Interesting findings with a PWM driven DC motor circuit

[EasyGoing1]

So out of curiosity, I connected my scope to a PWM output from an Arduino and I pulsed it at max value for a duration of 100ms on and 300ms off ... here is the output



What intrigues me about this is how noisy it is ... there is no load, the leads are connected directly to ground and the PWM pin.

Next, I used this circuit



and ran the same exact pulse to the gate. Here is the signal at the gate:



and here is the signal at the cathode from ground ... im posting two because I thought it was interesting how much the frequency jumped around:





Is this circuit noisy because of my power supply? Or what? And why does the frequency jump around so much when I have a load on it?

(Images also uploaded into the thread for STJ

[budm]

Re: Interesting findings with a PWM driven DC motor circuit

What does the waveform look like if you measure between two GND points?
How is trace laid out for the GND path?

[EasyGoing1]

Re: Interesting findings with a PWM driven DC motor circuit

What does the waveform look like if you measure between two GND points?
How is trace laid out for the GND path?

It's an Arduino UNO ... for the images without a load on them, the scope leads are connected directly to the Arduino using typical breadboard jumper wire.

When the load was connected, I just used a breadboard and standard jumper wire...

[EasyGoing1]

Re: Interesting findings with a PWM driven DC motor circuit

What does the waveform look like if you measure between two GND points?
How is trace laid out for the GND path?

I think I found the culprit ... here is a snapshot with the scope connected directly to the output of my power supply with no Arduino and no load on it what so ever ...

[redwire]

Re: Interesting findings with a PWM driven DC motor circuit

A brushed motor makes a lot of EMI due to the brushes arcing. It gets into everything, the scope, the Arduino.
Adding ceramic caps, the usual three can help. One across the motor (+) and (-), motor (+) to motor frame, motor (-) to motor frame, and the motor frame is usually grounded. Around 22-100nF each.

The scope is picking up noise due to a ground loop.
I would say between the scope grounded to earth-ground, while the Arduino is earth-grounded through the PC USB connection. Sometimes the noise is just due to the PC power supply.

Because a MOSFET can switch very fast, into the MHz there can be RF generated when switching.
This is due to circuit stray inductance and wiring. Usually a "gate stopper" resistor is used to slow down the MOSFET a little, 22-1000 ohms. You might also want a 47k pulldown so the MOSFET (motor) stays off when the Arduino pin is floating, because a port default is input until you hit setup code.

[PeteS in CA]

Re: Interesting findings with a PWM driven DC motor circuit

budm's point in his post is that your measurement may include noise that is not real. but is picked up by your oscilloscope ground lead. You should do as he suggested, just to see how much of your noise is not real.