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Grounding LOPT/Flyback secondaries

Thread starter #1
When building a HV power supply should one ground the anode (or cathode) of an FBTs secondary output?
Or neither?

I've been using the saintsmart ZVS/FBT combo to drive arcs and "vacuum" discharges.
Neither the ZVS nor the FBT has a ground connection so the outputs of both are floating.

My first concern is regarding using oscilloscopes:
I can measure the potential difference of the FBT output using a HV probe (/1000) and
a voltmeter but I also endeavoured to analyze it using my scope. Somewhat naively
I hooked up my TDS-2014B using generic P220 probes through the HV probe and got
some readings. Fortunately everything worked out fine but should it have? Because ...

The scope instructions say "don't float measured values more than 30V above ground". I just
measured the potential between the FBT anode and my house ground and (using voltmeter
plus said /1000 probe) and it was ~100V.

Why didn't this fry my scope? (other than it being "high quality w/ overvoltage protection" :)

My guess is that when I had my rig hooked up to the scope it ended up
having it's anode grounded and the 100V floating potential vanished.​

My second concern is just a matter of curiosity regarding the meaning of ground.
I ran my rig, ZVS+FBT+1cm arc, with the FBT anode grounded w/o problems.
However, when I grounded the cathode (red wire out of FBT), things went bad.
Things go "bad" even if I keep the FBT output circuit "open" and just measure the
voltage.
I actually have a hard time characterizing the "bad". The first time I had
a nik in the wiring of the FBT primary (external, not the FBT original) find a path
through the core to the FBT anode and burn up the primarys insulation.
The second time, when I only had the probe/voltmeter hooked up, I got misc
unlocalizable spark sounds.​

Why this asymmetry? If the outputs of the FBT secondary are floating why should
it matter if I ground the anode or the cathode?
If anything, it's the anode that has the physical excess of electrons so grounding the
cathode should be the logical choice.
 
#2
You do not need to ground it for driving tube. Only if you need some ground referenced output.

The transformer has built in diodes and other windings which will probably cause the issues you describe when grounding the cathode. It is not really designed to be a cathode and anode, but rather a cathode and a HT Return. The insulation and winding arrangements will be such that the HT Return is physically closer to other windings and connections and can not sustain high voltage without something breaking down.
 
Thread starter #3
Thanks,

Right, the grounding is more for the sake of the scope.

I hadn't looked at the schematic for the FBT in a while and I see what you mean by ground-return. It's a BSC25-T1010A.
The end-points of the secondary are the HV cathode and ABL (Automatic Blamking/brightness Limiter). (I wonder how the ABL
returns to ground in a proper TV). Then the divider network for focus and screen outputs does have a ground return which it
shares with the HT primary except that in my case none of the primaries pins are connected and I have my own primary
winding on the core. The overheating was near the GND and HT pins as evidenced by cracks in the epoxy.

So my guess now is that my customized primary induces a current in the secondary which induces a current in the
built-in primary which shares a ground with the secondary (albeit through the divider network) and that
can't be good.
 

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