Hi all,
I am going to give some details on how to modify the common "classic"
circuit out there, so a SSR (solid state relay) can be used for cathode
switching when one is using the very common Level Shifted Power Supply
usually with a MOT.
This circuit does not work as is for a SSR. I did try even though what I
saw on scope I knew would not work, just to verify. Just the usual SCR or a
triac works for switching the cathode ground.
First I will go over some things and details I have learned the past couple
of years on interrupting VTTC's. Not going to cover everything, so here we
go. Probably still be messy to explain though, sorry :-\
Going to copy and paste a portion of a private email exchange I had
somewhat recently covering several things and this was one that is related
and was on my 304 VTTC:
"BTW, I mentioned at the end of this video about switching to the regular
triac I was also using at the time, a little while back, also to switch the
cathode for comparison, but this video was never shot. In initial first
testing of this new controller on a regular triac (before shooting some
video of this) in comparing to the solid state relay I was just using
here...it was initially working somewhat okay, but then a few late timing
"nasty snaps" then killed my brand new, just the day before finished
interrupter. Decided then, I am done destroying 555's and transistors in
all my 555 type interrupters I have. Finally one time too many, when
something goes too astray on this indirect method of zero crossover
detection. I have now myself have done enough various tests, that it works
much better sensing the real load using the SSR.
Last real run was when after I waited on some parts and then repaired this
new interrupter and just of course used the SSR only and thoroughly tested
and all was good, no video shot. Then shut down and start to dismantle for
enclosure project."
Video link again:
https://photos.app.goo.gl/vokvkCZXPWmvCSWm9
So when I tried using the triac, this controller still detects zero
crossover on normal mains AC sine wave. But the coil is using a level
shifter supply.
Sine wave form is now different. So controller is actually switching late.
"Hard" switching is occurring instead of staring at 0V. You also do not get
full advantage of the "ramp" available now since switching late. Sometimes
when you are changing especially the PRF (pulse repitition frequency) it
doesn't do this smoothly and it will "hiccup" I call it, a really late
firing...but passes and smooths out again at new rate. When you are unlucky
it will sometimes cause a harsh "kickback" and since the gate of the SCR or
triac is not really isolated...it will kill various components in your
interrupter.
Now when you use a SRR in conjunction after being set up properly, the SSR
will "correct" any late pulses by not firing at all, since it has true zero
crossover detection on the actual load itself. And there is the benefit of
optical isolation with our interrupter now also.
So this modification is really easy with an added benefit more control
options for running the VTTC. When I looked at the waveform on the scope,
when I was trying to figure best/easiest way to do this I looked at the
threshold/discharge capacitor on the 555 for PW (pulse width)(Or the PW
half if a 556). Immediately realized with just a little experimenting on
this capacitor, that I could achieve 3 things if done right in conjunction
with varying adjustment of the PW variable POT checking this.
1: I could cut out the coil from firing.
2: Adjust to fire a single pulse.
3: Obtain a large variable range of "bursts". (Successive pulses) (Another
note here is an EVR Staccato controller does burst modes by adding another
555 to the circuit, but this is not really necessary I see now.)
I am going to attach several pictures of this modification I did to a
somewhat fairly new German Staccato controller I put together earlier this
year as an example that could also be done to other "classic"
controllers.(I have already done this also to 2 other different ones I have
and they work as well.)
This controller is marketed by this fella for SSTC use.
I came across this accidentally on eBay. Saw the board and it caught my
eye. Recognized some things...then realized it is still basically the
classic circuit but done differently and a few minor changes. I knew this
would work for VTTC's. And the price is very reasonable for a kit.
I am not going to attach the complete manual or schematic. The seller sends
a temporary, for an amount of time, a link to download after purchase and I
will honor that.
I will just show what area I changed.
I want to metion here, that this classic based controller will work as is,
if you are going to run your VTTC on just regular mains AC. It will work
just fine. No ability to cut out most likely. And no bursts available
though as is.
Pictures of population of board and putting in a custom enclosure after
carefully locating holes. And gutting out an old GFCI plug housing to put
the small "AC trigger" transformer inside for a custom AC "wall wart". And
the DC "wall wart" I found in my stuff around here. The DC one is not
included in kit.
https://photos.app.goo.gl/3y35HPHsmQ1sHBdz9
Figuring out connections and scope the output. Does not look good for SSR
when level shifted supply:
https://photos.app.goo.gl/BMtcDh7kM7J4J1Nh9
Took apart to make the simple modification and scope the results. Ah...
perfect :^) for the SSR now:
https://photos.app.goo.gl/M9MR1WLdyoC53NNj9
This new minimum PW, the coil is cut out:
https://photos.app.goo.gl/DZSbAfT23FG7eKN18
This adjusted PW is about where the coil will now fire on the lever shifter
supply. (SSR "ignores" majority of PW except just "last little bit" and
fires at true zero crossover of level shifted supply.):
https://photos.app.goo.gl/vVJjNdywya4APC3H7
The new maximum PW with the now real long burst (Several successive cycles
and adjustable any where in between:
https://photos.app.goo.gl/zz4k98pqSWmCCjuB8
Just a note here for the not possibly obvious for some...but all scope
pictures are off of normal AC mains waveform. Checking how well it's own
zero crossover is doing for proper and smooth function when changes made to
PW and PRF. The SSR will "correct" output to true crossover.
Just "visualize" shifting whole sine wave straight up to zero all positive
for the lever shifter.
So as "long winded" as this is... actually very simple one capacitor change
to do after figuring everything out and understanding the different
behavior of the lever shifted supply wave form besides the well known
voltage increase. Also there is another way I could have done all this too,
but this way gives me the most desirable results.
Again see video link above and see successful SSR use.
Chris Reeland
Ladd Illinois USA
Sent from my Samsung Galaxy Tab® S