Go Down

Topic: PWM Trouble Shooting (Read 243 times) previous topic - next topic

radcliff

Sep 24, 2019, 02:38 pm Last Edit: Sep 24, 2019, 02:44 pm by radcliff
I have an IRF 540 set up for PWM. Power source is 12v. battery The mosfet is being fed by a frequency generator at 20khz with varying duty cycle. The battery also supplies power to the frequency generator.

Can the mosfet be without a load during testing?


With the duty cycle I have set up I am measuring 1.6v (with a multimeter) from gate to ground.


Shouldn't it be 1.6v also from the drain to the positive side of the battery?
I'm getting 12v.

MarkT

A power mosfet just looks like a low-value resistor to the load when on, resistors don't mind having no load!

Without a load there's no point looking at the drain voltage though.  Add a resistor/LED test load if you want,
gives positive indication of state.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

DVDdoug

Quote
With the duty cycle I have set up I am measuring 1.6v (with a multimeter) from gate to ground.
Multimeters don't accurately read pulses so the reading is normally useless unless your meter reads the average or the peak and you happen to know how it reads pulses.

If you don't want to connect the "real load" yet, try testing it with a regular little LED (with the usual current limiting resistor).

Quote
Shouldn't it be 1.6v also from the drain to the positive side of the battery?
I'm getting 12v.
You do need some kind of load, but it can be a "light load" (like a 1K resistor).

gilshultz

You need a scope to properly analyze your problem.  Simple things you can do is turn the FET 100% on and measure the Vgs to be sure you have enough. You can get that value from the MOSFET data sheet.  Add a steady state loads (restitive, approximately the same as the real load if inductive) and measure Vds. First and foremost be sure all connections are good. Watch the power supply voltage through all the tests. 

There is a way to use your audio card as a scope but injecting more then 5V destroys them. You can find this on line.

Good Luck and Have Fun, Gil

TomGeorge

Hi,
You need a load of some sort, even a 1K resistor.
Then when you measure drain to gnd voltage, it should change with PWM duty cycle.

Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

Go Up