Re: Using SCR's For Regulating Input Current (fwd)

["High Voltage list" <hvlist@xxxxxxxxxx>]

Original poster: <sroys@xxxxxxxxxxxxxxxxx>



---------- Forwarded message ----------
Date: Thu, 3 Feb 2005 15:50:25 -0700
From: Gomez Addams <gomez@xxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Using SCR's For Regulating Input Current (fwd)


On Feb 3, 2005, at 3:28 PM, High Voltage list wrote:

> Original poster: <sroys@xxxxxxxxxxxxxxxxx>
>
>
>
> ---------- Forwarded message ----------
> Date: Mon, 31 Jan 2005 10:57:56 -0600
> From: Carl Litton <Carl_Litton@xxxxxxxxxx>
> To: High Voltage list <hvlist@xxxxxxxxxx>
> Subject: Using SCR's For Regulating Input Current
>
>
> Would someone please explain how the brick-like high current 'power
> module' blocks containing 2 SCR's are used to regulate current?

  I didn't know they could be used to regulate current!  (only average
voltage)

> We are
> interested in pursuing this as an alternative to heavy/bulky variable
> inductors.

These are used just like a triac dimmer circuit.  Two back-to-back SCRs
are preferable for high power circuits, because there is one fewer 
junction
in the current path, this yields a much higher temporary surge rating 
vs.
using a triac.  For example:
http://www.epanorama.net/documents/lights/lightdimmer.html

> We have been given to understand that such a module may be placed in 1
> leg of the AC input line (along with a potentiometer and such) to
> regulate the input current as a sort of pulse width modulator.

  Um, this sort of phase-angle control limits average power by limiting 
average
voltage.  I don't see how it can be used to limit current but not 
voltage.

  Perhaps someone smarter than myself will point out how I am wrong.  I 
can
imagine using a dimmer circuit across the high impedance winding of a 
beefy
transformer, and putting the low impedance winding in series with your 
load.
But you might have to provide some sort of extra power supply rather 
than a
simple diac-triggering circuit, since the firing voltage would vary 
with the
current through the ballast winding.