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Supplying enough current to servo motors

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Echelon 01

New Member
I have two possible power sources: either two 9v batteries (in series or parallel) or 8 C-cell batteries configured to produce 6 volts. These are design constraints and I can not get power any other way but these.

Now, I want to power two servo motors at once they are for the right and left treads of the robot. They both draw 680mA max, and less than 10mA idle. They will not always be on at the same time, but will be on at the same time when the robot is going straight. I only need the robot on for 2 minutes so draining the batteries after this time is more than acceptable. The minimum voltage that needs to supplied to the motors is 4.9 volts and the max is 6.1 volts. Any kind of voltage regulator or current limiting device can be used.

Anyone have an idea of what is the optimal configuration of the batteries - or what kind of regulation i can use to get 5-6 volts with enough current for both motors (1.33 Amps im assuming but that is the max they would ever need combined) ? Any comments or ideas whatso ever appreciated.
 

nettron1000

New Member
Have you considered using a 7806 voltage regulator IC ? The 78xx series regulators can handle a max current of 1 amp and they have thermal shut-down abilty built in. Should be fine for your needs.

If you want higher current handling ability from the regulator just add a pass transistor to the regulator, this would of course require a power transistor usually mounted to a suitable heatsink, rated for the expected max current draw of your load.


Edit: Configure your batteries for 8 volts to the regulator using the C cells.
 

DirtyLude

Well-Known Member
You don't need to regulate power to them at all. If these are standard modified RC Servos, they can be over powered at 9 volts for short periods without any danger. The optimal setup of the batteries still depends on information we don't have. Is there weight constraints? Do you need max/power speed, or is just moving adequate?
 

Echelon 01

New Member
The 7806 Looks like a great Idea, I have also looked into the LM7808 as well as some adjustable voltage regulator ICs. The only problem is how much current do these regulators waste? I really dont know except thats some are considered low drop out and some have other current saving features (but not the 7806). Here is a wiring diagram in jpg format that might shead some more light on what im trying to do, please keep in mind this thing must have both the right and left tread motors on at the same time to go straight. S1,S,S3, and S4 are the servos for right tread, left tread, pivot, and griping arm respectively.
 

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crust

Member
9V batteries are usually less than 300mAh whereas a C cell is nearly 7800mAh. I would opt for the C cells driving the servo motors. Using the parallel combination at 6V would allow you to directly drive the motors without the losses of a voltage regulator (though the speed/torque would decrease as the batteries ran low). The motor current required (0.68A) is not too high for C batteries. The total power in 8 C batteries far exceeds what you would get out of 2 9V batteries.
 

ivancho

New Member
I would recommend you to go with the 6V pack.... so you don't have to waste anything with a regulator. And the normal R/C servos will take the 6V easy.

:idea: What I've done in the past, to get more speed, is to remove the circuitry inside the servo and made my own controller. This way I have power those servos at 8.4V. I used one of those L293 SMD to make it small enough for my application.

I hope it helps you some
 
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