[GreenKeys] Loop power supplies

[Bob kb8tq]

HI

> On Mar 5, 2018, at 6:53 AM, Jeffrey Angus <jdangus at att.net> wrote:
> 
> On 3/4/18 10:41 PM, drlegendre . wrote:
>> @Jeff
>> 
>> Ok.. so by using a tube as the pass element, they're eliminating
>> the inductive rise time of the high-value rheostat(s) which would
>> otherwise be in series with the output current. Is this the whole
>> consequence?
> Compared to the inductance of the selector magnet, the inductance
> of the meter and 2K resistor is statistically zero.
> 
> On 3/4/18 12:33 PM, Nick England wrote:
>> FWIW - here is the manual/schematic for the TMC constant-current
>> loop supply, which uses a 6Y6 tube as the pass element.
>> http://www.tmchistory.org/tmc_manuals/manuals_db/psp/tm_psp-1-2_2_1_59.pdf
> Again, this is a regulated current supply, not a constant current supply.
> 
> On 3/4/18 10:51 PM, Bob kb8tq wrote:
>> The big deal with a constant current source is that it keeps “full
>> voltage” on the magnets longer than the equivalent resistor based
>> loop.
> Not exactly, they both have have full voltage available.
> The difference is the source impedance of the voltage.
> What causes the faster rise time is effectively having the high voltage
> across the inductor with no current limiting until the circuit reacts and
> increases the tube effective series resistance.
>> A less significant benefit is that you *might* put a bunch of magnets
>> in the loop and it would still stay at 60 ma. It would be slow, but it
>> would have a 60 ma current at idle.
> With a true constant current source, yes. But the above unit is a current
> regulator. The current has to be set every time the load changes.

We have wandered off to solid state constant current sources as well ….

If I have a “normal” loop supply and the 60 ma magnets are up to the 30 ma
point, I only get 1/2 the loop voltage on the magnets. They don’t “see” the 
full loop supply. 

If I have a proper constant current loop supply, even at 55 ma, the magnets
still get the full loop voltage applied. They “see” the supply right up to the
point they are at full current. 

The fact that full voltage is applied over a wider range of currents is why 
the constant current source is going to get the magnets energized faster. 

====

Back in the 70’s being very proud of all this stuff, I brought the topic up
to one of my professors. His observation after going through the details:

“Don’t you think they designed the timing for slow build up, since that was
the standard system?” 

So indeed, one can make the argument that faster may not be better. It
is easier if you are flipping configurations. It also will allow lower voltages
for the loop supply. 

Bob


> 
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> 
> -- 
> "I am a river to my people."
> Jeff-1.0
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