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/vokvkCZXPWmvCSWm9So 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/3y35HPHsmQ1sHBdz9Figuring out connections and scope the output. Does not look good for SSR when level shifted supply:
https://photos.app.goo.gl/BMtcDh7kM7J4J1Nh9Took apart to make the simple modification and scope the results. Ah... perfect :^) for the SSR now:
https://photos.app.goo.gl/M9MR1WLdyoC53NNj9This new minimum PW, the coil is cut out:
https://photos.app.goo.gl/DZSbAfT23FG7eKN18This 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/vVJjNdywya4APC3H7The new maximum PW with the now real long burst (Several successive cycles and adjustable any where in between:
https://photos.app.goo.gl/zz4k98pqSWmCCjuB8Just 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
