Society of Robots - Robot Forum

Electronics => Electronics => Topic started by: 28 on February 04, 2011, 11:47:34 AM

Title: Motor does not work right
Post by: 28 on February 04, 2011, 11:47:34 AM

I use this motor
http://www.pololu.com/catalog/product/180 (http://www.pololu.com/catalog/product/180)

And it works fine when I connect it directly to my alkaline 9V battery. However, when I connect it in series with the battery and a 330 ohm resistor, it fails. Also it goes from 64mA without resistor to 25 mA with resistor.

I can't use it in my robot because the circuit adds resistance and it adds more than 330 Ohm.

So why does one 330 ohm resistor screw it? What am I doing wrong?

Edit: I'm trying to build Sandwich, maybe someone knows him. =p

Title: Re: Motor does not work right
Post by: waltr on February 04, 2011, 12:09:35 PM

See the motor spec:
# No-load current: ~ 40 mA

This is the minimum current the motor requires and does require a higher current to get started.

330 Ohm and 9 V limits current to 27mA by Ohm's Law.

27mA is less than 40mA so motor no go!

Can you post a full schematic of what you are trying then we may be able to offer suggestions.

Title: Re: Motor does not work right
Post by: Soeren on February 04, 2011, 01:05:20 PM

Hi,

Looking at some of the parameters of the motor:
Free-run current @ 6V: 50 mA
Stall current @ 6V:    710 mA
and
Free-run current @ 3V: 40 mA
Stall current @ 3V:    400 mA

This means that average current draw under load should be around 380mA on 6V and 220mA on 3V (ideal).

Calculating for this...
The motors start up resistance is probably around 8 Ohm (+/- 0.5 Ohm).
A 330 Ohm resistor in series means you make a voltage divider which (at the instant you apply the voltage) will be:  9 * 8 / 338 = 213 mV  (0.213V), which won't make it go of course.

A more suitable resistor  would be close to 8 Ohm (2W) for 6V to the motor, as the, motor will look more like 16 Ohm when drawing 380mA (peak efficiency).


You won't get it running like you expect on a 9V PP3 battery, as its internal resistance is too high - 4 AA Alkaline cells in series will give you 6V and then you don't need to add any resistor.

Title: Re: Motor does not work right
Post by: billhowl on February 05, 2011, 12:05:56 AM

Here is the circuit wiring diagram for Sandwich, the line-following robot.
http://www.robotroom.com/SandwichPCB.html (http://www.robotroom.com/SandwichPCB.html)

Where did the resistance 330 ohms in the circuit come from?

Title: Re: Motor does not work right
Post by: 28 on February 05, 2011, 03:35:00 PM

I used three of them as 1K since I didn't posses a 1K-resistor. I also made other modifications like that.

But thanks for the help everyone I got it working .