hi
i have unipolar stepper motors from bigger floppy drives. they are rated at 12 volts and 200 ma. can i get higher torque from them in some way, other than increasing voltage? i mean is chopping possible? what exactly does it do?
thanx

 

The easiest way is to feed them off a higher voltage via wirewound current limiting resistors, this increases both speed and torque.

To avoid the resistors, but make it more complicated, you can use PWM to restrict the current rather than the resistors - the 'chopping' you mentioned.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

how will resistors help? wouldnt the voltage difference across the motor be same?
thanx for your reply anyways.

 

Try searching for "stepper motor tutorial", you will find plenty of info about it - but essentially the higher voltage allows the full current to be reached MUCH faster.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

Chopping is a fairly straightforward concept. It works by sensing the current through the coils and turning off (chopping) the voltage when the current rises past a preset limit. You can construct your own with comparators and sense resistors. There are a number chopping unipolar stepper ICs out there. Allegro has several.

Using current limiting resistors can get expensive though a 200 mA stepper probably means something in the 2W range. Not super pricey. a cheap 10% 2 Watt will run about a dollar. You will need 2 and will have to calculate the value based on the coil resistance of your stepper and the driving voltage. If your motor was in the the 1A range, I'd say get a chopper as the resistors will cost more than the chopping IC.

 

my steppers are 160 mA per phase..
what are sense resistors? i could not get this part... please elaborate on that.
actually i am using batteries(12 ni metal hydride batteries) as my power source. the motors i am using are from the older floppy drives. the motors are already 12 volt(and they apply very meager torque at this voltage) and applying 15 volt to them wont make much difference. should i step up this voltage in some way? but that would lead to power wastage too.. please ask me more details if you need any but suggest me some solution that i could implement. i am tinkering myself too but maybe you all could tell something better. thanx for your quick responses till now.
thanx

 

I think you are trying to get blood out of a stone. Those floppy motors are pretty weak. I'd say get a stronger motor. maybe a NEMA17 class motor.

a sense resistor is a very low value resistor. In series between ground and the load, it will develop a voltage drop that is proportional to the current. For example a 1 Ohm resistor with 1A through it will develop 1V (ohms law). 500mA through it will develop .5V and so on. You can use that to feed a comparator or an ADC.

 

any suggestions from where i can get some good motors, like some old equipment or something, we dont get gud motors here and have to import them online, which is quite a costl affair...

 

You don't give any clue as to what you're trying to do with them?, so how can we make suggestions?. As I said before though, you need to feed them from a much higher voltage (48V would be reasonable) with current limiting.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

i am trying to build a micromouse. so i dont have much option of using power source of higher voltage..

 

Don't use steppers then! - or use low voltage steppers, where 12V is high enough for decent current limiting.

I thought MicroMouse was essentially a race?, using steppers is the MicroMouse equivalent of starting on crutches for a marathon!.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

any suggestions from where i can get some good motors, like some old equipment or something, we dont get gud motors here and have to import them online, which is quite a costly affair...

 

5-1/4 inch floppy drives, printers, photo-copiers - those are usually the most useful.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

but i already have them from 5-1/4 inch floppy drives. they are not powerful enough...

 

can you give me some indications on how to spot good steppers, as i might have to buy them from some repaire shop and checking wont be an option...