Dick Green WC1M wc1m73 at gmail.com
Wed Jul 15 11:05:02 PDT 2009

That's great info, Jerry. So, if I understand the design correctly, if one
of the two wires in a pair is accidentally disconnected, and it happens to
be the return current line, the chip will burn itself up. Is that correct?
Of course, SteppIR's instructions for checking resistance on each pair at
the controller end of the cable would prevent an installation error of that

However, there has been much talk on the SteppIR reflector about never
unplugging the cable without first pulling power from the controller. Some
have claimed that doing so damaged their driver chips. The conventional
explanation is that one or more pins can get shorted in the process of
removing the connector. (Those readers not familiar with the controller
design, who are thinking that all you have to do is avoid unplugging while
the controller is operating, may be surprised to learn that even when off,
the controller periodically pulses the stepper motors to compensate for
drift when the motors are idle, and this feature can't be disabled without
removing power from the controller.) Anyway, I've always thought shorted
pins are very unlikely, given the design of the DB-25 connector.

However, if one pin in a pair can get disconnected a fraction of a second
before the other pin in the pair, and that pin happens be on the return
current wire, *and* the controller is doing one of its drift compensation
cycles, then perhaps it would be possible for the chip to burn up because
it's not sensing the return current. Sound plausible? While I can't see pins
in a DB-25 getting shorted, I can imagine the pins coming out of the female
connector unevenly if the connector is removed at an angle.

73, Dick WC1M

> -----Original Message-----
> From: K4SAV [mailto:RadioIR at charter.net]
> Sent: Wednesday, July 15, 2009 12:40 AM
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] STEPPIR QUESTION
> 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
> >> driver chips.
> >>
> >> Wonder what happens if you replace each output with a thousand-volt
> ?
> >>
> >>
> > 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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