View Full Version : Stepper Motor help

03-21-2008, 09:43 AM
hi friends..
i am very new to robot making, rite now i am working on a general purpose robot driven by stepper motors.I am using a PIC 16f877a as the uC and uln 2003 as driving circuit. I am able to run everything with a 12v adapter with 1A current.But my problem is arises when i come down to the battery. I am using a 12 V lead acid battery with 1.3 AH rating. This battery provides too much current for the uln 2003 and they burn out. So basically i need help for limiting this current.I thought of using a 7810 this would automatically limit the current to 1 A or i could use another circuit but i dont know which would be a good solution. So please could someone help me with a solution, looking forward to it.thanks in advance..

03-21-2008, 09:47 AM
What are the ratings /specs of your stepper motor?

Also are you using any other form of current limiting?

refer http://www.cs.uiowa.edu/~jones/step/current.html

03-21-2008, 10:12 AM
I am not a 100% sure of the rating. cause they arnt wriiten on the motor, its a shinano make model STP-42D162 1.8 deg step angle. and as per the current , i am just using an 7805 to power the pic circuit , n that has been working fine

03-21-2008, 12:21 PM
The measured resistance / phase is =30 ohm n these motors for those resistances are made for= 12 v so the current/phase is =0.4A.
Please help out. Have no clue as to which circuit to use for limiting current

03-21-2008, 12:33 PM
ULN might be burning out because you might be 'Holding' the motor for some time. PWM might be the solution for you. or,

1) try reducing the voltage, at the trade off of torque.
2) Measure the current consumed. - If the current is a bit too high, (than ULN can handle ) switch to transistors.
3) Lower current = lower torque, so decide on your torque requirements.

03-21-2008, 01:46 PM
Iam not sure as to what u mean by holding the motor on for too long.As per my torque requirement i used an adapter 12v,1A and powered both my stepper using that, below that the torque was not enough for the bot to move.
The problem now is that lead acid battery can provide all the current as per load, so basically i think the stepper's are extracting more current than the uln2003 can handle which is burning it out(burn within seconds of providing voltage). Also i am confused, my motor resistance in each coil is 30ohm,so basically it should draw only up to .4A this should not burn the uln2003. please help

03-21-2008, 04:46 PM
am i getting it wrong?

pwm with steppers??!!

03-21-2008, 07:00 PM
If ULN keeps on burning, it simply means, more current is passing through it.

I repeat, measure the current using a DMM. It has to be more than ULN can handle.

or there has to be something wrong in the circuit. can you post the schematic?

And why not PWM with steppers?? You switch the current ON and OFF, to limit the current. And you will have to do this if you need more toque out of your motors. Remember - the 'pulsed' rating will be always greater than the 'continuous' rating.

Anyway, I wont personally suggest ULN for anything close to .5A. Use L297-L298, or transistors/MOSFETS. My personal fav being TIP122-127.



ps: try to avoid steppers when you need good torque!!

03-22-2008, 09:15 AM
Sir the circuit works when i give 12v 1A supply, what i will do is give the motor 12v From that battery ,it should sink in all the current it needs,Ill measure this current this should give me the current the stepper is sinking/phase, rite. Besides this,how would it be if i regulated the current using an op amp.what type of resistance i should use in this case, i don't want them burning out.