5144
« on: March 01, 2018, 02:48:57 AM »
Hi,
using shielded cable certainly helps but if you think it will defeat electrical noise you're dreaming. Sorry to be blunt but this myth about shielding has to be busted.
Imagine a signal to a stepper driver, say a 5V pulse working into an impedance of 1kOhm, a fairly typical set of numbers. The energy in a 1 millisecond pulse is
(5)squared/1k *.001= 25uJ, yes only 25 microjoule. An electric motor cable lying next to our signal cable may have a pulse of 100V and 5A for the same 1 millisecond.
Its pulse energy is 100*5*.001=0.5 joule, that is 20,000 times the energy in our signal cable.
If we wish to retain the signal as relatively 'unpolluted' by its electrically noisy neighbor the shielding will have to reduce the radiated energy by something like 50,000
times, just about impossible.
Worse electronically controlled motors tend to have high frequency switching currents and all those jagged current pulses radiate very strongly. Physical separation even
of only an inch or so will markedly improve the situation, separation AND shielding now starts to look like a real solution.
Another area that is overlooked is working impedance levels. You noted that I used a reasonably low impedance (1kOhm) to model the input of the driver but
manufacturers can and do use impedances of 10K, 20K and 50K. The current required to signal the device is markedly reduced, a good good thing, but the pulse
energy is now so low that it will be very very VERY easily swamped by any electrical noise.
The upshot is that currents of around 5-20mA are appropriate for signaling stepper/servo drivers in amateur service, they will often work at very much lower currents
but become increasingly susceptible to noise.
Craig
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