Stall current is the current (amperage) that the motor will draw when you prevent the motor from spinning. For brushed DC motors the stall condition if often the condition where the motor consumes the most current. This is your worst case for picking a motor controller.
Since Voltage, current, and resistance follow Ohm’s Law (V=IR), you can measure two and compute the third. Since you do not have the right power supply, then measuring the resistance with an appropriate ohm meter will give you a good idea what the resistance will be when the motor is stalled. It will not be perfect because with lots of juice flowing through the motor, the resistance will change as the wire heats up. But it would get us in the right ball park.
Trying to measure the current with a 9V battery will be a waste of time. The poor little thing just does not have the ability to source the current those motors will take. As the battery tries to source the current and can’t the voltage will start to drop. It would be like trying to measure the water flow through the Colorado river by an analogy to a soda straw. Not worth the time.
If you can measure the resistance on the motor, then we can use V/R to get an idea what the stall current is. The V will be 7.2 if you buy that RS battery.
so for now i should just check the resistance without any power going through it
Correct. Use your multimeter probes across the motor terminals with the meter set to ohms. If you have multiple ohm scales set for the lowest range. I suspect the resistance will be less than 100 ohms. If this is an analog meter, set the zero level on the ohm scale before taking a reading. Take several readings with the output shaft at different positions. Some positions may show near infinite resistance as the commutator is open.
Then take 7.2 and divide by the lowest reading in ohms you got. That should be approximately your stall current in amps.
I got 2.3 ohms at the low end but to be safe what is the normal amperage for an rc car and i will test it again in a bit to see what i end up getting with a set of rechargeable AA batteries
Again, using AA batteries will not prove anything. Even if you got enough AA is series to create the 7.2 volts, the AA has different chemistry with a much higher internal resistance. It would be like taking your RS battery and adding in an extra resistor in the circuit.
At 7.2V and 2.3ohms gives you 3.1 amps. If you stall the motor that is what it will suck down. Now given that a ‘standard’ one amp motor controller is a bit anemic. But you could use one that had thermal protection that would shutdown if it got too hot. Tomorrow I will look at the SFE offerings to see what makes sense for you.
Thanks so much and do u work for sparkfun?
Your are welcome and no I do not work for SFE. In fact very few posts on these forums are answered by SFE. This in truly a community lead Forum.
I found the arduino motor shield which seems to have a high current rating (5 amps with external power supply) and it might work but could it hurt the motors being that the current from the board could be 5A
It also has a voltage range within the needed parameters
Here is a link to the board
http://arduino.cc/en/Main/ArduinoMotorShieldR3
also their are many tamaxx batteries and chargers do u think they could work I’ve found them almost everywhere
If uou find anything else it would be helpfull
A controller than can handle 5 amps does not mean it will source more than what your motor can handle. In this circuit, the motor is the consumer. The battery is the source. The controller is a traffic cop. If the cop can handle 5 amps, but the motor only wants 3, then 3 it gets. The only way to have to motor consume more current is to increase the voltage or decrease the internal resistance.
At 7.2V, it was never going to be the case that your motors were going to need an external power supply. The dinky regulator on an Arduino can’t push 1 amp let alone twice 3.1 amps. Also the voltage would be wrong.
The link you provided is for a controller that will source 2 amps to each of two motors. What I did not see was any mention of thermal protection. We would have to look at the chip’s datasheet to determine that.
Not familiar with the tamaxx batteries. Could you provide a link?
Need to leave the house now, but I will look for a suitable controller for you later today unles someone elase would care to make a recommendation?
Spec: 7.2V, Max 3.1Amps, two channel, Fwd/Bkg, PWM, Arduino 5V capable,
Possibilities:
http://www.sparkfun.com/products/9670
Only 2A, but if memory serves me correct, the ship has a thermal shutdown if it gets too hot. Not a shield, but easily conectable to an Arduino. Max volts is well above 7.2V
http://www.sparkfun.com/products/11080
Peak current of 5A with a sustained of 2.5A. Volts to 28. Not a shield and only one channel so you would need two.
http://www.sparkfun.com/products/9107
Current to 15A, Voltage well above 7.2. Not a shield and only a single channel.
http://www.sparkfun.com/products/9815
Shield, uses an L298H 2A steady state up to 3A peak. Does not seem to have thermal shutdown and the shield has not implemented the current sense you could use to shutdown the drive if you went over current.
From the above and having a general idea of your skill level, I would consider the Shield just making very sure you do not stall the motor. You could add in a small power resistor to limit the current if you wanted to be safe.
Pololu (and others) may have other choices that might better meet your needs.
IThe Arduino Motor Shield is based on the L298 (datasheet)
As stated on the arduino website
This is the same chip u mentioned multiple times above and this shield (link in past post) seems to have
A higher max current would it work better
How big of a power resistor and also the arduino sheild i found allows me to measure the current through the analog channels would that make it a better option
Can you give a link to the specific controller you were considering?
As for the resistor, it is fairly easy to compute. Your battery is 7.2 volts and the motor is 2.3 ohms. 7.2/2.3 is 3.1 amps. But saw the controller could only handle 1.5 amps. Then the total resistance in the circuit would be 7.2/1.5 = 4.8 ohms. The motor is 2.3 leaving you 2.5 ohms for a power resistor. Now this resistor can’t be a 1/4 watt Radio Shack resistor. That will vaporize with the 1.5 amps running through it. You need to size the resistor for power. Power is voltage times current. In your case 2.5 * 1.5 or 3.75 watts. I would go to DigiKey and look for a resistor as close to 2.5 ohms as possible and at least 4 watts. Now adding that resistor will mean that some of the energy will be lost as heat and not turned into mechanical energy. You would be best finding a controller than can handle your 3 amps.
Are you in for a learning experience and making your own controller from parts? A couple of good FETS would easily handle 3 amps. Arranging four per motor in an H-Bridge format would give you forward/backwards/coast/brake and speed control.
How hard would it be could you provide some reading material and some possible schematics
I was thinking of more of a online resource or tutorial
here is the other one that i was looking at http://arduino.cc/en/Main/ArduinoMotorShieldR3
with its current sensing ability i could program it to shutdown when it got near its max current
I might even be able to add a small heat sink to it
Ive got another option that i think might work which was brought to my attention earlier by another electronics enthusiast but it is a bit more pricy and im on a budget and still have to find a battery and charger
what do you guys think about it
do you have any ideas about batteries
inkspell4:
do you have any ideas about batteries
I have lots of ideas about batteries. Like I wish they were free considering how many I use in a given month.
What is the frame of reference for this questions? I though you had already picked the RS one.
No im still looking around for the best deal ive seen some traxxas one on ebay that are relatively inexpensive did u get time to look at the link i provided above and do you have any online resources for making an h bridge from mosfets that i could take a look at as another option
So to specify a battery you need to know the average current draw and how long you want the battery to last. For your motor, I will guess each motor will draw about an amp for a total of two amps. Now if you want to run for an hour, you need a 2amp-hour battery. If you want to run for 3 hours, you would need a 6amp-hour battery. If you want to run for 15 minutes you only need (0.25*2) 500maH battery.
As for H bridge references, there are a million on the web. I do not have any in particular that I like. You should read a dozen and start to pick up on common threads.