Need help choosing a chip

I'm new to this been at it for about 3 - 4 weeks now and loving every minute. Currently stuck on choosing a Mosfet or ? to complete my prototype. Hate to type all this so I made a video. Hope it's exceptable!

Keep it simple - use some relays with 5V coils, switch the full 12V across the motors. Then can ditch the optocouplers also, as the relay will provide the electrical separation for you.

The problem with using relays is limited space, mechanical failure, and I will need to use 13 relays to control everything. I will however use relays to control the blower motor, didn't test it but I'm sure it draws considerable amperage. This system will replace a control in a GM chassis. I'd really like to keep it all self contained and easy for anyone to just plug the unit in to an existing chassis

Where did 13 come from? You only showed 2 motors in the video, and no where near 13 switches.

logic level P-channel MOSFETs would be your best bet then to minimize voltage drop across them and prevent overheating.

The Actuators require (6), fan relay (1), Clutch Relay (1), A/C control relay (connects to the ECM to increase idle speed when the compressor is active (1), The blower motor uses a relay in (High position) (1) ahhh and the last 2 will be relays to control Low and Medium speed of the blower motor so that makes 10 actually. they will be controlled through the software activated bt the Shift registers.

I have some TIP102, TIP120, TIP35c, RFP30N06LE, and IRF520PBF I'm having trouble wrapping my brain around all the NPN, PNP, Mosfet, and Darlington info that I've been reading.

This is the car its going in

Think of NPN & N-Channel as parts that like to sink current from your circuit to ground. So connect the NPN Emitter or the N-channel Source to ground, they sink current to ground.

Think of PNP and P channel as parts that like to source current from the supply to your circuit. So connect the PNP collector or the P-Channel Source to +V, they source current to your circuit.

Darlingtons are just 2 transistors sort of in parallell the first one drives the 2nd one on harder, but the input just looks like 1 transistor.

NPN & N-channel generally have lower voltage drop (NPN) and lower Rds (N-channel) than their counterparts so dissipate less heat when conducting current.

Ok so if I'm understanding this correctly I need a PNP to source + to my actuator NPN's to source - (ground) to my relays [they are ground triggered] for the actuators do you think some thing like this would be ok

Sure, but look at this: VCE(sat) = 1.0 V (Max)

The power dissipated in NPN/PNP will be Power (P) = Voltage (V) * Current(I) so P = 1CV * 3A = 3W, can get warm Voltage = Current * Resistance, so can be rearranged as P = V*V/R , or I*I*R for a MOSFEt Power = I*I*Rds-on = 3*3*0.2 (for a 200mOhm part) = 1.8W and if you can find a better part, say 20mOhm, that drops even more: 3*3*.02 = 0.18W, not even getting warm, better reliabilty & no heat sink needed.

Since you are driving motors, look for logic level P-channel MOSFET low Rds (drain-source resistance when on), the lower Rds the better, especially for your higher current motors.

off to bed for me ...

Thank you Robert you have been of great help I truly appreciate it.! I'll start digging through the parts sites and see what shakes out. Again I can't stress enough how much I appreciate your help!

You’re quite welcome. If you go to, you can do some nice filtering starting with in-stock parts, get it narrowed down pretty nice.

Not sure if I got this all down. But I think I'm getting close. What do you think of this one?

This would be better

Static Drain?to?Source On?Resistance (Cpk ? 3.0) (Note 3) (VGS = 5.0 Vdc, ID = 25 Adc) RDS(on) typ 0.020 max 0.025 ohm 0.025 ohm vs 0.185 ohm

Want the low Rds with Vgs = 5V, vs 10. RDS(on) = 0.185? @VGS = -10 V

3A with Rds @ 0.185 = I*I*R = 1.665W and with 5V driver, you wouldn't get Vgs of -10V, so it would be >0.185 ohm

3A with Rds = 0.025 = 0.225W

So that's what you need : logic level => Vgs of 5v, and low Rds. And affordable if you're going to use a lot of them.

Thank you Robert I ordered some today. I'll keep you posted on my progress!