[SOLVED] PWM Circuit - Regular Opamp or Comparator

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hshah8970

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Hello everyone.

I'm unclear as to whether the U1B and U1C components in the attached circuits are regular opamps or voltage comparators. They are all taken from MAX4094 which is a single IC with 4 opamps on it. But U1C clearly is a voltage comparator.

I do not have MAX4094. What I have are LM358 (regular opamp) and LM397 (comparator). I need to know which one to use in place of U1A, U1B, and U1C.

I'm attaching the circuit image. It was taken from **broken link removed**.

 

U1C is a regular opamp working in open loop configuration, so effectively exerts the voltage comparator function. It doesn't matter if you use an opamp or a comparator in this position. Use that one with the larger skew rate (δV/δt).
 
The only strict requirement is to use a (unity gain compensated) OP for U1A. Slow OPs like LM358 will still work as comparators due to the low kHz PWM frequency, even for U1C if the driven circuit doesn't necessarily need fast edges.
 
I tried using the LM358 for the above circuit and it didn't work. I used 1k for R and R1, 2k for R2, and 330uF for C, to make the frequency nearly 0.66Hz. However the triangular waveform generated by the first opamp has a magnitude in milivolts which messed up the entire PWM functioning. Furthermore, the frequency wasn't 0.66Hz either.

According to the article I got the circuit from [ **broken link removed** ], the values of the triangular waveform should be 9V upper peak, 3V lower peak, and 6V center.

Any idea why this could be? Should I use a 741 instead of the 358?
 

The circuit should definitely work with LM358 and the said component values. You have 12V single supply voltage, if I understand right.

Check the resistor values and connections.
 

Yes. This is the circuit configuration I simulated in Multisim which works fine for a virtual opamp but not for the builtin model for the LM358. [attaching image]



Update: I shall recheck my connections tomorrow and update the post. I think I might have forgotten to connect a particular pin.
 
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I was talking about correct function in real hardware. You didn't mention a simulation (and this isn't the simulation forum).
 

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