# Current of a Power Antenna DC motor

Joined Feb 11, 2007
5
Hello.
I'm building a robot and would like to drive it using 2 of these DC motors.

It's the power antenna of a Volvo wagon.
It works beautifully, and there is a big gear inside that spins at 38.5rpms at 9V. It has HUGE torque, you cant even stop it with your finger.
Anyways, I measured the resistance of it and it came out to 1.68Ohms. What stumps me is while I had it running off the wall powersupply, it displayed that it was only drawing .3A. Whats going on? Shouldnt it draw 9V/1.68ohm = 5.36A???
I need to know how much current it draws so I can start making a battery pack... Whats going on?

#### beenthere

Joined Apr 20, 2004
15,819
It might try to draw a few amps if stalled. When running, motors develop a "back emf" that limits their current draw. That's why stalled motors burn up.

Joined Feb 11, 2007
5
So when designing my battery pack...should I design it for 8A or less than 3A?
Back EMF...hmm I remember I need to put a Zener Diode in there or something.
Should I design my battery pack for 8A then?

#### beenthere

Joined Apr 20, 2004
15,819
You could just fuse the motors to prevent excess stall current (I have no idea of your application) or design in some active current limiting.

The more current your batteries can supplu, the longer your robot can run.

Joined Feb 11, 2007
5
You could just fuse the motors to prevent excess stall current (I have no idea of your application) or design in some active current limiting.

The more current your batteries can supplu, the longer your robot can run.
I'm building a robot that needs to find a line in a 4'x6' box. It is to start in any arbitrary corner. There are 3 notches in the line and each notch corresponds to a hole at some distance away from the line/notch. The bot is to shoot at that point, a pingpong ball into the hole. It must do all this in 2 mins or less.

I'm sorry I'm a newb....what do you mean "fuse" the motor? Active current limiting...is that like an op-amp buffer circuit?

#### mrmeval

Joined Jun 30, 2006
833
fuse is a glass encased piece of wire that opens when the current gets too high.

I'll just fork out the  and buy the servo's my team mattes want.