unexplained short circuit with PWM signal?

Question

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,

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

Answer 1

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.

Answer 2

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.