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Re: [TowerTalk] SteppIR problem

Subject: Re: [TowerTalk] SteppIR problem
From: K4SAV <>
Date: Mon, 11 Jun 2007 07:16:06 -0500
List-post: <>
The question of the power supply's average current capability requires 
further investigation since this is the most logical cause of the 
problem.  This would exactly explain the symptoms reported.  The item in 
question is not the peak current capability, or long term DC average 
from the supply, but a current averaged over a shorter time during which 
the system is tuning.

One way to verify this would be to look at the power supply with a scope 
and look for voltage droops during tuning.  The minimum value acceptable 
would have to be a number supplied by FM (SteppIR).  Since the system 
operates properly with a 24 volt supply, it is probably something less 
than that.

Another way would be to substitute a power supply with higher current 
capability and see if the same symptoms occur.

Measurement of supply current with a DVM and comparing that to the power 
supply current rating, or measurement of power supply voltage with a 
DVM, would probably not provide an answer.  The average measurements 
would show a change with line length and with tuning, but the problem 
will exist for a shorter time than can be observed on a DVM (I think).

Jerry, K4SAV

K4SAV wrote:

>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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