Dick Green wrote:
>I remember reading on the SteppIR reflector about the driver chips being
>current sources and how this makes the cable length irrelevant, except for
>added capacitance. I understand that added capacitance messes up the pulse
>waveform, but I don't understand how increased resistance doesn't matter.
>
>OK, so the chips increase the current in the face of a higher resistance,
>and the load sees the same current. Fine. But the chips have to get the
>extra power from somewhere, and that's the power supply. If the resistance
>is high enough to cause the current draw to exceed the capacity of the
>supply, the first symptom is likely to be a voltage drop. I would be willing
>to bet that the controller's CPU can't tolerate less than a certain supply
>voltage, so it crashes.
>
>
>
An interesting observation. One can easily make the mistake of saying
that the current to the motor is regulated so the supply current doesn't
change due to the extra resistance, because the motor current doesn't
change, but this is not exactly true, as you are alluding to.
Think of the load as an RL network. The pulses delivered to this load
are both width and frequency modulated, in other words they change to
provide a fixed amount of average current to the motor. So for an
increased resistance the pulse width has to be wider than that for a
lower resistance. This is because the rise time to this network will be
slower. Looking back at the power supply this requires more average
current from the supply because of these wider pulses. (The rep rate of
these pulses also changes but that's just another detail.) However the
peak current required from the supply is the same in both cases, because
the regulator IC shuts the current off when it reaches a preset value.
It is this max peak value that is independent of the amount of
resistance, not the average current from the supply.
That does raise the question of what the power supply's capability is,
in terms of peak and average current capability. It is possible that
the power supply filter cap can deliver the high peaks but that the
supply itself cannot sustain the higher average current required.
Jerry, K4SAV
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