R. Kline wrote:
>There has been mention that the source is a current source, and depending
>upon the length of the control cable, the current would increase, etc. This
>couldn't be correct, since the load (motor), cable resistance, and source
>are all in series - so the current would be the same everywhere. What may
>be happening is that the voltage increases in an attempt to keep the current
>constant. Perhaps the supply "senses" the voltage output and shuts down if
>it exceeds a predetermined maximum voltage.
Apparently you didn't read my explanation, in answer to WC1M's question
about how this works. Here it is again.
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.
TowerTalk mailing list