[GreenKeys] 60ma power supplies

[John Nagle]

>> Lee Mushel wrote:
>>> ...
>>> a Triad N-48X 115v:115v  15VA transformer (available back then) and then
>>> ...
>> Actually, Mouser still lists it, and shows one in stock and 7 on order.
>>
>> Here's a question from the electronically challenged (me) - in the
>> power supply you describe, what value would you suggest for the
>> filter capacitor for a 60mA (not 20mA) supply?
>>
> 
> Use what Hoff did in his ST series.  A 100 uF at 250 volts.  YOu can go up 
> to double the uF value if you want to and a higher voltage also.  That is if 
> you are using around a 100 to 150 volt transformer and about a 2000 to 2500 
> ohm series resistor to limit the current.

    If you're driving a Model 15, make sure you snub the 400V spike coming back
from the selector magnet so it can't hit the power supply caps.

    I've done a more advanced design, which is available as an LTspice
simulation file:

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

Download the free copy of LTSpice and play with this.

    This is just a design study at this point, but it shows that it
should be possible to run a 60mA current loop off a USB port,
with suitable switching power supply design.  You only need 120VDC
for the first millisecond of each MARK period. The sustained voltage
for a 4H 55 ohm selector magnet is only 3.3V.  In traditional designs,
over 90% of the energy used is being dissipated in the ballast resistor
in the DC loop.


					John Nagle