Maintain constant voltage across a variable load (op-amp)

Thread Starter


Joined Sep 28, 2021
Hi readers :),
Vin is fed with the PWM signal coming from an op-amp to be amplified and fed into the load R1 (1 Ohms). The purpose I design this is I want the voltage source to be a constant 5V which is independent of the load (1 Ohms or 100 Ohms). So basically, this circuit has an op-amp which acts as a PWM controller. When voltage is high across R1 (lets say 8V) , the OP07 output duty cycle would decrease which means PWM decrease, this helps to lower the voltage of R1 back to its original voltage (5V). However, this circuit doesn't really seem to work.

Question: How to maintain constant voltage across R1 Load that is independent of the ohmic value of the load? :)

Thank you for reading :)



Joined Sep 17, 2013
I want the voltage source to be a constant 5V which is independent of the load (1 Ohms or 100 Ohms)
PWM does not provide a constant voltage: it switches between two voltages. You need a fixed 5V reference for the op-amp input.
A 2N4403 cannot handle more than 600mA, so is unsuitable for the load range.
The op-amp feedback is the wrong polarity. Swap the + and - inputs.


Joined Aug 7, 2020
It looks as though you are trying to design a buck regulator. You could use a buck regulator IC, such as LM2678. it would be a lot easier.

OP07 is not suitable for generating PWM, because it has a very low slew rate of 0.3V/μs. A comparator would be much better. You will also need an error amplifier, which is wired as an integrator.
A buck regulator IC will also contain both PWM generator and error amplifier.


Joined Nov 6, 2012
What is the overall purpose of this "Regulator" ?

"V1" says 10-Volts,
what is supplying this Voltage ?,
how much Current can it provide ?
A complete description of the Power-Supply is required before the discussion can go further.


Joined Mar 14, 2008
Please remove the dangling wires and unnecessary jogs in your schematic wiring.
It looks sloppy and makes it hard to read.

A wire should have no more jogs than absolutely necessary.


Joined Jan 23, 2018
Constant voltage with PWM and a random load resistance seems rather confusing.
Is the intent to have a given duty cycle produce a specific average voltage across the load? Otherwise, a regulated supply can easily deliver a constant voltage at any current within its capability
So really, the request is ambiguous, and more explanation of the requirements is needed.


Joined Jan 23, 2018
Actually, providing a constant 5.0 volts across a variable load is simple if you select a switching mode regulated power supply able to deliver up to 5 amps. That will hold 5 volts into one ohm. OR you can use a linear regulator scheme, simpler and less noise but much less efficient.