Strange oscilloscope pwm reading

[Nickedemuz]

Hi
Might be I bought a really cheap (and probably crappy) pwm motorcontroller from China. I hooked it up to my scope and got some very strange reading. There is also a possibility that I’m doing something really stupid with the scope cause I’m pretty green at this
The motor is running ok and the speed control works. I’m just very curious about the strange waveform. I expected it to be a square wave.
appreciate any help or info

 

[Nigel Goodwin]

If you've got the motor connected to the controller, then you won't see the PWM on the motor.

 

[unclejed613]

motor windings act like inductors, so when a pulse is applied the current ramps. it's the same effect as in a CRT monitor deflection yoke.

 

[Nickedemuz]

Oh, thanks! So I guess we’re looking at a noob mistake here At least I learned something new.
Could you please tell me what might be the correct way of hooking up the scope?

 

[Nigel Goodwin]

Oh, thanks! So I guess we’re looking at a noob mistake here At least I learned something new.
Could you please tell me what might be the correct way of hooking up the scope?

If you want to view the PWM then you need to use a non-inductive load (not a motor).

 

[Nickedemuz]

Then I’ll try hooking it up to a led instead and see if that does the trick.

 

[Nigel Goodwin]

Then I’ll try hooking it up to a led instead and see if that does the trick.

An LED isn't any kind of a load, an incanescent bulb would be better, presumably a car bulb as you look to be running a car type motor?.

 

[Nickedemuz]

Thanks a lot for helping me. I really appreciate it! I thought the LED would be the same. I’m learning a lot here And you are correct in the assumption of a car motor. It’s a wiper motor.

 

[Nigel Goodwin]

Thanks a lot for helping me. I really appreciate it! I thought the LED would be the same. I’m learning a lot here And you are correct in the assumption of a car motor. It’s a wiper motor.

Over the years I've found car bulbs an invaluable source of 'dummy loads', a combined rear/brake bulb gives you three possible loads, and for higher loads a front fog light bulb or headlamp bulb does the job nicely.

 

[Nickedemuz]

that’s a really good idea! Will be getting those to have some options. I had a small 12v rear lamp and...success!

Next thing to watch on YouTube will be how the loads differ from each other

 

[Nigel Goodwin]

that’s a really good idea!

Much as I'd like to claim it as 'my idea' it's a standard technique dating back a VERY long time, no idea how long, but back before me anyway

 

[Nickedemuz]

First time for this noob at least