Driving 3V motor on low voltage AA batteries to close a valve?

[mike_17]

Im trying to drive a 3V stepper motor from a discrete dual MOSFET H-Bridge(P Channel - Highside) (N-Channel - Lowside). I used PMV130ENEA and PMV250EPEA logic level mosfets.

One side of the dual h-bridge looks like the one on bristolwatch.com shown below:

The issue I have is that when the battery voltage goes below 2.5V the motor is not able to close the valve. I believe this is cause there is not enough current for the motor to push the valve closed at a voltage of 2.5V. I want to know how i can push the valve closed at the low voltage?? Will increasing the voltage using a step-up converter allow more current to be drawn from the battery? Any help is appreciated.

 

[duke37]

There are two possibilities.
1. There is not enough voltage to turn the fets fully on.
2. There is not enough voltage to drive the motor.

I do not see where there is a 2.5V supply. Stepping up the voltage is the way to go.

 

[mike_17]

The image in the post is not the exact circuit, its just to give a representation of the h-bridge configuration. The 2.5V supply is the battery voltage as it drops from 3V as it discharges. The supply is straight to the motor, so in the image it replaces the 12V.

The mosfets i used (pmv130enea and pmv250epea) are logic level mosfets with typical turn on voltage at 1.7V so i dont believe it could be that.

And yes i think that the lack of voltage is why the motor isnt drawing the current it needs. When under load at 3V the motor draws 400-450mA in order to move. So when the voltage drops to 2.5V so does the current it draws. Do you think by stepping up the voltage back to 3V, the motor will automatically try and draw 400-450mA again from the batteries successfully even if the batteries are still at 2.5V? Will it have enough current in the batteries at that voltage to provide for the motor?

 

[duke37]

If you look at the data for the pmv130enea you need about 3V gate to source to get a low series resistance (fig 9). The motor current has to pass through two fets, the supply is not straight to the motor.

If you use a voltage booster with a low voltage input, then more current will be drawn from the battery. 400mV seems plenty, I wonder whether the batteries could manage 600mA.
Remember that the convertor will not be 100% efficient, three cells would be better.