Simple Current Limiter (LM317)

[BillTodd]

voltage across the coil will also be fixed as V = I x R.

Erm, It's a coil Irving, the R's a Z ;)

The heatsink it's self is 85x90x4mm if you include the cooling fin's it's 85x90x24mm so the fins are 20mm, I do also have a 80mm 12volt fan bolted to the heatsink.

Ah Missed that bit! - Irving's right, that's nowhere near big enough for 4 devices.

Have you got a couple of Intel P4 coolers kicking around? the old 478pin cooler could dump 50-60w with the fan screaming. Using Irving's figures, you could clamp four lm316s to each which, if you ran the limiters in series, would give you the desired voltage drops for one motor.

[edit]if you ran the limiters in series - brain-fart! they're not going to work in series, unless the currents are exactly matched. You'd have to parallel them at half an amp each.

[irving2008]

Erm, It's a coil Irving, the R's a Z ;)

True, but if you read the Astrosyn spec you will see that the DC resistance of the coil is 6.2ohms so at 1A the static voltage across the coil is 6.2v. The current through the coil can never be greater than 1A because of the current limiter so while the voltage across the coil may be higher at t=0 you get no benefit from it, as the magnetic field is proportional to current. A chopper or PWM will drive a higher current through the coil faster due to the higher voltage but average it out (i.e. the RMS current = 1A) but the magnetic field build up is faster therefore torque at high revs is better.

Here is a graph showing the coil current for different applied voltages. A good rule of thumb is a maximum step rate of 3x the 95% energised point , which at 6volts is about 3 x 6.5mS or 51steps/sec = 15rpm. At 12v the maximum rate is 90rpm. At 18v its 142rpm. With a 5mm leadscrew these equate to 75mm/min, 450mm/min and 710mm/min. Realistically for a small router/mill using these lightweight motors 450mm/min is probably the fastest you want to go.

So there again this shows there is no point going above 12v with a linear current limiter; there is little to gain and much to lose....

What these also show is that with a chopper-based limiter there is little point in going much above 18 or 24v as you won't gain much in a unipolar configuration - the law of diminishing returns applies.

You might sensibly go to 36v in a bipolar chopper configuration where you gain the benefit of the higher 1.4A current capability and the full motor torque.