H-Bridge - removing resistor to base of MOSFET decreases power to motor

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Don23

Joined Feb 22, 2011
10
I've built an H-bridge circuit on a breadboard that powers a Traxxas EZstart motor. The motor runs optimally at 7.2 volts. I'm running it at 6v. It's based on the attached schematic. I'm using TIP122, TIP127 mosfets and 2N3904 transistors instead of what the schematic shows, but it all works fine.

I'm puzzled by one thing. I noticed that if I replace the resistors leading to the base of the PNP mosfets, I actually get less power out of the motor. See the attached schematic. I don't get the logic of why increasing the current to the base of the PNP Mosfets would have this effect. Can anyone explain?
 

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mcasale

Joined Jul 18, 2011
210
So, you are running this at DC -- not PWM, correct?

Can you post your "new" schematic? I'm guessing there are some unseen voltage drops that are preventing the MOSFETs from turning completely on.

Also, what voltages are you driving the inputs with?
 

crutschow

Joined Mar 14, 2008
24,741
If you changed from BJTs to MOSFETS int that circuit, then you need to add a resistor from each gate to source to insure proper turn-off of the MOSFET. The resistor value depends upon whether the bridge is just on/off or PWM.
 

Thread Starter

Don23

Joined Feb 22, 2011
10
You guys are all over this! Thanks for all the intel. I have to confess, I retried this test and the motor works fine with straight wires or resistors up to about 3K ohms. I'm assuming loose connections in the breadboard were the cause of my original problem.

This is straight DC power supply. No PWM. My most recent test was with 5.7 volts from 4 D cell batteries, before being connected to the circuit. At this voltage level, I had to twist the gear of the motor to get it run, but once it ran, it ran well.

FYI, see the attached image of my breadboard. I'm connecting a wire between the left and middle arrow, or the right and middle arrow to complete the circuit.
 

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