Re: DC power

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

Original poster: "LWRobertson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <LWRobertson-at-email.msn-dot-com>

Hi Chris

I've been experimenting with a charging system
much like yours. One thing to think about is that
all the charging has to take place while the arc
is still lit, which means for normal gap speeds
there is only a few hundred microseconds to get
your cap charged. A good reason to stay away
from big charging inductors, at least for this application.

I am using a 42 mH air core inductor wound with
#22 wire, 12.5 inches in diameter and 12 inches long.
It limits charge current to 40 amps at 20kV. A diodes
peak repetitive pulse current is about 1/4 to 1/5 of the
non-repetitive peak usually listed in the spec sheet,
so the charging diode should be somewhat robust. I'm
using an 8 Amp RMS, 400 A. peak,  48 kV diode with
no failure yet.

Works quite well, too - the cap is charged to almost
2 X the supply voltage. At the moment I'm fighting
with flashovers at the gap at higher power.

Cheers
Larry Robertson



----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Thursday, February 28, 2002 11:09 AM
Subject: DC power


> Original poster: "Chris Swinson by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <exxos-at-cps-games.co.uk>
>
> Hi all,
>
> I've been trying to work out a normal Tesla system but converting AC from
> the NST to DC. This basically works but with some problems which have left
> me a bit clueless.
>
> The first tests were done with a "out of sync" spark gap. To explain
better
> this spark gap is like a double throw switch. First the DC supply arc
across
> a gap to the tank cap. then that arc stops and the gap moves on and
connects
> the tank cap to the primary ( as normal ).  Then as the gaps move around
the
> DC supply is then connected back to the tank cap and so on...
>
> That basically works but where as before I could gain almost 20" with AC I
> can only hit about 4" with this set-up. I made the "charging" gaps as
close
> as possible to make sure there was as little loss as possible across the
> gaps. Which does seem to help. I did try a fixed contact to short the
supply
> to the tank gap but building this proved to be a lot harder so I had to
keep
> a spark gap here.
>
> I have talked to a few people and I was put onto DC resonate charging from
> richie B's website. This looked good but the large inductor needed is a
bit
> impossible as I worked it out to 1600H, or 200H at probable best. I read
> this as apparently you can't short 2 capacitors out and get them to
charge.
> This seems a little strange since the DC smoothing cap is 2MFD so it
should
> charge the small tank cap easily. But so it seems I was wrong to think
> this...
>
> Today I soldered a bucket load of resistors between the 2 capacitors as
this
> seemed to be plan B.  Its hard to say if it helped or not but with a lot
of
> messing around I could maybe hit 6". It seems best to leave the charging
> gaps across the resistors, though the resistors don't really seem to help
> that much. I have 200K between them, which according to calculation they
> should be 660K.  I am not sure if adding more resistance will help matters
> much. my thinking is that 200K will give about 50ma at 15KV or so which is
> what my NST was giving out.
>
> I now am a little lost to what to try next.  my only ideas which probably
> wont help much but...
>
> 1) use actual contacting gaps to short the DC supply to the tank cap.
> 2) the voltage might be lower due to the load of the smoothing cap.
> 3) try lower and higher resistors.
> 4) change tank cap value.
>
> I did try half size tank cap but that worked very bad, only about 1" spark
> then.  I might try a higher value tank cap, double value, but it worked
fine
> with that value on AC so it should be about right for DC power also. The
odd
> thing is that the primary tap has moved inwards half since running from
DC.
> Rotary speeds are a nightmare with DC.
>
> One good thing which has come out of it is the spark can go really high
> pitch and starts to become more flame like than sparks. I noticed this
with
> a car ignition coil driver that it sparks all over the place at one
> frequency but as it was turned up it turned into 1 flame like arc. Very
> interesting and when the Tesla coil rotary gap runs at a high speed I see
> this happening also. It also looks like it gains a extra inch or 2 in
> length. It does however start to pulse the sparks on and off rather than a
> constant arc.  I would love to push the gap further but it will probably
fly
> apart as my construction skills are not to great. But at least I have had
> something new to watch despite the lower power....... anyone got any
> suggestions apart from go back to AC :-) ?
>
>
> Chris
>
>
>
>
>
>
>