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PWM output mirror??

famosas

Jan 1, 2014
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Hi all, i was building a led driver simple buck converter circuit (second picture)using only discrete component. i want to ask if anyone know why my PWM wave output become mirrored? like in the picture. the yellow one is before i connected to the buck circuit. and when i connected to buck circuit it show 2 wave. the new wave is like a shadow to the 1st wave. and in the oscilloscope the frequency and the duty cycle go haywire. please help me. before this it works fine. what broken component in the buck circuit that can possibly caused this? please help me.
thankss
 

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Rleo6965

Jan 22, 2012
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Use the Triggering Function of your Oscilloscope to have steady displayed waveform.
 

KrisBlueNZ

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Hi there and welcome to Electronics Point :)

like in the picture. the yellow one is before i connected to the buck circuit. and when i connected to buck circuit it show 2 wave.
Connected WHAT to the buck circuit?
the new wave is like a shadow to the 1st wave. and in the oscilloscope the frequency and the duty cycle go haywire.
Please start by posting the schematic of your buck regulator.
 

KrisBlueNZ

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I would guess a finger, since the schematic has a pushbutton switch.
I don't think so. That schematic is a buck regulator, and the switch is a MOSFET. But without an actual schematic of the circuit, and some answers, who knows!
 

BobK

Jan 5, 2010
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I am being facetious. It thought it was showing a generic switch. Of course we need the full schematic.

Of course, if he is using a P MOSFET, it explains why the signal is inverted.

Bob
 
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KrisBlueNZ

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I am being facetious.
Yeah, I wondered. Sarcasm doesn't work well in a text medium. That's why we have emoticons :)
Of course, if he is using a P MOSFET, it explains why the signal is inverted.
I got the impression that the drive signal changes when he changes the load. Which of course it should.

I don't like it when people post a vague question then disappear. It leaves us guessing, and wastes our time.
 

(*steve*)

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Hi all, i was building a led driver simple buck converter circuit (second picture)using only discrete component. i want to ask if anyone know why my PWM wave output become mirrored? like in the picture. the yellow one is before i connected to the buck circuit. and when i connected to buck circuit it show 2 wave. the new wave is like a shadow to the 1st wave. and in the oscilloscope the frequency and the duty cycle go haywire. please help me. before this it works fine. what broken component in the buck circuit that can possibly caused this? please help me.
thankss

I suspect you're using a P channel mosfet and that it turns on when you pull the gate low.
 
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