Automatic Window Mechanics H-Bridge Operation

Other Hardware General Electronics
1

Hi all,

I'm building a vending machine type closet. It's a tall box with a window and a coin-slot. The idea is that
after someone throws in a coin the window slides open. For this purpose I use a scissor style powered car-window mechanism.

The H-Bridge I'm using handles a current of 10A with a 30A peak. The motor I'm using draws 5-10A.
Though it should be able to handle the motor, one of the MOSFETs on the H-Bridge becomes extremely hot when turning one direction, and another MOSFET will become extremely hot when turning the other direction.

The motor turns significantly faster on DIR == LOW (counter clockwise) than DIR == HIGH. When turning slow the MOSFET heats up very fast. When connected to the power supply directly the motor turns at the same speed both directions, and a bit faster.

Does any of you know what could be the problem here?

This is a photo of the circuit I use for testing:

#define pwm 11
#define dir 10
#define button 6

int buttonState = 0;
int ccw = false;

void setup() {
  Serial.begin(9600);
  
  pinMode(pwm, OUTPUT);
  pinMode(dir, OUTPUT);
  pinMode(stop_button, INPUT);
  
  digitalWrite(pwm, HIGH);
  digitalWrite(dir, HIGH);
}
void loop() {
  stopButtonState = digitalRead(stop_button);

  if (stopButtonState == HIGH) {
          digitalWrite(pwm, 0);
          ccw = !ccw;
          delay(1000);
          digitalWrite(pwm, HIGH);
  }
  if (ccw) {
      digitalWrite(dir, HIGH);
  } else {
      digitalWrite(dir, LOW);
  }
}
2

Maybe you need a different H-bridge, or you can use relays. Did you get the H-bridge from a reputable supplier?

Are you sure about the current rating on the motor? Is the power supply voltage "holding up" or is it dropping when the motor runs?

You might also want to check the voltage out of the bridge... The power loss (heat) is Voltage x Current. (That's the voltage across the H-bridge, not the voltage out of it.)

3

Thanks DVDdoug,

I got the H-Bridge for cheap so it might be that. I did already order a new one capable of handling 43A which should arrive in the next few days. I hadn't considered relays yet but since I only want to drive the motors at full speed I will definitely look into using them instead of an h-bridge.

At the moment I do not own a multimeter :o . I will get one tomorrow so I can test for the things you are suggesting.

4

igorbobeldijk:
At the moment I do not own a multimeter :o . I will get one tomorrow so I can test for the things you are suggesting.

Do not do anything more until you have the multimeter! :astonished:

5

igorbobeldijk:
Hi all,

I'm building a vending machine type closet. It's a tall box with a window and a coin-slot. The idea is that
after someone throws in a coin the window slides open. For this purpose I use a scissor style powered car-window mechanism.

The H-Bridge I'm using handles a current of 10A with a 30A peak. The motor I'm using draws 5-10A.
Though it should be able to handle the motor, one of the MOSFETs on the H-Bridge becomes extremely hot when turning one direction, and another MOSFET will become extremely hot when turning the other direction.

You probably haven't read the small print about MOSFET H-bridges - you must use PWM with a duty cycle
less than 100%. Trying to drive it with 100% duty cycle completely prevents the bootstrap circuit
working.

So use analogWrite (pwm, 240); instead of digitalWrite (pwm, HIGH);

6

MarkT,

You're right, this is the first time I read that I cannot use 100% duty cycle for this purpose. This solved it for me. It now turns full speed both ways. Thanks a lot!

p.s. I went out and got myself a multimeter anyway :smiley: