Re: Multi-Mini Transformers

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Stephen Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>

At 21:34 19/06/03 -0600, you wrote:
>Original poster: "Jolyon Vater Cox by way of Terry Fritz 
><teslalist-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>
>
>Has anyone on the List ever used a "multi-mini transformer" approach to 
>implementing a high-voltage power supply for TC use?
>
>
>But looking at this again it appears there may a problem with the 
>interwinding/ core insulation
>which may be insufficient to withstand the high voltages involved; for 
>with a (non-grounded) voltage of 9600V on the secondary and 240V on the primary
>the interwinding insulation on the transformers nearest to the ends of of 
>the "stack" would be stressed by a maximum of (9600-240)/2 =4680 volts. 
>Are mains transformers likely to be rated to withstand this?



I just got out my Rapid Electronics catalogue and it lists a whole bunch of 
mini transformers from 1.5VA to 30VA, all vacuum potted in epoxy and 
claiming 5kV primary/secondary isolation. Therefore you could generate up 
to 10kV centre tapped, if you had the time/money/patience to wire up 42 of 
these little dudes in series :) Using the 3.3VA units, you would get about 
12mA output. And the transformers cost 3.74 GBP each plus VAT in 40 
quantities. Hardly seems worth it since you could buy a brand new NST for 
the same money :(

On the other hand, if you were to make a DC supply, you could put a voltage 
doubler on the secondary of each transformer (as Jim Lux suggested) and get 
2*sqrt(2) times the bang per buck. Then you could use DC resonant charging 
to double the voltage again. Hence you could get a 10kV charging voltage 
from 8 transformers, which is a bit more attractive. By centre tapping the 
output, all transformers would be safely within their isolation ratings.

Since we only need 8, we could go for a bigger transformer, like the 30VA 
12+12V: 240V unit costing 9.50 GBP. 8 of these would give at least 240 
watts output continuous and could probably be pushed to at least twice that 
on short duty. You would connect all the 12V windings in series and hook 
them to the 240V line.

This would make a pretty mean tabletop coil <evil cackle> but again by the 
time you have bought 8 trannies, 16 450V electrolytic caps, 16 diodes, and 
two charging chokes, you'll have spent more than 100 GBP and got about the 
power of a 12/30 NST.

Or you could try the little 3.3VA 24+24V:240V units, at 4.15 GBP each. An 
ignition coil would make a perfect charging choke for a little setup like 
this. We are talking bug zapper power though.

http://www.rapidelectronics.co.uk/

Steve C.