24v Stepper at 12v?

[McFly007]

Hello all,

Stay with me here...

A few friends and myself have decided to build a motion-controlled time lapse rig. After consulting a different mate he pointed me to PICAXE.

So I've been mucking about with a few PICAXE 08Ms for about a week. They seem to be quite capable little chips.

So far I've got the 08M to fire the camera shutter, and tonight I've got it powering a little stepper motor I bought from Oatley Electronics (they're in Sydney, but I'm in Melbourne, Aus). My haphazard trial and error approach to design has resulted in me purchasing things as I feel they are required.

In getting this far I've realised that the motor I've got requires 24V DC. It gets pretty warm humming along at 12V DC. The final rig will be running at 12V DC.

So now to my question - is there a way to increase the voltage to 24V DC from 12V DC? Or should I invest in some new stepper motors?

 

[eclectic]

Welcome to the forum..

Is it this one...?

http://secure.oatleyelectronics.com//product_info.php?products_id=591&osCsid=4351029406a4c7aa35d9fb531e5d3643

http://www.eminebea.com/content/html/en/motor_list/pm_motor/pdf/pm35s048.pdf

Could you post some more details as to
how it's wired and to the programming?
Then others may be able to give more advice.

Bottom line is, if it has to be replaced,
well it's only $7.

e

 

[Andrew Cowan]

Swap the 12V power supply for a 24V one.

OR

Add another 12V battery in series with the existing 12V battery.

No other (simple) way, sadly. It will be cheaper to replace the stepper.

A

 

[gengis]

There's a reasonably easy way to double voltage using a LM555 timer IC. Basically the 555 runs as a square wave generator at some relatively high frequency then is applied as a charge pump to add the input supply voltage to itself.

It is good for about 100 milliamps or so. If you needed more than that you'd have to beef up the output stage of the 555. For 24 volts out at 100 milliamps you need to put in 200+ milliamps at 12 volts.

http://www.elecfree.com/electronic/voltage-doubler-with-ic-555/

 

[McFly007]

Hello folks, thanks for your responses.

Eclectic; that is indeed the motor. I'm running it as a unipolar stepper with a ULN2803A driver IC powered from an old 12V model railway power supply. The PICAXE is powered from a separate 5V plugpack but I've got them grounded together. The PICAXE program is basically the example stepper.bas that comes with the editor, modified a touch to run on an 08M.

Although the data sheet gives the coil resistance as 50 ohm, I'm reading it as 135 ohm from coil to ground, 270 across both coils. I'm unsure why this differs from the sheet.

Andrew; I have considered using two batteries, but given I'd like to run the whole rig from a standard 12V I don't think this will be convenient. We may be taking the rig into bush or up mountains, and don't want to carry more batteries than required.

Flooby; that is an interesting proposition - although I think 100 mA won't be enough to drive the motor with two coils live.

I've been looking around for some 12V steppers as the best solution, but so far all I've found is a box of used 6V steppers at Truscotts in Croydon. I was thinking of perhaps running these inline with a resistor from the 12V supply, or maybe they can run at 12V?

However, the rig needs to be reasonably efficient, as it needs to be capable of continuous operation for 12 hours at a time drawing from a battery supply (at night). So I guess the resisitor option will be drawing more current, and therefore using more power.

 

[westaust55]

That old 12V model railway power supply may have quite a bit of ripple (ac voltage superimposed on the dc) at the output which does nothing useful and could be the culprit in warming up the stepper motor.

Stepper motors are a little different to "normal" motors. From my past review of datasheets they have much higher resistance and lower inductance.

My initial, but untested, theory is that as the torque available is proportion to the current in the motor windings, and with a stepper motor having high resistance so that, current is fairly proportional to voltage, so the torque may be proportional to the voltage. Thus running the stepper motor at half rated voltage would result in half the available torque to turn the shaft (against a load).

The PM35S datasheet indicates 50 Ohms, whereas the Oatley Electroncis site indicates 70 Ohms. My guess is they buy in a batch of stepper motors and sell whatever they get supplied as being fairly nominal.

At the risk of incurring comment from certain others here, have a look on Ebay.
While I have purchased a 7.5degree/step 12V stepper motor from Jaycar in preparation for some experiements, I also purchased a secondhand heavy duty 1.8degree/step 3V 1.7Amp stepper from Ebay on teh grounds that I could run if from a couple of D-cell batteries if desired.
There are many second hand motors out there (on Eek bay) taken from printers and other equipment at assorted voltages and step increments from which to make a selection.