Small motor driver

[Commanderbob]

I am making a small robot that is using 2 DC 3V motors that will be powered with a Li-ion battery. The problem is most of the electronics run at 5V using a step up circuit and I need a way to control the motor (forward and reverse with PWM) at 3V. I have used H-bridges for this but most have a voltage drop of 2V, so I could not use them with the 3.7v battery. I prefer one that uses a PWM input and a direction input (Ex.dir=0 CCW dir=1 CW, PWM=1 On PWM=0 Off), that makes it easier when the AVR only has two PWM channels. Has anyone been able to use motors like this before?
Thanks,
Justin

[TrickyNekro]

I don't think you will have any problem with choosing the microcontroller as
most of them run as low as 2V7...

For the H-bridge... did you considered any MOSFETs...
MOSFETs are beginning to have less and less internal resistor, meaning that your voltage drop
will be very very low... and you don't even need any external components...


Cheers,
Lefteris

[bens]

I am making a small robot that is using 2 DC 3V motors that will be powered with a Li-ion battery. The problem is most of the electronics run at 5V using a step up circuit and I need a way to control the motor (forward and reverse with PWM) at 3V. I have used H-bridges for this but most have a voltage drop of 2V, so I could not use them with the 3.7v battery. I prefer one that uses a PWM input and a direction input (Ex.dir=0 CCW dir=1 CW, PWM=1 On PWM=0 Off), that makes it easier when the AVR only has two PWM channels. Has anyone been able to use motors like this before?
Thanks,
Justin

You have a few options that I can think of:

1) Build your own low voltage H-bridge out of discrete MOSFETs, as TrickyNekro suggested.
2) Use an existing H-bridge with a 2V voltage drop and a 5 - 6 V power source (e.g. four NiMH cells).  Such a power source can be problematic if you want it to power electronics at 5 V, but you could use it just for the motors.  How committed are you to that Li-ion battery?
3) Consider a  pre-built solution, such as this low-voltage dual serial motor controller.  This contains two low-voltage H-bridges constructed out of discrete MOSFETs and controlled by a microcontroller that accepts serial commands.

- Ben

[Commanderbob]

I was thinking about making my own but that would require 2 PWM channels per motor correct? I was looking for something more like http://focus.ti.com/lit/ds/symlink/tpic0107b.pdf.

I do have a 7.4V 1100mAh Ni-MH pack that I could use but this is a small robot and the pack will be MUCH heavier then the robot.

I am using an AVR that is run off 5V for speed. The 5V is generated from a converter.

The robot consists of two bump sensors, ATmega32, 3 axis accelerometer, a servo, and a sharp IR sensor. Everything except the motors and the 3 axis accelerometer runs off 5V.

I could fit the Ni-MH pack under the robot if there is no easy way to use the li-ion.

Thanks,
Justin

[TrickyNekro]

I think MOSFETs are the best solution....


I weird question to you...
Have been making a coil gun?!?! I want to make one this summer or at least experiment...

If you are willing to help and have enough info should I pm you?!?!

[Commanderbob]

Have been making a coil gun?!?! I want to make one this summer or at least experiment...

  yes. I have made a few.

If you are willing to help and have enough info should I pm you?!?!

Post your questions on [ur=http://forum.coilgunpower.com/l]my forum[/url].

I decide to use a H-bridge IC and the 7.4V battery.

Thanks,
Justin

[Admin]

You can use a 3.6V or 2.4V battery for your motors/driver, and your 7.2V battery with the microcontroller.

Nothing says you can't use two batteries

(just make sure you keep grounds common)

Oh, and many AVR's can run at 3.3V, no problem. Just check the datasheet.