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I want to use some DC-DC LM2596 stepdown power converters available from EBAY. They have a pot used to set the output voltage.

Several questions arise.....

First, is related to adjusting the voltage to the desired levels. Is the output voltage dependant on the input voltage? The input voltage may vary from 12 to 18 volts or so when connected to different systems, so having to adjust for this manually is a non-starter.

Second, assuming these are usable, what input filtering is needed for correct function? The input will be from a rectified square wave that's about 16v created from an H-bridge which is reversing polarity of the signal. Basically a 50% duty cycle square wave around a ground reference.

I initially used an LM317 regulator, but the thermal requirements to supply 400MA dropping the voltage to 6VDC is excessive.

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First, is related to adjusting the voltage to the desired levels. Is the output voltage dependant on the input voltage? The input voltage may vary from 12 to 18 volts or so when connected to different systems, so having to adjust for this manually is a non-starter.

No. That's why it's called a regulator because it regulates the output voltage. Regardless of the input voltage the output voltage will be the same.

Second, assuming these are usable, what input filtering is needed for correct function? The input will be from a rectified square wave that's about 16v created from an H-bridge which is reversing polarity of the signal. Basically a 50% duty cycle square wave around a ground reference.

A full-wave bridge rectifier, ignoring the ground, as long as you then never use that ground for anything, with a large capacitor. Since it's a square wave there should actually be very little ripple.

Of course, one has to ask, what is this H-bridge square wave generator and why are you trying to power an Arduino from it?

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    \$\begingroup\$ The input is from a Digital Command Control (DCC) system used to both power trains on a track AND send commands to those trains by varying the width of the square waves to preoduce protocol based packets. \$\endgroup\$
    – crusader27529
    Dec 23 '16 at 17:01
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    \$\begingroup\$ Because it's a square wave, as long as it's full-bridge rectified, the input capacitor on the regulator board may be sufficient smoothing. \$\endgroup\$
    – Majenko
    Dec 23 '16 at 17:03
  • \$\begingroup\$ The Arduino is a speedometer mounted on a car, and obviously will be powered from the rails. \$\endgroup\$
    – crusader27529
    Dec 23 '16 at 17:05
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You COULD use one of these modules, but you need to take some precautions.

Firstly you should ensure the module is isolated from the other circuitry with a series diode. This depends on exactly what else is connected, but I assume putting a filter capacitor across the other circuitry may affect its operation.

Secondly a square wave will result in very high ripple on the input. The capacitor included on the module is likely too small, and is unlikely to be rated for such high ripple. I would use a larger capacitor, with a high ripple current rating. Again the voltage rating of the included capacitor (which is probably 35V) may be inadequate. I would use another isolating diode after the added filter capacitor to minimise ripple in the included capacitor.

If the input is balanced around ground, you may be better to use one rail and ground (if this is possible).

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