View Full Version : help with stepper motors

06-11-2008, 12:41 PM
i am using a 12-0-12 transformer, half wave rectifier, 7824, and uln2803 for driving two steppers ( single power supply for both the steppers)....... i think everything here can handle upto 1A and uln upto 500mA per pin.... am i right????

i am using sequence 1100,0110...... for driving. so at every instance 4 pins of uln are active.... now could it provide 500 - 500 mA on four pins at the same time ????

another confusion: my calculations are
voltage- 12v
phase resistance- 75ohm
so current per phase - 160mA
if i am using above sequence will the single stepper draw double current??? i.e. 320mA for 1 stepper ,, means 640mA for two steppers???? can u help me out????

06-11-2008, 01:35 PM
I haven't done steppers at all so can't help you there. However, my suggestion is regarding the power supply... Mobile phone shops stock a huge variety of SMPS adapters these days. I just got myself a fake LG replacement adapter that is 12V @ 1A.

The shop also had some heavier rating SMPS for laptops/printers (24V @ 1A/1.8A/2A etc.).

If you feel that a linear power supply like yours hasn't enough juice, then you could try one of these.

06-12-2008, 06:50 PM
Remove the 7824 if it is in path of supply to motor.
It is limiting current to motor.

if its 78L24 --> 100mA
78M24 --> 500mA
78H24 --> 1A
All these are "theoretical" max limits.
(actual could to lower when the ic gets hot.)

you dont need a "precise" 24V regulated by an IC to motor.
unregulated but well rectified and with WELL Filtered witha Huge capacitor .
(1000uF min, i recommend 2200uF) is required.

if it still doesnt have enough torque after removing 7824, then increase current rating of transformer.

Good luck

06-21-2008, 07:29 PM
thanks ..... i will keep ur suggestion as my last weapon

i checked the datasheets and its true what u said..... L78xxC is making problem..... but i have a doubt..... when i am drawing more current (almost 3 times) than rated , my ic should boil.... but its at normal temp..... is there any current limier inside it????? i cant figure it out in block diagram...........

i am getting similar problem with my sensors.... they are working fine separately ( upto 9 cm wall detection) but when i joined it with uc its range decreased to 3cm.......

well , even now with low torque and reduced range my micromouse is running fine after some adjustments but still i would like to know the reason......

also can u suggest me any replacement for 78xx..... i have heard of something "MAX"....... will it be better?????(my total current requirement is about 500mA)

06-21-2008, 09:39 PM
i checked out datasheets of max220-249........ i think it is meant for some other work and not for voltage regulation..... i think i misunderstood someone saying about it.........

well, i also tried to search 78xx series of 'M' or 'H' type but they r not available...... so how can i make a high current power supply....... OK for motor i do not need that much regulated supply so i can omit 78xx. but what to do for my sensors and microcontroller??????

06-22-2008, 09:49 AM
Motors - As Vivek said, you don't need to be using a regulated supply. Infact in most cases (esp. with linear regulators like 78xx) you shouldn't. Current limit and some other issues being the reason. Instead feed the motors voltage directly from the batteries. Use a high value capacitor, as Vivek suggested. These act as charge reservoirs during peak current surges and avoid resetting the circuits.

Generally, most electronics circuitry will need 5volts for operation. You should use a 7805 in this case. This has a theoretical maximum current limit of 1A and needs atleast 2.5-3volts more than 5volts at the input side.

If you need to draw more current than 750mA you should seriously think about using switching regulators. Search for switching voltage regulators. You can look for them at www.ti.com or www.maxim-ic.com or just use Google. While on the search, do read a few documentations on these sites about buck-boost (step-up/step-down) modules.

If you can't afford to have the input voltage more than 7.5-8volts, you just think about using low drop-out voltage regulators in this case. MAX603 is one such example.

Once again, sensors and all electronics should ALWAYS have regulated supply. Motors, in most cases, should be given the voltage directly from the battery (however, do check the current limit of the motor coils before doing so) and use a high value capacitor in your power supply.

06-22-2008, 07:17 PM
thanks ppppking04

i think max603 will do the job...... i will check it tomorrow and will post the results....... thank again to all.....