Simple Current Limiter (LM317)

**Mad Professor** · 13-07-2009

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.

**BillTodd** · 13-07-2009

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** · 13-07-2009

Originally Posted by Mad Professor

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...