No longer lurking, stepper motor question.

[irving2008]

Hi Irving..

The Stepper Motors are : E-Bay item 271431291269, no datasheets that I could find, searching the links the seller put with them, as with you no 112, or for that matter the ending W which supposedlt designates Unipolar or Bi-Polar is not listed

And L298 Driver boards : E-Bay Item 301045130656

I did have a breakthrough last night and got the steppers moving using a 5v source, and the heatsink did get a "tad" warm ! So will look at upgrading it and maybe a small fan off an old graphics card put to use.

The problem with those driver boards is there is no current limiting so your ability to drive any serious stepper at any useful speed/torque is severely compromised.

Basically you're relying on the nominal resistance of the stepper motor winding to do the current limiting. I don't know the resistance or inductance of those motor windings (can you measure them?) but for an example calculation lets assume 1.5ohm and 10mH (educated guesses looking at other frame sizes with a 112 winding designation).

The L298 will drop about 3v @1A and 5v @2A. At 2A the motor winding will drop 1.5 * 2 = 3v so any supply voltage to the L298 >8v is going to fry it, so on 12v its going to get very hot very quickly!

At 1.2A the voltage across the motor is 1.2 * 1.5 = 1.8v and, on a 5v supply, across the L298 its 3.2v. The L298 will be dissipating (both windings active) 3.2 * 1.2 * 2 W = 7.7W. That tiny heatsink is probably no better than 5degC/W so the heatsink/case will be @ 20 + 7.7*5 = 60degC and the junction @ 60+ 3 * 7.7 =88degC

So 5v is do-able. But thats static holding torque, not motion.

As soon as you start stepping the inductance comes into play. Your torque will, crudely, be halved when the impedance of the inductor equals its resistance. At 1.5ohm and 10mH that's about 240rpm.