[GreenKeys] Loop power supplies

[John Nagle]

On 03/05/2018 12:44 PM, greenkeys-request at mailman.qth.net wrote:
> From: Jeffrey Angus<jdangus at att.net>
> Subject: Re: [GreenKeys] Loop power supplies
> 
> On 3/5/18 9:36 AM, Nick England wrote:
>> My point was that the thing we call a ?voltage regulator? keeps a
>> constant output voltage even when the load varies (also when the
>> supply voltage varies). So it is a ?constant voltage source? (within
>> limits obviously). No load voltage =15v, 100 ohm load voltage =15v. Right?
>>
>> So what do you mean by a ?current regulator? that must be readjusted
>> when the load changes?? If so it isn?t regulating current. By your
>> definition, an adjustable voltage regulator is a ?current regulator?.

    I had to measure this when building my boards.

    What my driver board actually does is charge up 2uf to 120VDC
and dump that into the selector magnet to pull it in.  Then a
15V supply provides sustain voltage to hold it.  That, by itself,
will run a 220 ohm selector quite nicely.

    1uf is not enough. 3uf is more than you need and the charge
circuit can't get it to 120V from the available USB power in the
time available.

    The actual inductance of a Model 15 selector magnet wired
for 220 ohms is about 5.5H.  Measured by applying a DC voltage
and watching the current ramp rate on a scope.  It changes
when the magnet plate pulls in, but by then the inductance
doesn't matter.

    I put in a final current regulator stage with a depletion
mode MOSFET and a sense resistor to series regulate the whole thing to
60mA. That's was to eliminate a jumper which allowed changing
from 220 ohm to 55 ohm selectors. Jumpers are so last-cen.
It's easier to do this  automatically, and it also provides
short circuit protection.  With a 220 ohm load, the regulator
isn't doing much.

    Yes, you can run a Teletype at much lower voltages if you reduce
the tension on the selector armature spring. But the mechanism becomes
less reliable when run underpowered. Do it right.

				John Nagle