Something happened that i haven’t been able to explain , I wrote a program to output a PWM signal, this to activate a transistors base using a PWM which I could change the frequency using a potentiometer, the program worked fine, and the circuit too, however something was happening,

1) When I lowered the frequency of my PWM the motor stopped spinning however a “buzz” could be heard from inside the motor

2) When I tried to verify my PWM signal using an oscilloscope, like in the picture, , a spark came out at the tip of the probe and the circuit stopped working, the microcontroller on my board died, (it just gets way to hot in seconds) however I still have no idea why this happened,

enter image description here

Hopefully someone here can explain to me what happened, I did plug my PWM signal directly to the oscilloscope before with no problems however when tried to debug in the working circuit this happened, as i recall the probe was gorunded with the circuit , but you might also want to consider the case of it not being grounded


When you drop to a low duty cycle, a motor will not be able to rotate due to the mechanical inertia. However, you are still pulsing the voltage onto the motor. So you are hearing the coils in the motor activate and deactivate rapidly but without enough energy to rotate the motor.

I usually put a lower limit on my PWM duty cycle in the software. I just turn the PWM off below that point.

As far as the scope issue. You may have had a static shock as it is pretty dry right now where I am. When it gets this way, I have to be careful to ground everything. Just touch the probe to a connector case, like one of those USB.

Or, you may have some sort of grounding issue that could cause a problem. Scope grounds are connected to earth ground, and that little metal ring near the tip is the ground, so if you are powered in a non-isolated way, you could easily accidentally short.

But since you specifically said the spark came from the tip of the scope, I would guess static is probably more likely.

  • \$\begingroup\$ Yeah, agree with the static. Also I can't think how such large voltages could be induced in the device. Maybe the OP just had that bad luck that just coincided with his experiments. If it were a scope grounding issue (good call, I've blackened a building once blowing a main fuse with an ancient scope ground), he would have had the damage when connecting the oscilloscope ground, not the tip. \$\endgroup\$ – PkP Jan 10 '15 at 7:25
  • \$\begingroup\$ I do know the osciloscope is mains earth bounded, however i did not know the little ring was ground too which makes me wonder why is it there in the fisrt place?, i might have shorted it by touching with that and not noticing, all my grounds where at the same node, the problem happened when putting the tip (positive) probe to the PWM when i made the motor "stop", to hear the buzzing, also the room temp was 17C with around 48% humidity, i have little knowlege about static discharges but isnt mains earth supposed to absorb that?, since i was working on a 10M Mains earth grounded ESD mat \$\endgroup\$ – GoatZero Jan 10 '15 at 7:37
  • \$\begingroup\$ As Alice would say, couriouser and couriouser. :-? \$\endgroup\$ – PkP Jan 10 '15 at 7:43
  • \$\begingroup\$ The little ring is there because it's used to provide a ground for some probe accessories. \$\endgroup\$ – EM Fields Jan 10 '15 at 8:50

I don't have a definite answer for you right now, but let's see if we can start to make sense to this, together with the community.

The first thing to note here is that the DC motor is an inductor. And you'll need to be careful when switching inductors; after all, that's how you do switching power supplies.

When Q3 is conducting, current starts to flow from 12VDC to the ground via the motor coil (let's forget the brush for a while). So you induce a current into an inductor. But when Q3 stops conducting, the current doesn't stop. A magnetic field has formed inside the inductor that has energy that wants to keep pushing electrons. If D3 wasn't there, a Really Large Voltage would build up across M1, since the current wouldn't have anywhere to go. Step up converters use this phenomenon to boost voltage.

But in your circuit there is D3, so when Q3 stops conducting, there is a path for the current to flow; the current flows back towards the 12VDC supply. Is the supply still working properly, by the way? If the power supply doesn't want to absorb the current, the current then keeps flowing into the motor inductor; it goes there round and round until the resistive and other losses absorb the energy.

Ok, now bring back the brush, the rogue element here, which might explain some of the phenomena. What the brush does, it switches the polarity of the motor coil during the motor rotation. And at this point it becomes difficult for me to think what might be happening, hopefully someone else can assist. Could it bring a large negative voltage to the collector of Q3? What would that mean?

A critical flaw in your design is that you should never adjust the operation of a DC motor to a point where it stops spinning. It's not meant to be used that way. You'll hear the coils buzzing but nothing happens. Sometimes it kills the motor, but perhaps not in your case.

Finally, it's a DC motor and you're providing AC, or Alternating Current, to it. That's basically not good. At least some filtering should be used to make it more DC. Motor experts, please help.
[Edit] Community suggests that it's no problem to use motors this way.

Hope this helps a bit.

  • \$\begingroup\$ yea, supply still working properly, Im not sure what you mean by feeding the Circuit with AC since i dont see any AC signal in the diagram \$\endgroup\$ – GoatZero Jan 10 '15 at 7:29
  • \$\begingroup\$ You make it AC by switching the current on and off with Q3. That's exactly what AC means, alternating current. Q3 alternates the current - switches it on and off. Hmm... well, at least the direction of the current stays the same in Q3, even though the amount of current is alternating. Not really sure what it should be called. \$\endgroup\$ – PkP Jan 10 '15 at 7:31
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    \$\begingroup\$ This is a pretty typical way of using DC motors. The PWM causes an average DC level. As long as the frequency is high enough, the current smoothing action of the motor coils handles it. \$\endgroup\$ – caveman Jan 10 '15 at 7:42
  • \$\begingroup\$ @Caveman, Right. Got it. Didn't know that, good to know. So no any special filter caps required? \$\endgroup\$ – PkP Jan 10 '15 at 7:44
  • \$\begingroup\$ You can add them if they make you feel better, but I don't think the motor cares that much. \$\endgroup\$ – caveman Jan 10 '15 at 7:45

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