Re: Tesla coil from a single microwave oven (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

Moderated and approved by: Gerry Reynolds <greynolds@xxxxxxxxxx>



---------- Forwarded message ----------
Date: Sun, 24 Dec 2006 13:05:05 +0900
From: Peter Terren <pterren@xxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Tesla coil from a single microwave oven

I have completed a Tesla coil made from the components of one microwave 
oven.  This was set up by me as a competition with a $50 prize on the 4hV 
forum.
http://4hv.org/e107_plugins/forum/forum_viewtopic.php?15477.0
but no-one one won it and my entry was 9 days late so I didn't either.

Here's how it was done:
Power supply is the MOT. This needs current limiting and this is achieved 
with a 150nF capacitor and the voltage is boosted by a one stage multiplier 
using the MOT diode to give 6kV peak. Note that power is taken off across 
the diode not across the cap to get the voltage multiplier effect. Hey 
what's this about a 150nF cap? Where is that in a microwave oven? Well that 
is the key to this project. The 1uF 10kV DC mylar and oil cap (that is in 
all non-inverter MO's) needs to be taken apart and unwound. Lengths of the 
multilayered dielectric and foil are taken out and cut to length. It took 
about 10 feet of the windings to make 150nF. This is perhaps only 30% of the 
total. New electrodes were added.
Tank cap. This is a 1nF cap. The technique is similar but I retained the 
original electrodes from the cap to use in this section as they were better 
suited to a high current. Construction of this cap was different in view of 
the much higher voltage/current and frequency stresses. It used about 30% of 
the length of the cap windings. The plates were only about 1 foot long by 2 
inches, however I used two sections of foil/dielectric between them. The 
foil was not connected and was kept to equalise voltage stresses. All in all 
something like 12 layers of mylar and 2 foil layers separate the main 
electrodes.
Being mylar they do warm up a bit.
Spark gap This is two aluminium electrodes (from the frame of the MO fan) 
bolted to part of the MO timer circuit board with all the components and 
printed circuit ground off. It really needs another section to give better 
quenching.
Primary is the MO transformer 6V winding which is heavy copper. This is 
stuck on to the 3 legged wheeled frame that the glass food tray sits on and 
rotates.
Secondary former is the cardboard internal cardboard insulation from the HV 
cap. It was washed but became a bit soft but still usable.
Secondary windings use wire from the fan motor and less than half was used. 
The hot glue didn't stick the cardboard former together properly and the 
windings slipped and overlapped. I have had some interturn shorting (and may 
still have) so I can't really push the power much without a rewind. No idea 
how many turns.
Toroid I did try winding lots of turns of wire on the fan blades but it 
didn't improve the performance. so I just left the metal mesh spacer that 
sits on the output of the magnetron for a nice colour.

Pics show the plan and sparks close up. Max sparks in low power testing were 
1 inch but performance has dropped probably due to tuning or a shorted turn. 
I can't push the secondary further due to the overlap.  Not fabulous 
performance but the proof of principle is there are plenty of opportunities 
to do better.

All the pics on my site here:
http://tesladownunder.com/Solid%20State.htm#Tesla%20coil%20made%20from%20a%20microwave%20oven

Peter