Questions about current limiting

[uplb]

Hi,
I have a very simple driver based on TIP122's darlington transistors ( http://www.freelabs.com/~whitis/soft...RIVER%20TIP120'). TIP122 has a max output of 5amps.

I will be using an 8amp (12v) power supply from a computer. I have three stepper motors rated at 3A(2.31V), 2.3A(1.98V) and 0.87A(1.7V).

My question is: Do I need current limiting using resistors?

Thanks!

[irving2008]

ummm, yes... otherwise the only thing limiting the current is the motor winding... and at 12v your 3A motor will be trying to take 15A! The 3A motor (2.31v) has a winding resistance of just 2.31/3 = 0.77ohm. Assuming that figure is for one half of one winding then you will need a current limiting resistor of (12 - 2.31)/3 = 3.2ohm, rated at (12 - 2.31) * 3 = 29W. A 3R3, 50W resistor is the nearest standard match. You will need two of them, one in each V+ line of the 3A motor.

For the 2.3A motor you will need (12-1.98)/2.3 = 4.3ohm and 23W (4R3, 50W) (2 parts)

For the 0.87A motor you will need (12-1.7)/.87 = 11.8ohm and 9W (12R, 15W)(2 parts)

Most of your power supply output is going into heating up the current limiting resistors which are not cheap either. This is why simple stepper controllers like this are only suitable for very low current motors. Once you are into the .5A+ bracket a better driver arrangement is needed. For the price of the resistors you could get a cheap driver on eBay from China!

[uplb]

Wow. Thanks for the detailed answer.

I was just wondering. Instead of using power resistors, is it okay just to use individial lower rated power supply for each stepper motors (i.e. 3.0A power supply for the 3.0A motor, 2.5A power supply for the 2.3A motor and 1.0A power supply for the 0.87A stepper motor)?



Or as an alternative for expensive power resistors, can I just connect many 1watt resistors in parallel?

Thanks again!

[irving2008]

Originally Posted by uplb Wow. Thanks for the detailed answer. I was just wondering. Instead of using power resistors, is it okay just to use individial lower rated power supply for each stepper motors (i.e. 3.0A power supply for the 3.0A motor, 2.5A power supply for the 2.3A motor and 1.0A power supply for the 0.87A stepper motor)? Or as an alternative for expensive power resistors, can I just connect many 1watt resistors in parallel? Thanks again!

Not really because at best all that will do is force the power supply into current limit which will drop the voltage to the point where you won't get much torque or speed out of the motor, at worst it will overheat the power supply...

You can parallel up 1W resistors (so for example 12R would be 10 x 120R giving 12R at 10W rating or 13 x 150R giving 11.5R at 13W) but its messy.

If you are going to that effort why not build a current chopper?