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DC Supply for Mark
SNIP
I don't get this. Surely the two NST secondaries, centre-tapped are
*in-phase* with each other. There can only be 3 phases, to have 6 would
imply phase shifting by the transformer ????
Imagine using paired MOTs on each of the 3 phases, running the secondaries
in series ....... why would they be out-of-phase ?
The real question is "what to do with the centre taps". Jim Lux, who
started this question, thinks I should still ground them.
Hi Mark, ALL
Anytime you run a multiphase setup you WILL have to rectify each AC unit
before parallel or series connecting them. You have two ways to do this.
First of all you can use one diode (or set of if the v rating isnīt high
enough per diode) in each of the outer transfomer legs. These two diode sets
are connected together at the cathodes for plus. The output is this "double
diode connection" and the center tap (-). This makes a full wave rectifier
which would give you half the voltage (15kv/2*1.4) and double the current (ie
30mA NST would result in 60mA current). The three neons (each on one phase: R
S T) can be connected in parallel after the rectifier to further increase the
current. The only disadvantage I can see (aside from half the voltage) would
be that each diode must have a PIV (peak inverse voltage) of at least twice
the peak output voltage. This is not the case in a 4 diode full wave
rectifier.
Second possibility would be to use a 4 diode full wave rectifier. This lets
you connect the outer terminals to the AC in (rectifier) and parallel all
three NST after the rectifier (+ to + and minus to minus). This lets you take
advantage of the full voltage (15kv * 1.4) at the current stamped on each NST
(times 3 for your setup). The centertap is left alone in this setup. If you
were to connect all ctīs and earth them, you would superimpose a phase shift
of 120° (x 3 for 360°) on the outputs (before the rectifier). This could lead
to a very large current draw on the diodes and burn them out.
So, Mark, Itīs really up to you. If you need current go for the first. If you
need voltage and / or donīt have high voltage dioides (for a 15kV NST you
would need at least 30kV HV diodes for suggestion #1) go for suggestion two.
Oh, I donīt remember if you or someone else asked about protection. If you can
find HV MOVīs use these across your rectifier. Otherwise you will have to use
saftey gaps and bypass caps. I would definately avoid chokes because they can
superimpose very high voltage on your rectifier and this would kill them. You
could also connect several HV diodes in parallel and / or series and put them
across the DC output (antiparallel, of course). If you have good diodes, they
should be able to catch any backward running current.
Hope it helped,
Coiler greets from Germany,
Reinhard