[GreenKeys] Loop power supplies

[Jeffrey Angus]

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 think-
> “Voltage regulator” = “constant voltage source”
> And
> “Current regulator” = “constant current source”
>
> All within limits of course, voltage regulators can’t feed shorts, and 
> current regulators can’t feed opens.
>
> IIRC, time constant for voltage across a cap is RC. Bigger R, slower 
> rise time. Bigger C, slower rise time. And time constant for current 
> through an inductor is L/R, so bigger R gives faster rise time. Bigger 
> L gives slower rise time. Which is just another way of saying we like 
> a 120v loop supply with a 2k series resistor, instead of a 12v loop 
> supply with no resistor.
>
> EE class was 50 years ago, so I have probably lost those neurons a 
> long time ago.
>
> Cheers
> Nick
Sigh.....Rather than my usual point by point, I'm going to reply down
here in full.

Let's assume you have a 120 volt source, and you want to drop it
down to 12 volts. For a 100 ohm load, you could just put a 900
ohm resistor in series with the load. You'd drop 108 volts across
the 900 ohm resistor and have 12 volts across the 100 ohm load.

Now, let's make that a 200 ohm load. What happens? The voltage
across the 200 ohm load rises to 21.8 volts.

Let's look at the TMC current REGULATOR again. Assuming a 100
ohm load, it's looking at the voltage across the load, 6 volts, to set
the bias on the vacuum tube so that it's resistance is high enough
  to drop 108 volts, so there is only 6 volts across the load.

The "how" of that is that it can put up as much of the available
voltage across that load as it takes to make 60 mA of current flow
through it.
This is to compensate for the inductive reactance of the coil.
However, it's not a true current source, because the bias has to be
adjusted each time for a different DC load resistance.

If you change the DC resistance to 200 ohms, it will have 12 volts
across it with 60 mA current. This changes the bias requirement to
control the vacuum tube to set the steady state current to 60 mA.

The difference between a current regulator and a current source is
that the "set" current for a current source does not care what the
load resistance is. It can vary from zero ohms up to just under 2000
ohms and it it will stay at 60 mA with a 120 volt source.

The current regulator in the TMC is NOT a current source, it is a
peak current limiter, or "current regulator" as they call it.
It relies solely on the compliance (120 volt source) to over come
the inductive reactance of the coil.

In that respect, it is faster than just a fixed resistor.





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