Simple Current Limiter (LM317)

[irving2008]

Good Day All.

I want to build a simple current limiter for my stepper motors.

The most simple design I have found on the internet so far is using a LM317 Adjustable Regulator.

As I am running a 36v supply this is above the limit for a standard LM317, But lucky for me there is a high voltage version.

I am going to be running 3 stepper motors in unipolor, so I am going to need 2 LM317HV's per stepper motor.

I know that using the LM317HV's with a supply voltage of 36v is going to give out alot of heat.

I am going to be mounting all 6 of the LM317HV's to a 85x90mm heatsink that has a 80mm 12v fan.

In the datasheet for the LM317HV it shows to use a 0.1uf cap on the input, but does not show anything on the output.

As said above I have found a number of current limter designs based on and around the LM317 all of them have caps on the input and output, from 0.1uf to 200uf.

I know the hole idea of the caps is to smooth out any noise to and from the LM317 but how can I work out what is going to be the best caps to use for my design?

Thanks for your time.

Hi,

If you are running these steppers unipolar why go for such a high power supply voltage? you are giving yourself so many problems by doing so.

Higher voltages only apply to designs using a chopper or PWM current limiter, not a linear design. The higher voltage can then give improved current ramping in the windings but this is not the case for a linear current limiter.

The Astrosyn steppers in unipolar configuration at 1A per phase need 6.2v. A 36v supply means the limiter will be dissipating 29W. For an LM317 design, the current set resistor will be 1.25ohm and will dissipate 1.25W, needing a 3W resistor. If you put 6 LM317 on a heatsink at 30W dissipation each (worst case) you are dissipating 180W.

For an LM317 the thermal characteristics are 4degC/W junction to case so the junction temperature will be 4 x 29 = 116degC above case, the limit being 125degC. In other words you could use an infinite heatsink and they'd still fry at room temperature!

Dropping the supply volts to 12v would run the LM317 at 4.6W, the maximum case temp could then be 106degC and the required heatsink (for 6 units at 20degC ambient) would need to be better than 3.1degC/W - say atleast 1.5degC/W for safety assuming some thermal grease between the case and the heatsink. An 80mm x 80mm CPU-style fan blown heatsink (if thats what you were proposing, typically 0.8degC/W) might just be sufficient if you can get the LM317 close enough to the centre and can get sufficient airflow. Note that a CPU-style heatsink is designed for a heat source about 20mm sq in the middle, not 6 TO220 cases bolted round the edges.

A normal extruded ally heatsink with 15 - 20mm fins will be around 3 to 5degC/W for a 100mm x 75mm piece and maybe 1 - 2degC/W if blown by a fan.

You can never have too big a heatsink with linear devices!