Re: Report - Dimmer as a Variac.

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

Actually, back to back SCRS (or TRIACs) should work ok... (although the
waveform will be a bit wonky, which will screw up sync gaps...and weirdly
change the timing on static gaps)..

The trick is deriving the SCR gate pulse from the line waveform, not from
the current through the load (which is what a two terminal dimmer does....).
If you have PFCs on the inductive load, it should make life easier... The
whole point of a PFC is that it brings the current more into phase with the
voltage, making it look like a resistive load.  What you will need is some
sort of snubber to keep the "initial current" spike when the device turns on
from cooking stuff (a "ballast" for your PFC....).

You also might not get the full adjustment range.... Say your trigger logic
essentially pulses the triac at some fixed time after the voltage zero
crossing (i.e. the typical RC phase shift/DIAC trigger network).  This sets
the "turn on time" for the triac.  However, the "turn off time" is
determined not by the voltage across the load, but when the current goes to
zero.  At 90 degrees phase lag (totally inductive load.... unloaded NST),
this could be well into the next half cycle, so if your trigger phase is set
to less than 90 degrees after zero crossing, the triac may never turn
off....

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Saturday, July 07, 2001 7:44 PM
Subject: Re: Report - Dimmer as a Variac.


> Original poster: "David Sharpe by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<sccr4us-at-erols-dot-com>
>
> Terry, All
>
> OUCH!!!    =8*][.  What was surprising to me was the integration (PF)
capacitor
> made the situation MUCH MUCH worse.
>
> To make a true solid state Variac, it appears that the following
conditions
> must
> be met:
> 1.  Continuous current flow throughout the AC cycle.
> 2.  4 quadrant power flow (power to load, load to power line), without
blowups
>      or excessive EMI/RFI generation.
> 3.  Continuous power conversion at a considerably higher frequency then
line
>      frequency, that is forced commutated.  This condition unfortunately
> eliminates
>
>      SCR's and Triac phase control from consideration.  Also, significant
> tolerance
>
>      to momentary and sustained overloads without catastrophic control or
load
>      detonation.
> 4.  Condition 3 allows for high speed response for constant current,
constant
>      voltage and instantaneous fault conditions.  In fact, constant
current
> control, with
>      voltage as a secondary control loop is an extremely robust
controlling
> method
>      that is finding much industry favor of late.
> Several recent power electronics topology circuits have appeared that may
> be leveraged to this problem...
>