PWM circuit...

(20

down

holiday

Thanks for the warning. You just saved me some research $$$. 8)

complementary

volt

Ok; I think I follow this. (You mean, use the 74HC4050 as a "pre-amplifier" for the mosfet gate, right?) But what exactly is a "complementary emitter follower"?

I will also investigate the possibility that the motor was originally designed for 6V, as opposed to 12V. That would probably explain the temperature rise, if I've been exceeding the design voltage for the motor...!

Thanks again,

Mike

To quote an AOL commercial, "Slowed way down. Slowed down like holiday traffic". Contacts can be the switching mechanism for a PWM output but they are not very reliable. All the heat that would normally be dumped into the mosfet during turn on and turn off will appear in the tiny gas pocket between the contacts, converting it to little blasts of plasma, which will quickly either heat the contacts till they melt together, or will oxidize them till they no longer conduct. That 20 amp rating is based on the contacts cooling after each break before the next make, and a PWM regulator does not meet that requirement.

You may cool a large mosfet quite a bit just by adding a complementary emitter follower between the 555 and the mosfet as a current booster, with something like a 10 ohm gate resistor. If this is for the 6 volt version, you might try paralleling all 6 buffers in a 74HC4050 to act as a high current gate driver. This will boost the drive considerably, compared to the LMC555 output. But I would probably still keep 10 ohms right at the gate or put a ferrite bead around the source lead.

Right. Lots of other noninverting chips with high current capability could work, too, including 3 state buffers (with the output strapped to be constantly enabled. These come in 4s, 6s and 8s per pack.

It is a current amplifier made up of a pnp and npn transistor with their bases connected together as the input and their emitters connected as the output. The PNP collector ties to the negative rail, and the NPN collector to the positive rail. Unfortunately, you lose ..6 volts in each direction to forward bias the base emitter junctions. And with only 6 volts available, you cannot afford to lose any gate drive voltage and expect to get minimum temperature out of the fet.

That would do it. Of course, you could limit the duty cycle of the PWM to no more than 50% and thus limit the average motor voltage to 6 volts, even though the circuit runs on 12 volts. Then you have lots of gate voltage and can apply the complementary emitter follower. :)