Hi from South Africa, a little help please

[irving2008]

Here is a picture of my L297/L298 drivers from a few years back, see the size of heatsinks for 1.4A operation



Sorry Dan, but you're on a hiding to nothing, a current limited supply simply won't work. With the L298 you must use the sense resistors/chopper circuit on each winding. A stepper motor works by phasing current through 2 windings. At any point in time current is flowing in none, one or both windings... to get constant torque and accurate stepping you have to current limit on a per winding basis.

The next 3 pics shows the voltage across the sense resistor (a proxy for the current in the winding) at different step rates for a fairly high inductance motor running on 36v supply and 1.4A. The inductance of the winding is what limits the rate of current change so you have to drive the winding hard with a high voltage to get enough current flow to get useful torque at any useful step rate, but then you have to limit the current to avoid frying the winding. If memory serves, these are at 75, 112 and 133rpm. You can see the initial slow current build up followed by the chopper action. In the last pic the current limit is never reached - above this speed the stepper will generate less and less torque (in fact by 300rpm it hasn't enough torque to overcome its own friction and stalls). This motor should ideally be running on 100v to be useful above 100rpm, and this is a 1.5Nm NEMA23 motor with a 1.4A/2.5v winding.

[daniebjoubert]

Irving,

Good to hear from you, hope you are well.

Thanks again for the information and pictures. The heat sinks are quite big I see.
As I am experimenting I don't mind using a lot of aluminium at this stage. (junkbox full !)

I took notice of the fact that current limiting should happen on a per winding basis.
On My circuit board I have allowed for the 2 current limiting resistors for the L298.
It's not clear in your picture whether your boards have them fitted or not.
Will the L298 perform the necessary current limiting with those resistors or should there be an extra current limiting circuit.?