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https://www.autonerdz.com/cgi/yabb2/YaBB.pl General Public Area >> PicoScope >> PWM sub zero ? https://www.autonerdz.com/cgi/yabb2/YaBB.pl?num=1374996996 Message started by Robski on Jul 28^th, 2013 at 12:36am

Title: PWM sub zero ?
Post by Robski on Jul 28^th, 2013 at 12:36am

I have noticed this now on several captures of PWM control circuits, the PWM drops below zero, why would this be does anyone know :-/ ?

Seat_Altea_circuit_design_switch_on_640x385.jpg (47 KB | 506 )

Title: Re: PWM sub zero ?
Post by brandonb on Jul 28^th, 2013 at 5:07am

its the flyback circuit in action, if it wasn't there you would see huge kick backs like injector wavforms
they use a diode reverse biased across the device or simular design, when the driver cuts off juice, the diode is forced to conduct cycling the electromagetic energy that is now being turned to current back through the device in a small loop until it dissipates, hence negative voltage

Title: Re: PWM sub zero ?
Post by Robski on Jul 28^th, 2013 at 5:26am

Thanks Brandon, that makes perfect sense [smiley=ok.gif]

Title: Re: PWM sub zero ?
Post by Darren Cotton on Jul 28^th, 2013 at 11:26am

Good question... I have seen this before, but never really thought about why it does this... Good question Robski
and thanks Brandon, I think I get it...

Darren

Title: Re: PWM sub zero ?
Post by Anthony Lamb on Jul 29^th, 2013 at 12:45am

If i am understanding it correctly it is like a diode in the coil side of a relay?

Pretty cool :)