Re: [TCML] measuring current going through a MOT to be used as ballast

[David Speck <Dave@xxxxxxxxxxxxxxxx>]

Thomas,

You need a clamp-on ammeter for a temporary measurement, or a current 
transformer and a matching panel  ammeter for a more permanent installation.

Harborfreight.com and eBay have relatively inexpensive clamp-on ammeters.

eBay is a good source for current transformers.  Most industrial current 
transformers convert their rated input current to 5 amps AC output.  
Most matching industrial current meters show a full scale reading for 5 
amps AC input.  If you buy a 200:5 amp current transformer, it will 
output 5 amps when 200 amps are flowing through the center of the 
"donut".  If you have a meter scaled for 200 amps full scale, then it 
will indicate the current in your primary circuit.

Watch out for older industrial ammeters that have nonlinear scales or 
compressed scales.  They are less useful for your sort of application.  
Also beware of special purpose current transformers solely for high 
frequency use (usually tiny, as in quarter or nickel size), or CTs that 
have unspecified output.

For your Tesla use, a 50:5 or 100:5 CT would probably be entirely 
sufficient.  If you can't find precisely that value, you can multiply 
the sensitivity of a current transformer by running the primary wire 
through the core more than once.  E. G., if you have a 100:5 amp CT, and 
run the primary through the center hole twice, it will read 50 amps full 
scale. If you run the wire through 4 times, it will show full scale at 
25 amps.

For example:

http://cgi.ebay.com/64-301-Square-D-Current-Transformer-300-5-Ratio-600VAC_W0QQitemZ380193191712

is a 300:5 current transformer

and

http://cgi.ebay.com/SIEMENS-600-AC-AMPERES-AMP-AMMETER-METER_W0QQitemZ270427435495

is a nice, inexpensive linear scale 600 amp full scale ammeter.

If you run your primary wire through the CT 5 times, it will output 5 
amps for 60 amps in the primary circuit.  Mentally insert a decimal 
point in the scale readings in the meter so it is reading 60.0 amps full 
scale, and you are in business.  (no connection to any of the sellers).

If you intend to use the current measuring setup for a running TC, then 
it is best to avoid electronic meters as they give erratic readings in a 
high RF environment.

Word of warning:  Never pass current through a current transformer that 
doesn't have its meter attached, or else its output terminals shorted 
(Yes, I know it's counterintuitive!). A current transformer "wants" to 
put out a certain number of amps, and will generate near infinite output 
volts in order to accomplish that end if it is not properly terminated.  
This can destroy the CT or make a severe shock hazard.

You will find many LEM current transducers on eBay.  These versatile 
devices use Hall effect detectors, and can measure both AC and DC 
current in isolation, without contact.  Only downside is that they 
require +/- 15 volts DC power inputs, and give an output voltage that is 
proportional to the current passing through them.  They are better 
suited to driving an oscilloscope than a panel meter, but they do come 
in handy for some applications.

HTH,

Dave

> I tried to measure the current going through the primary of an
> open-secondary MOT, to see how if I could use it to limit current in another
> MOT in series. It must have been a lot - it blew my 10A fuse. How can I
> measure this?
>
> Many thanks
>
> Thomas
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla