Thread: Simple Current Limiter (LM317)
13-07-2009 #11When I contacted Astrosyn I was adviced for the best speed and torque to run the steppers at 30-40volts not exceeding 1amp per phase.
One alternative is to use resisters instead of the LM317s. Resisters wont give you a perfect linear current drive, but you may be able to increase the supply voltage to compensate and, since they'll run to a higher temperature before failure, the extra dissipation is less of an issue.
The real answer is to get yourself a better bipolar driver with switch mode current limit (pwm ideally).
While Astrosyn are correct that higher volts = higher torque at speed that doesnt apply when you have a fixed linear current limiter as the voltage across the coil will also be fixed as V = I x R. By the same token a resistive limiter wont change anything... plus you'll need 6 x 30ohm 50W resistors and they'll be considerably more expensive than the LM317s! apart from mechanically their external temperature will get to around 95degC which would need considerable protective casing.
A Chopper or PWM current limiter can make use of the higher voltage by driving a higher peak current (thats where the big capacitors, 25000uF+on a 10A supply, in the PSU come in) for shorter times into the coils but maintaining the average current at the defined value.
Short and long of it... with the unipolar, linear current limited design there is no point (nor any sensible way) in running above 12v on the power supply.
If you want to go this route then you will need to build a chopper current limiter which is more complex (but not overly so) and I'd be happy to suggest an approach for you...
Ok so what's involved with building a chopper or pwm current limiter?
13-07-2009 #14voltage across the coil will also be fixed as V = I x R.
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.
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.
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.
Last edited by BillTodd; 13-07-2009 at 03:48 PM.
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.
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