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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
I cant get the PWM to vary anymore. ive killed one IC by putting more than 7 volts in the comparator. so I replaced it an I'm getting a 50% duty cycle out on the drive pins. but when I vary the comparator input from 1.5 to 6.5 volts I should see a PWM % change right? I saw a change in the above oscillograms.
I dont know how I could be doing it wrong, in 2 ad 16 are tied together, and pin 1 is the input from the varying voltage. I don't want to move on to etching a new board until ive got this problem solved.
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
No, I don't think so - the op-amp needs some feedback to set the gain.
I've just been looking at other adjustable PWM designs and found this: Pin 9 on the diagram is the output of the error amp and this is being looped back to pin 1, which is the same as your pin 1, the inverting input of the error amp. Pin 2, the NI input, is pulled from GND to Vref by a potentiometer.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
your right op-amps need feedback, duh, I overlooked that. but the original circuit just has a capacitor and resistor in series, and I think that for damping. Ill put in the components you suggest.
I cant even begin to get a feedback circuit to work as you suggest, the PWM just sits there,
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
Can you measure the voltage on pin 3? That is the key, and determines where on the PWM ramp the pulse latches The ramp pin (7) has a nice ramp waveform on it then, from the Ct pin? (might be tricky to measure that one without effecting the oscillation) If you're getting an output it can't be current limiting...
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Need help with current limiting.
