Re: Report - Dimmer as a Variac.

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Dave,
         The sort of approach I'd use would be offline active PFC 
followed by a PWM H-bridge directly feeding the load. Cycle by cycle 
current sensing/control would be a must. The PWM would of course be 
set by finger tip control (a crummy old carbon pot :). The above 
approach (together with the appropriate common-mode filters) should 
address the issues you raise below. Some differential filtering of 
the H-bridge output may be necessary - mandatory for motors to avoid 
micro-machining the shaft, but probably not necessary for running a 
NST (self-filtering). Am I reading your post correctly?

Regards,
Malcolm

On 7 Jul 01, at 20:44, Tesla list wrote:

> 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...
> 
> Back to the drawing board, let me see what I can come up with (with a
> little help
> from some colleagues...)
> 
> Regards
> 
> Dave Sharpe, TCBOR
> Chesterfield, VA. USA
> 
> > Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
> >
> > Hi All,
> >
> > I wired up the dimmer and hooked it to a 250 watt light bulb and got the
> > following voltage waveform:
> >
> > http://hot-streamer-dot-com/temp/250W-R.gif
> >
> > The dimmer is holding off the voltage turn on until what appears to be a
> > timer triggers it to turn on.  The dimmer remains conducting until the AC
> > reaches zero volts (perhaps amps).
> >
> > I then hooked the input of my small coil in parallel with the 250 watt
> > light and tried again:
> >
> > http://hot-streamer-dot-com/temp/250W-RandSTC.gif
> >
> > This is the voltage waveform I caught but the 2.5 amp fuse on the coil's AC
> > input protection blew right away.
> >
> > I wired in the Pearson current monitor and tried again with a 4 amp input
> fuse:
> >
> > http://hot-streamer-dot-com/temp/VandI.gif
> >
> > Looks like I am getting current spikes...  Big ones...  At first I though
> > the MOVs where clamping voltage spikes but the fuse is on the other side of
> > the MOVs and the peak voltage is lower than the MOV clamp voltage.  Also,
> > some of the lower voltage peaks are getting just as big of current spikes.
> >
> > I clicked the gain down and tried again to see how high the spikes are:
> >
> > http://hot-streamer-dot-com/temp/VanI-2.gif
> >
> > Only 58 amps!! =:o
> >
> > I suspected the 60uF of PFC caps are not getting along with the sudden
> > dimmer voltage turn on which is about 180 volts.  So I pulled the
> > connectors off the caps to take them out of the circuit and tried again:
> >
> > http://hot-streamer-dot-com/temp/NoPFC.gif
> >
> > Neat!  The fuses don't blow and the current and voltage are reasonable now
> > :-))  There are still 11.8 amp current spikes which may be due to the
> > capacitors in the small coils dual stage line filter:
> >
> > http://hot-streamer-dot-com/TeslaCoils/MyCoils/SmallCoil/small_ACwiring.jpg
> >
> > I can hear high voltage on the NST output with just the NST protection
> > circuit attached.  The input voltage spikes may be pumping high voltage
> > spikes onto the output too.  I will have to hook up the high voltage
> > fiber-optics to check that but that may not be a problem once it is hooked
> > to a primary cap.
> >
> > So PFC caps cause big current spikes with he dimmer's sudden turn.  The
> > output voltage seem to be mirroring the input signal giving high voltage
> > spikes on the NST output at a low power setting on the dimmer.  The line
> > filter may be also causing current spikes, but small ones.
> >
> > Sort of bad not to be able to use PFC caps but the output spikes should not
> > be a problem and the line filter perhaps can be moved before the dimmer or
> > those spikes just ignored.
> >
> > Some additional pics of the "stuff" are at:
> >
> > http://hot-streamer-dot-com/temp/P7070006.jpg
> > http://hot-streamer-dot-com/temp/P7070007.jpg
> > http://hot-streamer-dot-com/temp/P7070008.jpg
> >
> > Thanks to Brain for letting me use his fiber probe that I have back here
> > for some "factory repairs" ;-))  I left mine on and toasted the
batteries...
> >
> > The science goes on...
> >
> > Cheers,
> >
> >         Terry
> 
> 
> 
>