Simple Current Limiter (LM317)

[BillTodd]

When I contacted Astrosyn I was adviced for the best speed and torque to run the steppers at 30-40volts not exceeding 1amp per phase.

Yes, if you don't have sufficient voltage it'll severely limit the top speed

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).

[irving2008]

irving2008: When I contacted Astrosyn I was adviced for the best speed and torque to run the steppers at 30-40volts not exceeding 1amp per phase.

I know that running at this kind of voltage is going to make alot of heat.

The heatsink is not a computer / cpu heatsink

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.

Kip: I am using unipolor as that is all my driver boards support.

The point I am making is you cannot run a LM317 at 30W dissipation. The heatsink you are planning to use will be, with the fan and some thermal grease, about 2.5degC/W thermal resistance. Together with the 4degC/W of the device you have 6.5degC/W from ambient at 20degC to the chip at a maximum of 125degC. Therefore you can afford to dissipate (125 - 20)/6.5 watts = 16Watts, i.e a voltage drop of 16v at 1A. Therefore you cannot run higher than 16+1.25+6volts = 23volts, and that assumes ONE LM317 per heatsink. With 6 devices sharing that heatsink the thermal resistance of the heatsink is multiplied by 6, i.e. the equivalent of 6 x 2.5 = 15degC/W therefore doing the same calculation (125 - 20)/(15 + 4) = 5.5W = 5.5v at 1A therefore max supply volts =5.5 + 1.25 + 6 = 12v! (and that is running ther LM317 close to their max operating temperature, so expect failures over time - you should never go above 75% max operating temperature as a general rule).

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...

[Mad Professor]

Ok so what's involved with building a chopper or pwm current limiter?

[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.