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High Power Dimmer, SS Relay



Original poster: "David Sharpe by way of Terry Fritz <twftesla-at-qwest-dot-net>" <sccr4us-at-erols-dot-com>

Terry, All

There has been an ongoing discuss of using dimmers and/or motor speed
controls for transformer voltage adjustment in lieu of a variac.  A recent
circuit was shown in EDN magazine (3/15/2001) using two SCR's in a
"phase-leg" configuration, with PUT's (programmable unijunction
transistors) providing necessary triggering.  The circuit was unique in that
it would work for incandescent as well as inductive loads, can be
optically isolated, and can operate as a phase control as well as a true on/off
SS relay.  If current and voltage feedback are provided, and an integration
capacitor is placed across primary of powering transformer, a quasi-adjustable
voltage AC active ballast system with high speed (<8ms) shutdown is technically
possible.  The mentioned circuit would provide about 80% of requirements
as listed above.  The cost of all components would be _significantly_  less
then an equivalent VARIAC, with enhanced circuit protection, in a much smaller
more serviceable package.

I much prefer SCR's over triacs in high power (read Tesla Coil) applications
because:
1. surge ratings are at least 10X better.
2. far more robust then a triac
3. higher temperature ratings
4. isolated devices (even in a "phase leg" or doubler configuration) are
available
5. voltage ratings of 1.4kV are available in isolated case, if you go hockey
    puck case (and a deep pocketbook), devices to 15kV -at- 1kA+ are
    commercially available.  Does require more expensive heat sink, cooling
    and HV insulation considerations at HIGH POWER (>50kW).
6. If input voltage ratings are held constant, SCR's are cheaper at same
current

    level as compared to Triacs.

I have schematic, will sketch up circuit in Excel and shoot to Terry to post on
web site...  Cost of all parts minus SCR is probably about $50-$70 for a
240 VAC

unit to control up to 20A.  Predominate cost above that will be heat sinks,
cooling
fan(s) and SCR's.  120 VAC service requires 400V SCR's,  240 VAC requires
800V devices, and 480 VAC apps (for us HIGH POWER junkies) would
need 1.2kV devices.  The circuit as shown is designed for currents of up to
100-200A according to author, but appropriate SCR(s) are up to designer to
select.  I would not exceed 50% Irms rating at full power for reliablity and
robustness.

Regards

Dave Sharpe, TCBOR
Chesterfield, VA. USA