One
of our popular renewable energy demonstrations is a simple device consisting
of a coil with a large number of turns of thin wire connected to two LED's,
red and green opposite ways around. When a large magnet from an old speaker
is moved towards it the red LED lights, and when it's moved away again
the voltage reverses polarity and the green one lights instead showing
how alternating current is generated.
Unfortunately the original version of this device uses
a focus coil from a 1950's television set - a nice coil but not very available
nowadays. Recently I needed to get a better magnet for it. Although there
are some nice ferrite ring magnets in loudspeakers, I thought it would
be interesting to get one out of the magnetron from a microwave oven for
a change. Whilst thinking about the idea, I realised that a microwave also
contains a high voltage step up transformer that produces about 2500V to
run the magnetron valve. It occurred to me that the secondary coil of the
transformer would have a large number of turns and if it could be extracted
without damage it might be possible to build a new version of the "big
coil and magnet" demo entirely out of modern parts.
I should point out that you shouldn't attempt to copy
what I've done unless you've got some background in electricity or electronics
and understand what's dangerous and what isn't - there are some more entry-level
projects in the DIY pages. This section of the site is just an account
of the things that happen in my workshop.
Someone like a school science teacher could do something
similar and produce a useful and safe energy demonstration at the end.
The microwave I got helpfully included a circuit diagram, this is the interesting bit. I'm sure there are plenty of explanations of how a microwave works if you search about on the net, but basically the magnetron is an old fashioned thermionic valve. It's got a cathode in the middle which gets hot and emits electrons. The anode is around the outside and connected to the positive side of 2500 Volts, so the electrons are strongly attracted to it and would shoot off towards it very fast in a straight line. They are stopped from doing that by a pair of magnets which create a very strong field inside so that instead they tear round in a circle. Electrons are very small and light so they go round at about two and a half thousand million times a second, producing the microwave radiation (about the same frequency as mobile phones but much more powerful).
It's worth taking notice of the warning plate, but with an understanding of how the thing works. For a start it only produces microwave radiation when it's running, so a dead one off the rubbish tip has no radiation hazard. Bits of metal that have been exposed to microwaves aren't radioactive or emitting anything, in the same way that shining a powerful light on them wouldn't make them luminous afterwards. There is a very considerable danger from the 2500 Volt power supply when it's on. The high voltage should go away within less than a minute once it's switched off as the storage capacitor has a built in resistance to discharge it, but you shouldn't assume that it's working properly.
With the case taken off the oven, here's the view into
the right hand side. The potentially dangerous
bit is the capacitor down at the bottom right. The first thing to do is
to deal with the possible hazard from the capacitor. If you're thinking
of having a go yourself and get an oven that's very different or don't
recognise the bits, just put it back in the rubbish and get another one!
The capacitor needs to be short circuited - I've
used a lead with crocodile clips on each end. When doing anything like
this I use one hand and keep the other in a back pocket. It's an old radio
ham trick for working on dangerous equipment. If something fell over and
you tried instinctively to grab it with both hands, hopefully the extra
time needed to extract one would give you time to stop yourself. A shock
from one hand to the other tends to result in cardiac arrest.
Remove the capacitor and wrap
wire round its terminals or solder a link across it before disposing of
it.
Having got rid of the high voltage capacitor, it was safe to extract the useful bits. The transformer has a high voltage warning but is incapable of storage so once the thing's on the bench with the Mains lead cut off it's safe.
Getting the magnets out of the magnetron was easy - the
end cover was just held on by a couple of tabs easily sawn
through. The pretty pink coloured bits on the ends of the magnetron
chamber contain beryllium, a particularly nasty substance, though it's
part of a sort of ceramic so it's OK as long as you don't shatter it.
After getting the magnets out,
they had picked up a load of metal swarf - gaffer tape is good for cleaning
it off. It's probably worth saying something about hazards associated with
the magnets. They're very powerful and could take a piece out of your finger
or at least a nasty pinch if they snap together. I chamfered the edges
of mine with the angle grinder as they were quite sharp. They should be
kept well away from things like credit cards with a magnetic strip and
anything with a hard drive in it like laptops or iPods.
With the easy bit out of the way, the next problem was
to get the secondary coil out of the middle of the transformer without
damaging the delicate thin wire.
The transformer core consists of 'E' and 'I' shaped bits
of soft iron. Old transformers were often glued together and it was almost
impossible to separate them but modern ones have just the edges of the
laminations joined with a shallow TIG weld. Here's
a close up of the join. The welds were cut out
using an
angle grinder being very careful to
keep it away from the windings - one slip would have trashed them instantly.
After removing the 'I' sections, the primary
coil came out easily which was a pity as it's the high voltage secondary
one that's really wanted.
The 'E' sections were fairly well stuck together so it
was necessary to hold the transformer in a vice
and carefully drive them out with a small chisel, initially one at a time
for the first few and then in groups of about ten. This was quite tricky
- they had to go out straight to avoid cutting into the delicate coil.
Finally after about half an hour I'd dismantled the whole
core and extracted the coil. The tape covering it was a bit chewed
up but a test with a resistance meter proved I'd still got a coil.
Finally, I cleaned the rest of the tape off and mounted the coil on a board connected to a pair of LED's to complete the AC generator demonstration.
Some of the leftover stuff might also be useful for educational
purposes. The soft iron laminations are interesting as they're strongly
attracted to magnets but impossible to permanently magnetise, and the primary
coil might have applications for an electromagnet but only on low voltage
as it's only got a resistance of a couple of Ohms.
