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.

 

Is it possible to increase gear ratio of motor drive?
Use larger capacity batteries?
Parallel battery with very high capacity capacitor?

 

Why 3V batteries?

 

Hi Bernard,

No i cannot use larger capacity batteries i have to use AA batteries, and currently i have two batteries in series to give me the 3V i need. I can only use two batteries max.

I want know if there is some circuitry that i can add to let the motor draw the current it needs at a lower voltage. Currently motor draws at >400-450 mA to close the valve. This is fine at 3V, but as the battery voltage reaches 2.5V it cannot draw enough current. There must be a work around for battery powered devices such as in toys that they use.

The mechanical aspect of gear ratio is one that i have not explored, the current gear ratio is 1:100. I need a electronic circuit solution because i cannot change the gearing on the motor at the moment.

 

I want to deal with this issue with circuitry.

I want to be Kate Uptons bra

 

Size constraint of the enclosure means it will have to be AA batteries, in order to supply the 3V needed. AAA bateries are a possibility but i dont see how they will help the situation. Other batteries are too big.

 

How about a different battery 1/2aa lithium 1200mA 3.6V


Or 18650 3.7V 5000mA rechargeable

http://m.gearbest.com/chargers-batteries/pp_22299.html?currency=GBP&gclid=CLOFqeH93soCFVQ_Gwodr4YKkg


http://www.buyabattery.co.uk/3-6v-c...tteries.html?gclid=CPP-m6T83soCFSoOwwodKNgCoQ

 

Hello,

@Dodgydave , could the shown battery handle the 400-450 mA the OP mentioned in post #4 ?
The continue current is 35 mA max, but what would be the peak current?

Bertus

 

Thanks, for that link Dave. Unfortunately i have to use AA batteries, since these are most common for any user. If you have any circuits ideas i will greatly appreciate it.

If i use a step-up converter to boost the voltage as it drops below 2.5V to 3V, will this mean since the voltage is higher, the motor will draw more current as it needs??

 

You cant create more current from nothing...

 

Ok. Surely at combined voltage of 2.5V of 2 AA batteries, it is still capable of providing a minimum of 400-450mA to the motor? When i connect the setup to a power supply and drop the voltage to below 2.5V the current it draws reduces as you expect, resistance of motor is the same and the voltage drops therefore current drops too(ohms law) right? So if the battery still has enough current capacity which it should, increasing the voltage will force the motor to draw more current. Am i making stuff up or does that sound correct like in my head??

 

Its in your head, the internal resistance of the batteries increases as it gets lower in voltage. You can put capacitors in parallel with the battery to increase capacity,.

 

If your used batteries can support 600 mA @ 2.5 V then a Boost-Buck convertor might work. From Electronics In-house, momo_w20, eBay, PM-1625MDU Fixed Frequency Buck-Boost, DC-DCX73440, 1.8 V-5 V to 5V ( adjustable ? ) @ 600 mA.

 

Try a Ltc3113 buck booster...

http://m.linear.com/product/LTC3113#overview

 

I want to be Kate Uptons bra

That's better than what Prince Charles wanted.

 

if i use a charge pump (since they are more efficient) to increase voltage from a lower battery voltage upto 3V seems like a good solution. The only problem is charge pumps dont provide high enough currents, say for example 100mA. The motor needs around 400mA, so another 300mA needs to be provided somehow for it to move.
If i use a large capacitor on the output of the charge pump, will it be able to provide the extra current it needs once its fully charged??? How exactly will that work?

 

It won't. You can't get more power out than you put in.