I'm working on a DC motor speed control circuit, but it's not working properly. The MOSFET (IRF240) isn’t switching because the op-amp (LM741) isn’t providing enough gate voltage (only ~172mV, but it needs 4V-5V).


Here's the circuit—any ideas on how to fix this?
i upload the file

 

I would like to try to help you but I never open zip files from unknown sources.

 

sorry i did not know that he will accept .sch files

 

Hi siwaffle,
Which program did you use to create the sch file?
E

 

its a schematic from orcad family 9.2

 

Post a screen grab of the schematic as I don't have orcad.

 

 

the op-amp (LM741) isn’t providing enough gate voltage (only ~172mV, but it needs 4V-5V).

1) The 741 is not a rail-to-rail output type of op-amp, it has a poor gain-bandwidth product (so can't switch quickly), and it can't provide enough gate current for switching a power MOSFET at high frequency.
2) At 4V the IRF240 may only be supplying ~ 250 microamps (see the datasheet). Vgs should be ~10V to turn it on fully.

 

You need a comparator, not an op amp to drive the MOSFET.
What is the V3 input control signal?
How is that circuit supposed to work?

That is a boost switching circuit.
What voltage does the motor need?

 

The TC4426 is a good power Mosfet driver.

 

You'll never get anywhere using the opamp in an open loop configuration. No matter what you do on the input the output will most likely be at one of the power supply rails. The 741 will not operate happily in single supply mode and the output will not get anywhere near the power supply rails. These are things you should be aware of. Have you received any instruction at all on using opamps?

 

See

 

I am wondering just what the TS is expecting the circuit shown to actually do. It looks a bit like it is intended to be a switch mode voltage regulator/driver, but that makes no sense at all to me.

So really, the very first thing we need is an explanation of the intended function of the circuit.