Bill,
I took a video of the PWM... at the very beginning and end the triangle wave seems to go haywire. The squarewave is pin 7 and the triangle wave is pin 6


http://youtu.be/Lpgv3nfWcho

Hope this helps ?

 

I still can't figure this out... any help will be appreciated!

 

What is the load on your PWM? Is it a mosfet gate? If so it could be ringing on the gate back feeding in to your circuit.

 

No, it feeds straight to a driver IC

 

Bill,
I took a video of the PWM... at the very beginning and end the triangle wave seems to go haywire. The squarewave is pin 7 and the triangle wave is pin 6

http://youtu.be/Lpgv3nfWcho

The waveforms look more or less normal to me. At the maximum and minimum duty-cycles you are going to 0% or 100% output with circuit noise causing some sporadic pulses. The "haywire" is likely from the oscilloscope sync. If you sync on the sawtooth waveform you should get a stable sync and will better see what is happening.

 

No, its not normal, where the pwm freaks out is where the mosfets or the driver blow up.

 

Just to let everyone know what happened with this, there was no problem with the pwm, the error was completely mine.

I made a really bad error that I didn't pick up on for ages. Floating inputs on the 5v rail cause random noise.... should have realised when I put my hand near it and it started to go wonky. Stupid, stupid stupid.

Didn't see it on the schematic because I'd drawn it so compact.

 

Happens to us all. And remember, experience is when you make the same mistake a second time.

 

Happens to us all. And remember, experience is when you make the same mistake a second time.

Or as I heard it "Experience is what allows you to recognize the same mistake twice".

 

"Education is what you get reading the fine print. Experience is what you get when you don't." Pete Seeger