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Re: Tesla and Measurements.
Greetings
Mark wrote:
> While
> experimenting with the FET coil, which was single tuned (no primary
> cap), it was found that the number of primary turns was critically
> important. Too many and you could not get the power in, too few and
> you blew breakers (or worse). However, the primary was VERY closely
> coupled.
Mark could you give me some details of the primary and secondary of
your FET coil. I just begining to see beyond solving the electronics
problems and looking at the next set of problems which is getting
my head around coils, and coupling. My present primary is just
9 turns of twin core flex round a plastic bucket, using both cores in
series. (I found that using just one core the current protection came
on very early limiting the amount of power I could put into the coil.
Frustrating but better than blowing FETs. (Secondary 1600 turns
0.4mm wire on a 4" pipe, 210khz resonance).
I did some thought to the induction of the primary and therefore
the rate at which current will rise in it but of course the presence of
the secondary knocks it all out.
I've gather that for FET coils the vertical helix is the correct shape,
and that it should run nearly the full length of the secondary, leaving
a clearance at the top slightly bigger than the expected spark.
But what radius, I feel my present 12" primary, 4" secondary is wrong
and I'm thinking about 8", or even a trumpet bell shape (can't spell
paraholic) starting very close to the secondary and opening out -
I'm also considering having two // primary coils and using them in
push pull. (hence the twin core wire - it seemed to work at low
powers.
I know that Full Bridge is more efficent but there is double
the expense in a full bridge, and twice as much to blow!
I made one accident discovery I forgot to mount the capacitor in
the snubber network and my output FETs ran cooler - I thought I
had done the right thing putting the snubbers as close to the FETs
as possible but the heat was obviosly traveling down the resistor
leg accross the circuit board strip to the FET, dah! Obvious
isn't it.
I have got two irf740's running at 150v, 1A and I can draw a
3" arc out of the toriod. (I know that is just a spark gap to
most of you). I tried increasing the current limit and they
blew! Replaced them and I'm adding another 6 FETs
on slave boards, change the snubber layout and mount them
about an inch above the board and increase the resistance.
I have a big snubber now accross the coil 840ohm plus
4000pf (6kv ceramics - caps seriously derate at high
frequencies. So I'm thinking interms of just 10k and 200pf
next to the FETs. And big fast diodes clamping the end of
the primary coil to both power rails.
Spent the afternoon in Birmingham (UK), at an electronics
trade show. Some companies are sniffy about dealing
with Joe public so I set myself up as "Alan Sharp Services"
(- well I do baptismal services, marriage services, funeral
services.....). Discovered Coil Craft have a division over
here so I have a source for high frequency transformers
(1000W) if I want to go back to bottom feeding.
Lots to do but it will have to wait until next day off -
probably after Easter.
Have fun,
Alan Sharp