Re: MOT Measurements

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

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>



Tesla list wrote:
> 
> Original poster: "Matthew Smith by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <matt-at-kbc-dot-net.au>
> 
> Hi All
> 
> I was hoping to be able to report success with a potential divider that
> I have just built for my DMM, which allows me to measure transformer
> output up to 7kV (Vout=Vin/10), but...
> 
> Despite having even labelled the wretched thing, I managed to connect
> the DMM probes across the wrong terminals.  There are now several tracks
> missing from the PCB inside my DMM - completely fried; it's going to be
> a long job with silver loaded paint to repair that!
> 
> Nevertheless, if anyone wants instructions for a cheap ($15 Australian),
> simple and safe (although not idiot-proof) voltage divider, let me know
> and I'll post construction details and design notes on my site.
> 
> Now, a question...  It looks like the MOT that I was testing is putting
> out about 6kV unloaded; is this normal?  Second question: what would
> make a good test load for the secondary to get a truer idea of the
> output voltage? 

Assuming you want something like a 1 kW load at 4 or 5 kV (i.e. 200 mA),
you need a bunch of 10-100 ohm power resistors.  The usual wirewound power
resistors are good for about 1-2 kV per inch of length, although one should
be prepared for catastrophic failures.  In a test lash-up though, they'll
work just fine.

Nichrome wire (scavenged from a toaster or hair dryer, for instance) can
also serve as a nice high power load, and you can mount it in such a way
that breakdown isn't an issue.  A typical 1200W hair dryer draws around 5
Amps (on 240V power), and is, therefore, comprised of a 48 ohm resistor.
Many hair dryers have a hi/lo setting which connects the elements in series
or parallel (say, a pair of 100 ohm elements, in series = 200 ohms (low
power), in parallel =50 ohms (high power).

Salt water also makes a fine high power resistor, although controlling the
resistance for precision is tricky, and finding suitable electrodes is
sometimes tough.

Carbon or carborundum rods




 I know the technique of reversing the transformer (feed
> secondary, measure primary) but I want to do things this way round as an
> academic exercise.  Any pointers would be appreciated.
> 
> Cheers
> 
> Matthew Smith