). The recommended replacement is the IRFP460 (Rds about .2, current capacity 20 amps).

The three mosfets are paralleled to drive the dc motor. I am wondering if I can use only two new mosfets instead of three because of the increased current capacity.

The PCB is an older design - I don't have schematic, but it is similiar to the newer controllers, except this one has no transformer. It rectifies line 110V ac and regulates it with zeners for various parts of the circuit. (This was a sample mill and did not have CSA approval).

The PWM signal from a LM324 circuit goes to two optos (TIL113) and these drive the mosfets. The supply voltage on the LM324 is 24 volts. The drive goes to each mosfet through a 100 ohm resistor to the gate. A 100K resistor goes from gate to source.

Any advice on changing the values of these resistors, or making other tweaks with the change in the mosfets?

(I know UncleRabid has a Yahoo group on these controllers, but no one seems to be around there these days. I'd like to see the chips fly, not fry )

Grant

[sghioto]

grant fair,

I would try it using two of the IRFP460. Shouldn't need to change any resistor values on the gate drive as long as Vgs is at least 10 volts and doesn't exceed it's 30 volt rating. If the mosfets are running "hot" then install the third one.

Steve G.

[grant fair]

I replaced two mosfets. It works fine, maybe better than ever (it sounds better). So far, no detectable heat on these.

Thanks for your reply.