# PWM Motor Driver output voltage is enormously load - dependent

I've built this PWM motor driver (for a motor which requires 5A at maximum 24V and which I want to drive sometimes with 10V and other times at 24V). Here you find the datasheet of the mosfets.

PS: there is also a missing 10k resistance between gates and GND

The device works and supplies correctly my motor. If I use a 50% of duty cycle, I get an average voltage of 12V across my motor.

But, If I measure the output voltage without load, or with a low - current load (such as a 200mA fan), the device works (it provides an average voltage which is determined by the PWM duty cycle) but not with the right proportion: without load (the only load is my volt - meter) I get 12V (instead of 6V) with 25% of duty cycle and 20V (instead of 12V) with 50% of duty cycle. With fan the result is slightly different but always in contrast with the right proportion.

My possible explanation: the output voltage is, in theory, a square wave. A Volt - Meter, a fan and my motor filter it in different ways, so the result is different. But, if this explanation is true,

• I'm surprised of such big differences between the behaviour with these different loads.

• I don't know how should I take care of this fact and design my circuit to work properly with a specific load (for my motor, it works perfectly, but maybe it is lucky....I've never thought at this aspect).

• I think that different loads will change the amount of ripple, and not the average value. But, in my measures, the average voltage is different.

Can you help me to explain this phenomenon in a more detailed way?