[GreenKeys] Power supply for Eric's demodulator

[John Nagle]

On 1/4/2011 8:32 AM, greenkeys-request at mailman.qth.net wrote:
> Subject: [GreenKeys] Power supply for Eric's demodulator

Anybody interested in building a more modern power supply?
Take a look at

	http://www.aetherltd.com/public/flashsupply6.asc

This is an LTSpice file of a circuit simulation.  You
need LTspice

http://www.linear.com/designtools/software/

to run it.

This is a proof of concept simulation for a modern 60mA
supply for a Model 15.  If built, it should be about a
1" square board running off 5V at 250mA.  I was looking into
integrating this with a USB interface, and powering it from
the USB port.

It may seem difficult to get 120V at 60mA from 5V at 250mA,
but it's not.  The circuit assumes the selector magnet is 55 ohms,
4 henries.  To get 60mA through that, 120V is required at start,
but once the inductance has been overcome, which takes about 3ms,
the sustain voltage is only 3.3V.  (E=I*R, for 60mA * 55 ohms).

The circuit uses an LT-3484-1 part, which is a switching power
supply controller for photoflash applications.  During SPACE
periods, using a tiny surface-mount pulse transformer, it
charges up a 1uF cap to 120V.  On a SPACE to MARK transition,
the capacitor is dumped into the selector magnet, until it
discharges and the fixed 3.3V supply (the LT-3080) takes over to
sustain 60mA.  The waveforms look about the same as they do
with a classic 120V fixed supply through a 2K 10W resistor.

In the usual Teletype power supplies, with a 2K 10W ballast
resistor, 95% of the energy is going into heating the ballast
resistor.  With a pulse design, far less energy is needed.

It requires surface mount parts and is an all-tiny-part design;
the transformer is 6mm x 6mm x 3mm high.
I did this as an exercise in low-power design, just to see
if it was possible to power a 60mA current loop device from
a USB port.  I haven't actually built one.  This requires
making up a custom board; layout for switchers is critical.
But it's clearly possible; the parts are being used at far below
the ratings needed to fire a photoflash tube.

Personally, I've been using 120VDC 200mA open-frame
linear regulated supplies, which I got from HSC Surplus in Santa
Clara, CA.

				John Nagle