3
\$\begingroup\$

In a previous question I posted to the exchange I asked for some instructions on trying to variable speed control a motor. Someone suggested utilizing PWM from my Arduino to open and close a Darlington pair to control speed. I wired it up and ended up frying my darlington pair. I am afraid that I screwed up something with the flyback diode... I am not certain. I am not sure where to trouble shoot what I did wrong. The transistors and flyback diode were very hot to the touch.

What kind of ratings and type of diode should I look for when looking to eliminate flyback voltage from a 18 VDC, 1.5 motor? How would I wire this correctly when using PWM from an Arduino to control a transistor pair?

I am a very new hobbyist and am having trouble wrapping my head around this issue.

\$\endgroup\$
  • 1
    \$\begingroup\$ Question titles should address the topic directly, rather than "does anyone know about topic?" I've edited accordingly. \$\endgroup\$ – JYelton Jan 24 '14 at 20:40
  • \$\begingroup\$ A major step in understanding is to realise that in most cases in electronics 'there is no magic'. Or, not at the top level where things can be understood 'well enough'. Dig deep enough and you find it's magic all the way down BUT you need little or none of it for a basic understanding. In most situations a diode can be considered as a one way valve. Apply voltage (More positive at the Anode end) and current will flow. Change polarity and no current flows. SO look at your circuit diagram. Will the diode conduct when the darlington turns on - if so something is wrong as the diode ... \$\endgroup\$ – Russell McMahon Jan 25 '14 at 11:18
  • \$\begingroup\$ ... is there to conduct when the inductor (here a motor) polarity reverses. And go from there. When you PWM a motor, when you turn the drive off the inductor (motor) current MUST go somewhere. The diode is there to ensure the current goes somewhere useful and/or harmless. The current is the current flowing in the motor when it is interrupted. The voltage is whatever you apply when the diode is reverse biased, and around a volt or less when the diode conducts. ... \$\endgroup\$ – Russell McMahon Jan 25 '14 at 11:21
3
\$\begingroup\$

Diode was probably inserted backwards around the motor - the purpose of the diode is to bleed a current spike from the motor's inductance, which flows opposite direction.

The diode has a line/bar that indicates the cathode - this side needs to be connected to + side of the motor.

You might need to replace the darlington and diode, unfortunately.

\$\endgroup\$
1
\$\begingroup\$

The flyback current will never exceed the motor current, so 1.5A is more than okay. It needs to be rated for more than the supply voltage (with some margin) PIV and a Schottky or ultrafast type is better for speed.

I agree with tariksbl that the diode you had was probably connected the wrong way- it has to block voltage when the motor is energized. If it conducts, the drive circuit gets whacked, and maybe the diode too.

A suitable part would be the Micro Commercial SR208-TP (Digikey SR208-TPCT-ND), which is 31 cents in 10's. It's an 80V 2A Schottky type in a through-hole DO-41 package.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.