A problem with Stepper Motors

[ibrahim03]

I am driving two stepper motors from the parallel port of the pc. The ratings of the two motors are 10V and 3A each. The computer sends pulses to FETs(z44n) which in turn drive the motor.

To give power to the motors i was using a computer power supply from a very old computer (386 or a 286 which i got from scrap). The rating of the power supply was 12V 8A and 5V 20A. Now I know that ideally this power supply should be giving 12V output but somehow it was giving 10.3V so I connected it directly with my motors without using any kind of converter(to convert from 12V to 10V). The motors ran fine for about 5 minutes but after that the power supply burned down (there was a lot of smoke and I turned it off immediately). After that I bought another old computer power supply of the same ratings. Coincidentally this power supply too was giving an output of 10.55V instead of 12V. So again I connected it directly without any converter and again the same thing happened, it ran fine for sometime and then the power supply burned down again.

Can anyone tell me what the problem is? Only the power supply burns.

 

[melc]

The answer is easy. PC power supplies are designed to work with complete load on all supply outputs. If you want to see 12V on the +12V output you *must* have at least 2A load on 5V supply.

A 3A stepper could sink up to 10A spikes. If your supply hasn't enough capacity filters on the output could be burned. Take another PC supply and load the 5V output with 2 ohm/20W resistor. Then supply your motor, use a 4A fuse in series with your V+ and try again. If the fuse will burn, your stepper command, or wiring is wrong (or you have a heavy mechanical load).
Use at least 470uF near every motor V+, as close as possible to the motor. Use suppressing diodes on every motor coil as closed as possible to the stepper.

Think twice and test once.

 

[Sal]

Yeah

I see two problems 1) You are using 12V instead of 10V which is the nominal voltage for your motors, remember that a multimeter gives you an average and your power supply is given about 4.2A to the motor, if the power supply doesnt burn, you will burn your motor anyway. Then you need to reduce this voltage, you can even drive the motors with 5V if it gives you the required torque. 2)The current transient currents may be overpassing the limits of your power source.

good luck

Sal

 

[ibrahim03]

The answer is easy. PC power supplies are designed to work with complete load on all supply outputs. If you want to see 12V on the +12V output you *must* have at least 2A load on 5V supply.

A 3A stepper could sink up to 10A spikes. If your supply hasn't enough capacity filters on the output could be burned. Take another PC supply and load the 5V output with 2 ohm/20W resistor. Then supply your motor, use a 4A fuse in series with your V+ and try again. If the fuse will burn, your stepper command, or wiring is wrong (or you have a heavy mechanical load).
Use at least 470uF near every motor V+, as close as possible to the motor. Use suppressing diodes on every motor coil as closed as possible to the stepper.

Think twice and test once.

Thanks for the detailed reply melc. I will surely try your suggestions but before I do I have a few more questions.
First, as I said that the supply is giving an output of about 10V instead of 12V, I am using that fact to my advantage as the ratings of both the motors are 10V and not 12V.(Running them at 12V might damage them as Sal suggested) Is it allright to do that?

And the 470 uF capacitor is for power factor improvement, right?
Also which suppressing diodes should I use? will 1N4007 work fine?

 

[melc]

One: a DC motor could work with large voltage dfference. For example a 10V stepper (where did you find such nonstandard stepper?) could work ok at 5V or at 12 with no problem as long the supply will deliver the requested current and the load will not be too big.

Two: 470uF is creating a low impedance point near the stepper, avoiding all spikes going away into driving circuit.

Three: at 4.2A load (if it's true, but I have real doubts, how big is your stepper -measured in cm?) 1N4007 could be too small as current suppressed, choose a 3A diode.

 

[manish12]

What is the resistant of one winding ?
It is better to go for emitter driver instead of collector driver.

 

[ibrahim03]

Ok my problem was solved ! (not as expected but nonetheless it works for me). I went back to the scrap store and found a power supply from a relatively newer computer. It was giving a full 12V on its output without any load on its outputs (is that "complete load on all outputs" thing only for older power supplies melc?).
I used 5V output because it was less then the rated 10v and my mechanical load was not large so there was no problem with that.

The new supply had some kind of built in protection against spikes. Whenever there were spikes, it turned off automatically. It was not physically possible to attach capacitors close to the motors so I attached the 470 uf capacitor at the output of the supply. But the capacitor was very large so when the motor was started it drew a very large current and turned off the supply. I tried to run it without any capacitor but even then the supply turned off due to spikes from the motor. So I tried a 100uf capacitor and it Worked !!! .The motors are running fine now without any problem.