Re: High Power Dimmer, SS Relay

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

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Dave,

Neat!!

There is another big advantage for those of use who have helped load/unload
the big 240VAC variac controllers.  It would weigh far far less!!  That may
justify the solid state variac all by itself!!  I wonder if it is possible
to make solid state choppers or whatever to control current to act as a
ballast as well.

Cheers,

	Terry


At 03:45 PM 7/6/2001 -0400, you wrote:
>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
>