[GreenKeys] sneak peek

[John Nagle]

On 08/21/2016 01:25 AM, Gil Smith wrote:
> The buck regulator you spied is not for loop use, it is for 48V (loop
> supply) to 5V conversion for the logic supply, so AC only goes to the
> (not shown) 48V enclosed switcher.  On units that do not use a 48V loop
> (some folks will insist on 120V, and the loops can do that), the buck
> reg cannot be used, so a USB adapter will be needed to provide 5V for
> the logic.  USB just because it's easy and there are oodles of them out
> there, and they put out at least 500 mA.  But a 5V supply generated
> elsewhere would be fine as well.  Can be a bit noisy (as usb adapters
> are) since the micro and such run on 3.3 LDO.
>
> Yeah, just a traditional loop supply with resistor design. I kinda like
> the idea of melding the new technology with the classic stuff anyway.
>
> I have settled on 48V loop supply for my use -- it is not only a
> non-lethal voltage, but the total power in the resistor pair is under 3
> watts so they can be on-board.  Not like I will have it running 24/7,
> but if I did, I would feel safe that it would not start a fire.

    I've designed some circuits to generate loop power from a USB port,
but they require surface mount parts, and my surface mount soldering
isn't good.  I have simulations in LTSpice for these.

    You really need 120V only for the first 1ms or so after a SPACE
to MARK transition.  Sustain only requires 3.3V or so to get 60mA
through a 55 ohm selector magnet.  So what's needed is a circuit
that charges up a 1uf capacitor to 120V and dumps that into the
selector magnet.  After the capacitor discharges, the 3.3V sustain
supply takes over.

    There are ICs intended to charge photoflash capacitors, and some
of them could be used for this.  They're specialized switching
power supply controllers.  But they're all very tiny surface mount
parts.

				John Nagle