[GreenKeys] Loop supply options

[Richard Knoppow]

     Its common for power supplies to have bleeder resistors to 
set a minimum current. Assuming an open circuit voltage of 150V 
the power dissipated by a bleeder drawing 15mA would be 2.25 
watts. If I understand how the loop works, and I may not, the 
magnets draw 60mA from the source. It is a current operated 
device. The voltage has to be sufficient to allow the 60ma to be 
drawn. Since the magnets are connected through switches or relays 
(which are just switches) the load will go from whatever the 
magnets present to an open circuit. The amount of overshoot will 
depend on the ratio of current drawn open circuit to that drawn 
by the magnets. A bleeder is a simple way to limit the ratio. 
While the electronic regulator evidently needs the minimum load I 
think its good practice to provide a minimum for a plain 
rectifier supply. Again, if my understanding is correct, the loop 
supply need be no more than a simple DC supply. It can consist of 
no more than a suitable transformer, a rectifier, probably a 
series resistor and filter capacitor. The series resistance is 
part of the filter. A parallel bleeder would then limit the 
voltage rise on open circuit and the voltage difference between 
no load and normal load conditions as it would on any DC supply. 
Some care may be necessary in the choice of a filter capacitor 
since the cap will tend to cause a current spike when the load is 
attached.
     It seems to me the transformer can be any isolation 
transformer. A bridge rectifier will deliver a DC voltage on the 
order of the AC in the secondary. The bleeder can also act as a 
trimmer for loop current or one can add another, variable, 
resistor in series with the magnets.
    This is not an efficient system but the amount of power 
dissipated by the supply is not very great, several watts perhaps.
    There has been an extraordinary amount of traffic about this. 
If there is something I am missing that makes it more complicated 
I would be glad to know.
    A note: evidently the 60ma value is taken from wire telegraph 
practice. In that situation the current supply was often from 
batteries that were tolerant of constant load. All the 
instruments on the circuit were on line all the time except when 
one of the stations was transmitting and opened up the circuit 
closer on the key. I am not sure how this translates to 
teleprinter practice.

On 9/9/2019 11:43 AM, Nick England wrote:
> Data sheet says minimum 15ma load required for regulation. 
> Unclear what would happen used with a loop that switches from 0 
> to 60ma.
> Nick
> 


-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL