Calculating resistance for driving motors off Arduino

Question

I'm planning on driving two motors off an Arduino. Here are the two motors I'm using, requiring 3VDC and 350mA. It was recommended I use this H-Bridge to control the two of them, capable of driving 4.5V-36V at 1 amp.

Considering the power output from the H-bridge is substantially higher than required by the motors, I'm assuming I'll need some resistors in the sequence

Arduino >> H-Bridge >> Resistor >> Motor

I used Ohms's law to calculate I'll need one \$\dfrac{V}{I} = \dfrac{3 V}{0.35 mA} = 9 \Omega \$ 1 watt resistor in conjunction with the power in on each of the motors.

Is this right? I'm a software engineer so I'm not very knowledgeable in this area.

Answer 1

I don't know where the 350mA comes from but the motors can take over 1 amp on load: -

Specifications

voltage:
    operating range: 1.5-3V
    nominal: 3V 
no load:
    speed: 12511rpm
    current: 0.29A 
at max. efficiency:
    speed: 10012rpm
    current: 1.16A
    torque: 15.7gcm
    output: 1.61W
    eff.: 42.36%
    stall torque: 78.4gcm 

On no-load the supplier states 290 mA but at max efficiency (somewhere about full load) the current is 1.16 amps.

This rules out using the SN754410. From a 5V motor supply, it will produce about 2V for the motor (due to gross inefficiencies in the transistor output stages) and, it will get too hot too quickly and fry. Any higher supply voltage and the device fries more quickly.

If you look at the full data sheet on page 4 you'll see that the absolute maximum power that is permissible to dissipate from the SN754410 is 2.075 watts. Given that the motor can take 1.16 A at or near full-load and the volt drops in the chip will be somewhere about 3V, the power dissipation will be nearly 3.5 watts just for one motor.

Note also other areas of the motor spec: The current of 1.16 A is at a torque of 15.7 gcm whereas the stall torque is 78.4 gcm - this means that if the motor stalled it is likely to demand a current of over 5 amps.

Here is a related question on the SN754410 that shows where on the data sheet the internal volt-drops come from. There are also recommendations for alternatives BUT you do need to state/know your motor supply voltage.

Answer 2

Power is not a problem, since the H-bridge won't force the motors to actually use all that it is capable of putting out. It can put out up to 1 Ampere of current, but the motor doesn't have to draw that much.

What might be a problem is that the H-Bridge can only work with a power supply starting at 4.5Volts for the motors. However, if you look at the specs, you will see that it only delivers about 3Volts of the 4.5 to the motor - so, you are good to go if you can provide 4.5Volts to the VCC2 input (pin 8) of that chip. If you use 5Volts, you will probably be OK.

Use the H-Bridge as is, no additional resistors in series with the motors. Provide 5V (or better, 4.5) to Pin 8 from a supply that can deliver enough current for two motors. Avoid powering the H-Bridge from the same supply as your Arduino, or provide plenty of bypassing (capacitors on the 5Volt supply to the Arduino) to filter out the noise.

---

I see Andy aka checked the specs on the motors. Yikes! That is too much current for that H-Bridge, so don't use that one. Sparkfun also supplies chips that can handle upto 4A, as well as finished modules using those chips.