Has anyone run any of the GM motors (GM3 GM9 etc) off of a 9v battery? I was thinking that after the 1.5v drop that the motor driver takes, the motor will only be getting 7.5v. --That’s only a little tiny bit above the 6v specs. Anyone tried this? Or does anyone have a idea of how bigga resistor to use to drop that last volt and a half? --Any thoughts you got, folks.
Well, using one resistor
Well, using one resistor will not work. One resistor will drop the current, but not the voltage. You need to make a resistive voltage divider. That’s a fancy way of saying “put two resistors in series between the battery terminals and create a voltage drop”. A more precise way of putting it is…
V = (R1/(R1+R2))*battery voltage
for example - you have 2 resistors in series between v+_ and G on a 9V battery… the resistor values are R1=1K ohm and R2=200 ohm
V=(1000/(1000+200))*9v
voltage across R1 is 7.5V and across R2 is 1.5V
Clear as mud?
What kinda 9 volt?
Not sure a 9 volt will last very long with the GM motors from Solarbotics, GM3, etc. They spec the GM3 motors at 62 mA free run at 6 volts, 45 mA at 3 volts. This progresses fairly linearly up to the 7.5 v or so you expect out of the h-bridge, so you might see 72 mA draw with the higher voltage. Apprently from the above data at Polulu, it will shorten the life a lot. What might cause your motors to heat up and burn out too quickly is the stall current, 389 mA at 3 volts, 733 at 6 volts. At 7.5 volts, 900 mA or so might be going through the motor, if you have the power source that can deliver it. A little alkaline 9 volt might not be able to deliver this higher current to heat the motor too much. So a little 9 v might actually lessen the damage. I guess a lot depends on the robot. If you are talking the 9.6 volt NiCad or NiMh rechargables, then they will probably deliver all the current needed to burn the motors in use. I had a minisumo robot that used the Tamiya twin gearbox, and tried to run it off 7.2 volt, then a 6 volt pack. The h-bridges used an Allegro A3951, which could lose around 2.8 volts driving the motor. Even at the lower voltage, 6 volts down to 3.2, the robot still would burn out a motor after a few matches. In minisumo there is a lot of running at stall to consider though.
Here’s an app that will help
Here’s an app that will help you out - you need to enter 3 arguments to complete a calculation.
http://ourworld.compuserve.com/homepages/Bill_Bowden/r2.htm
this one is similar, but calculates for load as well -
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/voldiv.html
Wow, thanks everyone!
Wow, thanks everyone!
I think with all this new info, I’ll just stick to the 3v I am feeding it now. I’m not liking all these pictures of burned out motors! Maybe I’ll just switch to some 143:1 motors instead of the 224:1 motors I am using now. The robot is not too heavy so I am sure I can trade the torque for some speed and still run with the smaller voltage.
–Oh and wicked thanks Casca for the link… That resistor calculator will be very helpful in the future for a lot of stuff!
I love this site!!
www.rocketbrandcustom.com baby!!
Welcome - have a look here
Welcome - have a look here as well…
Motor differences
One thing I saw in another thread somewhere here on LMR, was that the motors used on the Solarbotics GM3 etc are a slightly different type than those used on the Tamiya gearboxes. The Tamiya ones are Mabuchi FA-130 that are meant to be driven at 3 volts, and have some scary stall current rating as well as being pretty noisy. The motors on the Solarbotics gearboxes are supposed to be RM3 original replacement
http://www.pololu.com/catalog/product/604
or RM2 for higher torque
http://www.pololu.com/catalog/product/611
made for 6 volt use. Just both have the same case size and style, but apparently different windings.
GM7
Could I run a GM7 off 9V, as my robot project requires an extra small battery. I don’t the 6V is a rating, just a example voltage, because I mean how can you burn out a motor? It’s just a big coil of wires. But if it is a rating, would some PWM help prevent any burn-outs? And if that won’t work, can anyone suggest a battery smaller or the same size as a 9V, and also provides less voltage.
As far as how higher voltage
As far as how higher voltage affects a motor, see the link that Frits posted above. Nice empirical data with pictures and graphs to show you exactly what happens when you run motors higher than their rated voltage. You can decide at which point the shortened life is no longer worth it, but there seems to be a pretty quick dropoff, at least with the FA-130’s. The RM3’s are rated at a higher voltage so I don’t think going up to 9v will be quite as bad, but there’s only one way to find out 
Just remember one simple way to drop a few volts is to stick a few simple 1N4004 diodes in series – each one drops about 0.7v, so I’d use about 3 in a row to bring 9v down to about 6-7v.
Dan
Zener Diodes?
Thanks very much guys, but could I just hook the motors to the same voltage regulator as my microcontroller? That way I can save a little space. Also, would Zener diodes be an option?