MOSFET h bridge problem, thing wont saturate!

This is my design for a simple mosfet h bridge. It uses IRF1405 N channel Mosfets (yes, the symbol is wrong, i’m sorry)

I added the 10k resistors for pull downs, so that the mosfet gates are low by default.

There are actually two of these, using all the same power sources to power two seperate motors.

I originally used the 5v from the arduino pins to turn on the mosfets directly, but it did not saturate them even though the saturation voltage should be like 2v (???)

anyway, this didnt work, so I used the 2N3904 NPN transistors to allow the 9v(this 9v is taken from the battery that also powers the arduino) to turn the mosfets on (an amplifier).

It doesn’t do anything- the motor turns very slowly like it did without the amplifier, and the mosfets become hot- i presume because of high drain-source resistance.

Can anybody tell me why this isnt working, and/or how i could make it work?

Well - I am unable to locate a datasheet for the IRF405 (got one handy?); but I can tell you that you can't make an h-bridge with n-channel mosfets in that fashion; generally you see n-channels on the bottom, and p-channels on top. It is possible to make an n-channel only h-bridge, but the upper side has to have a voltage boost system so that they will turn on properly, and you have to always use PWM to the mosfet inputs (unlike a combine n/p-channel h-bridge).

My best guess, sans datasheet (and disregarding your circuit - which won't work) - is that a) the mosfets aren't being turned completely on, and/or b) the RDS is too high.

There are others here, though, who can better help you on this, as I am fairly newb when it comes to mosfet h-bridge understanding...


Thanks for the quick reply!

The h bridge works fine if you plug the gates directly into the 9v battery, and so my question is more asking why the amplifier isnt doing its job.

looked back at the transistor, turns out they are actually called IRF1405 (apologies for the mistake) hence your difficulty finding a datasheet. Here is the link:

That FET will not turn on fully until you have a Gate source voltage of 10V. You need to use a logic level FET if you want to drive it with smaller voltages.

You also need to get yourself a proper circuit, that one is just rubbish, it will not work. Even if it did you would never be able to turn the motor off.

Ok- you guys are misunderstanding. The circuit WORKS fine- the h bridge, that is. when you put 9v to it by attaching the battery, you can turn the motor on and off fine. The only thing I don't understand is, why isn't my way of amplifying to 9v working? Please can someone tell my WHY this circuit shouldn't work- I don't see whats wrong with it.