[GreenKeys] Loop power supplies

[Bob kb8tq]

Hi

> On Mar 5, 2018, at 8:49 AM, Jeffrey Angus <jdangus at att.net> wrote:
> 
> On 3/5/18 6:55 AM, Bob kb8tq wrote:
>> 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.
> That's not how it works.
> At initial turn on, the coil sees close to the full loop voltage
> because of the reactive impedance of the coil. As soon as it starts
> to draw current, the voltage drop across the resistor increases.
> For a DC coil resistance of 100 ohms, this means 6 vdc at 30 mA
> and 12 vdc at 60 mA.

Nope, not at all. 

The inductance is why the current does not go to 60 ma immediately. 
This is not just a bunch of resistors. At 30 ma, you have half the loop
voltage on the series resistor and half the loop voltage on the coil. If
it’s a 120V loop, you have 60V on the magnets. Pop a scope on it and
take a look. 



>> 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.
> Absolutely not.

Build one and put a scope on it. They most certainly *do* saturate
the pass transistor right up to the point the set point current is reached. 
With the transistor “full on” you have the entire loop across the magnets.

Bob


> The whole point of operation of a current source, is the compliance
> voltage needed to make the current a set value over a wide range of
> of resistance. In the case of a large inductance, it's a combination of
> the DC coil resistance, which remains fixed, and the AC reactance,
> which varies over time, dx/dt.
>> 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.
> Again, no.
> The reason it is faster is due to the variable resistance of the current
> source, rather than a fixed value. The lower initial source impedance
> of the 120 v loop supply allows for a slightly faster rise time of the
> current through the selector magnet.
> 
> 
> -- 
> "I am a river to my people."
> Jeff-1.0
> WA6FWI
> www.foxsmercantile.com
> 
> ______________________________________________________________
> GreenKeys mailing list
> Home: http://mailman.qth.net/mailman/listinfo/greenkeys
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:GreenKeys at mailman.qth.net
> 
> 2002-to-present greenkeys archive: http://mailman.qth.net/pipermail/greenkeys/
> 1998-to-2001 greenkeys archive: http://mailman.qth.net/archive/greenkeys/greenkeys.html
> Randy Guttery's 2001-to-2009 GreenKeys Search Tool: http://comcents.com/tty/greenkeyssearch.html
> 
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html