[GreenKeys] Loop power supplies

[Jeffrey Angus]

On 3/3/18 10:35 AM, Jim Haynes wrote:
> I haven't tried it, but there are circuits that use the "constant 
> current"
> properties of a transistor rather than a fixed resistor, and are said to
> give equivalent results with lower voltages such as 48V.  For instance
> QST, Jan 1972, p. 40.  Maybe someone who knows how can do a SPICE
> simulation to compare such a circuit with a plain resistor. 
As has been said in the past, "An elephant is a mouse designed by
committee."

The selector magnets in a Model 15, in parallel for 60 mA operation look
like 4 H of inductance. That's kind of "A lot" in scientific terms.

There are two types of sources in "basic electronics" they are the voltage
source, and the current source.

For a voltage source to be able to impress the desired voltage across any
load, it hast to be able to source a lot of current. Think of hooking a 
battery
across a discharged high value capacitor. It's going to take a bit of 
time for
The voltage across the capacitor to come up to the voltage battery.
Because a discharged capacitor looks like a zero ohm resistor. The internal
resistance of the battery limits the current and the time it takes is 
roughly
t = r * c.

Now the fun part. Current sources. Instead of current capacity, the term for
a current source is called compliance. It refers to the peak voltage 
available
to maintain the load current.
In the case of the selector magnets, they're approximately 100 ohms, so you
would think "That's easy, it only takes 6 volts to push 60 mA though that."
Except, for the total inductance. Inductance resists changes in current the
same way capacitors resist change in voltage.

A 120 v source with a series resistance of 2000 ohms acts like a very simple
current source. When you initially connect it to a 4 H inductance, the 
initial
current through the coil is near zero. This allows almost the full 120v 
to be
impressed across the coil. As expected, that gets it's attention so to 
speak,
and the current through to coil rises rapidly. As the current through 
the coil
rises, the voltage drop across the resistor increases. By the time the 
current
through the coil is up to 60 mA almost all the voltage is dropped across the
2000 ohm resistor.

Now, a true current source would look like a variable resistor with a high
source voltage. That 2000 ohm resistor would initially look like close 
to zero
ohms, and would adjust upwards towards to 2000 ohms as the current through
the inductance of the selector magnet increases to 60 mA.

Would a current source make the rise time a little faster? Yes, but it's 
a matter
of diminishing returns. Aside from the brutal simplicity of a 2000 ohm 
resistor,
the consideration is the power capability of the transistor acting as a 
variable
resistor. At some point, when the current reaches 60 mA, it has to dissipate
in the neighborhood of 7 watts.

What is the effect of rise time for the current through a selector magnet?
It's called marking distortion. The time between you start a marking signal
and when the selector magnet pulls in. Enough of a delay and the magnet
will be in the wrong position when the machines decide "What condition?"




-- 
"I am a river to my people."
Jeff-1.0
WA6FWI
www.foxsmercantile.com