Can anyone advise on what sort of current limiting to use? The post below, dealing with a similar but not identical motor, seems to use a much more complex method to run the motor. As I am very new to using these motors I'm not sure if I'm missing something http://forum.arduino.cc/index.php?topic=212653.0
Resistance of both coils is 21 Ohms, drops to around 13 as rotor is manually rotated.
My concern is that just connecting the 5v out from the Arduino to the motor voltage pin referenced in Igoe's notes on the L293D might be drawing too much current to handle. I have no real idea whether this is likely to be a problem with such small motors.
The steppers will still work, albeit with lower torque, without the 5v connected at all, running the L293 at 3.3v is enough for the motor to function. They seem to skitter about a bit more, am I right in thinking that the higher the voltage used in the stepper (to a safe limit) the more controlled the steps will be?
There are two separate issues.
Can you power supply (the Arduino 5v pin?) supply enough current.
Will 5v cause too much current to flow through the motor coils and damage the motor.
As far as I know there is a large voltage drop in the L293 and if so (check the datasheet) the full 5v or 3.3v will not be seen by the motor coils. In this case, that may be a good thing.
I have no idea what current those tiny motors can handle. The only way to be sure might be a destructive test.
Without knowing what current is actually flowing in the motor when the L293 is powered from 3.3v I can't answer your question about a higher voltage giving more control. That would be true if the low voltage just doesn't provide enough current. One way to test might me to see if the 3.3v works better at lower speeds. Generally stepper motors need a higher voltage for higher speed running (to overcome the inductance of the coils). That's why bigger stepper motors are driven by specialist driver boards - for example the Pololu A4988 - that have current limiting circuits.
BernardMarx:
I am more than happy to test the motors to destruction so that my final product works well.
Any pointers as to how to get the most from doing so?
Obviously if you connect (say) 30 volts to a motor and it goes "poof" you don't learn very much so you have to start from the other end - with low voltages.
I think I would write a sketch that puts a steady current through the motor using the L293 - i.e. current flowing but the motor not moving. Since the 3.3v supply seems not to release the smoke I would try that first and see (a) what voltage is across the motor coil (b) what voltage is across the L293 and (c) what current is flowing in the motor coil. It would also be useful to see if the motor is getting hot.
Then I think I would try the same arrangement with 5volts.
The critical information is the voltage across the motor coil and the current in the motor coil.
If the motor is not showing any distress - i.e. getting too hot - I would perhaps increase the voltage to 6v, 7v etc. Power from a bank of AA alkaline or NiMh batteries would be a simple way to increase the voltage in steps.
I don't know whether it applies to those small motors but bigger stepper motors seem to work normally at a temperature which is just about uncomfortable to touch.
From what I've been able to glean off websites, people seem to quote a range of 3-5V for this motor. Coil resistance of 20 Ohms gives 150-250mA current.
However I've been using Sparkfun's "Big Easy" and limiting the current to 150mA (bottom of its range) - and the motor runs very hot.
So I would be interested in the experiences of others - what kind of voltage/current has worked best?
I will probably be mucking about with these quite a lot since at 20p they are about as cheap as it gets
Stepper motors (as are nearly all electric motors) are thermally limited - try
driving via a L293D at 5V supply and see how hot the motor gets. If its too hot
to touch its probably too much, if its just hot that's likely perfectly normal. These
motors focus digital cameras and are intermittent duty rated, not continuous. Expect
about 100mA for a 20 ohm winding (I've some with 50 ohm windings and they
need more than 30mA - so I can't run them direct from Arduino pins alas (with
diodes of course).
I suspect 20 ohm windings are designed for 3.3V, 50 ohm for 5V, both give 0.5W
to the motor winding.
As you say, 5V seems too much - there's a burnout-about-to-happen smell once it's run for a bit and it gets too hot to touch pretty quickly.
Anyway I just accidentally put 12V through and that burnt it out pretty quickly. Time to get the soldering iron out for motor #2 of my 10
From reading around a bit more [ World’s Smallest Stepper Motor with Arduino and EasyDriver - Electronics Blog ] one way to go seems to be putting the EasyDriver power input on the 3.3V regulated level - make sense given what I've seen so far so I will try that (together with keeping EasyDriver current at the lowest 150mA level by adjsuting the trimpot).
Will be interesting to see how much torque I can get. I have a plan to have lots of small objects moving so having a stepper at a price point < 1 GBP is potentially very handy.
I've done some more testing (all 10 to destruction, got some more though )
They have pretty low torque at all voltages, though they probably won't need much for the sort of micro projects you would consider using these for.
Their biggest limitation (for me anyway) is their coarse step resolution, maybe only 8 steps for full 360 rotation.
This makes sense I suppose, given that they are from camera lenses originally; I'm guessing its for f stop control.
I might have a go with the other microsteppers available on ebay, the ones for controlling the laser position in cd/dvd etc drives might have finer control, but then given they all seem to use a screw drive too, I suspect they also have quite a low number of steps/rotation.
B