[TowerTalk] STEPPIR QUESTION

[K4SAV]

If you read the data sheet for the L6219DS bridge driver IC, this will 
answer a lot of your questions about how the SteppIR drivers work and it 
should then be obvious why adding simple buffers or clamps on the output 
lines won't work, unless the circuitry is made extremely complicated.

The low frequency pulses you see on the output lines are the data 
patterns used for driving the motors.  There is a much higher frequency 
(above the audio range) that is used to regulate the drive current to 
the motor.  The bridge is a current source driver.  The bridge connects 
one wire to the supply voltage while it senses the return current on the 
other wire in the pair.  This current does not rise instantaneously 
because it is driving an inductor.  When it senses the proper current it 
turns off the driver for a fixed amount of time. This continues at a 
high rate, maintaining a constant current for the duration of the high 
(or low) pulse of the data pattern. When the data switches to the 
opposite state, then the bridge swaps the lines, driving the opposite 
line of the pair and sensing on the remaining line. 

Now you should be able to see why you can't just add a buffer.  It won't 
work because the bridge won't be able to sense the current.  Also you 
should be able to see that when the return current doesn't come back 
down the wire pair (maybe it goes to ground instead, or to some other 
line) then the driver can't sense the output current so it stays turned 
on and burns itself up.  There is no current limiting on the high side 
of the driver.  It must operate properly in this servo-loop 
configuration to regulate the current. 

People that add C, RC, or LC filters on the output of these drivers are 
also risking make the driver operate improperly, because a cap on the 
output causes the current level to be sensed prematurely and the driver 
shuts off before ever driving the motor with proper amount of current.

Providing protection for the driver requires a circuit that limits the 
output current on every line to a maximum of 1 amp which is the level 
above which the manufacturer of the part says that damage may occur.  
That requires bi-directional current limiters on every line, which also 
has to be able to handle the peak voltages expected on those lines due 
to external sources.  Undoubtedly this will also require some clamp 
circuits to protect the bi-directional current limiters.   That's what 
you need if you do an add-on to the existing circuit.  Something 
completely different is possible if the modification is made on the 
controller PCB, which will change its design.

I have not looked at the new controller circuitry.  Does anyone know if 
they are using the same driver IC?

Jerry, K4SAV

> Dan Zimmerman N3OX wrote:
>   
>>> Is there a way to build an external box with something like opto-isolators
>>>
>>>       
>> to better isolate the driver chips from getting nailed so easily?
>>
>>
>> Well, you need to build something that can source as much current as the
>> driver chips.
>>
>> Wonder what happens if you replace each output with a thousand-volt MOSFET ?
>>
>>     
> jimlux wrote:
> Circuits made to take big transients would do something very much like 
> that.. They use a fairly decent FET or IGBT with 100V or more. Then a 
> fast clamp on the output (like a back to back thyristor/DIAC) to limit 
> the voltage (zeners might work, but because their voltage stays high, 
> they dissipate more power).. Finally, a series transient suppressor (a 
> LC circuit of some sort) to limit the di/dt, and then a vacuum gap 
> across the whole thing where it goes to the outside world.
> _______________________________________________
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