Re: Ammeter Question

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

Original poster: David Speck <dave-at-davidspeckmd-dot-org> 

Aaron,
The most common approach to measuring higher AC currents is to use a 
current transformer (CT) to drive an AC Ammeter.  A CT keeps the high 
voltage primary currents away from your front panel and eliminates the need 
to carry heavy high current wires up to the panel as well.

A current transformer is a doughnut shaped device with two output 
terminals.  Most industry standard current transformers produce 5 amps 
output for their stated input current rating, and then standard panel AC 
ammeters read 5 amps full scale.  You run one wire of your primary circuit 
(the circuit you're are actually measuring) through the center of the 
current transformer and connect the two terminals of the current 
transformer output to your 5 AC amp panel meter.  Ideally, you can find a 
panel meter whose full scale readings correspond to the full scale output 
of your  current transformer, but you can make your own scale, or convert 
in your head if you don't find a perfect match.

Most current transformers are specified by their output ratio, something 
like 50:5 or 100/5., meaning that they will try to put out 5 amps for a 50 
amp input in the first case, or 5 amps for a 100 amp input in the second.

eBay is a great source of current transformers.  I just searched for 
"current trans*" and found 44 hits, some as low as $3.00 each.  You would 
also have to find a matching 5 amp full scale AC Ammeter, but there are 
plenty of those available as well.

A little trick with CTs is that you can make them more sensitive if you 
want.  [Making them less sensitive is not practical.]  For example, you 
might buy a really cheap 100:5 transformer, but find that you are only 
measuring about 20 amps most of the time.  If you loop your primary wire 
around and pass it though the CT doughnut twice, the CT sees twice the 
current, and now reads as a 50:5 CT.  Pass it through three times, and it's 
a 33.3:5 CT, 4 times, and it's a 25:5 CT, and so on.  You can start small, 
and take out turns as your tesla coils get bigger.

One warning:  Current transformers can generate very high output voltages 
if there is no load across their outputs.  Do not pass current through the 
sensing hole of a CT without a properly matched load (AC Ammeter) connected 
to the CT, or you may burn out the CT or get a nasty shock.

Note that current transformers only measure AC currents, probably what you 
need for your coiling work.  Also available on eBay are LEM current 
transducers.  These look like CTs, but use a Hall effect magnetic sensor 
and require +/- 15 volts DC (most of the time) for power.  LEM sensors can 
measure both AC and DC current, but you must have a meter setup which can 
either rectify the AC component to display it, or show the DC 
component.  Probably more complex than you want to get into right now, but 
they are out there for future reference.

HTH,
Dave

>Where can I find an AC ammeter to go inline with my hot power supply? I
>would like to be able to measure the amps the transformers are drawing.