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On 6/21/18 10:02 PM, Raymond Spigot wrote:
Hi, thanks for all the interesting discussion about NST testing and safe
grounding, it's all been instructive for a noob
I have this grey resin block neon transformer in the shed I was thinking of
experimenting with. It's a 15kv/30mA one with a midpoint ground
connection. Is there a way to rectify the output so I can use it for
trying out powering a dc tesla?
I have a big bag of diodes (1N5408 I think) that were pretty cheap. If
they'll handle 1000v each I could solder them up into long strings and use
4 strings of diodes to make a bridge rectifier? I guess I'd need a
capacitor on the other side of the bridge as well though to smooth it out a
bit.
Then I was wondering if there was another way to do it with a midpoint
grounded transformer like mine. I'm thinking the 2 secondary windings on
the transformer must be out-of-phase, but could I rectify them
individually? Between each HV output and the midpoint ground I mean. Then
I'm thinking I'd need 2 bridges but they're only trying to rectify half the
voltage each and it would be easier to cope with.
Basically I'm hoping there's a way to get like 10kv dc out instead of
closer to (I think) 20kv Because then I can buy a main MMC capacitor that
doesn't have to be rated for the highest voltage (so I can put less caps in
series)
hope this makes some sense to someone! thanx for advice anyway
Ray
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A single bridge works, ignore the midpoint ground. Just remember that
you can't ground either of the DC outputs of the bridge.
I wouldn't go with long, long strings of 1n5408. If you do, you need to
use something like 30 of them in series, so you'd need 120 to make your
bridge.
You're MUCH better off getting something designed for the purpose. There
are 6kV diodes designed for HV rectifiers that are fairly inexpensive.
They typically are designed with good reverse recovery characteristics.
Put 5 of those in series (20 total), sink it oil for cooling/insulation,
and you're good to go.
If you can find some 25 or 30kV diodes (they're typically little plastic
blocks or sticks) even better. Microwave oven diodes are generally not
HV enough (they're rectifying a few kV)
if you ARE going to series lots of diodes, make sure they're from the
same mfr lot (if they came on tape, they're probably good). You don't
need to do the resistors or capacitors in parallel with each diode thing
- that was what folks did when manufacturing consistency wasn't great
and reverse recovery times and leakage current would vary.
The thing you worry about in series strings is when the diodes start to
turn off. The first one to turn off takes all the voltage. Now, on a
sine wave circuit, the reverse voltage isn't applied instantaneously, so
you've got time for the slower diodes to turn off before full reverse
voltage is applied.
Note that this is not the case for some other HV applications,
notoriously TV horizontal flyback.