[TowerTalk] SteppIR problem

[Jim Lux]

1000 ft of AWG22 wire is about 19-20 ohms

Based on some stuff on N8LP's website, I'm going to guess that the 
motor current for a single element is about 300 mA.  I'm going to 
guess that they use standard 2 phase motors, so 150mA per winding. 
The voltage drop in the cable is going to be about 3 Volts, which 
seems quite low.

It might be that a higher voltage is used to get the di/dt to be fast 
enough to get reasonable step rates (on most stepper drives, you need 
substantially more voltage than you'd calculate from the winding 
resistance, so that you can get the current up high enough fast 
enough.. the voltage waveform looks like a spike with a back porch... 
see, e.g., TV horizontal flyback waveforms)

The series L from the cable is going to be substantial, even with the 
twisted pair, and the driver has to charge the entire transmission 
line up, too.  I seem to recall something like 0.5 to 1 
microhenry/meter for twisted pair of "reasonable size".. Call it 200 
uH total.  Then, you've got that surge suppressor with a ferrite bead 
and some parallel C (0.001 uF).

I can easily see the driver electronics being unstable/unpredictable 
into this sort of load, especially if it's one of those slick PWM 
drivers like the ones from Allegro that try to drive constant current.