Re: Ammeter Question

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

Original poster: "Peter Terren" <pterren1-at-iinet-dot-net.au> 

Hi Rob
I agree it is a simple current transformer. Did I get lucky - really now.
Actually I figured that I would not saturate provided I was getting out less
than the previous mains voltage (250 in Oz) but I needed a moderate voltage
to negate the bridge rectifier drop. 1 1/2 turns seemed to do it ok with the
meters I had. I also agree that the capacitor might not be needed but there
may be a lot of hash and spikes on the line so I figured a bit of averaging
would be fine as long as it didn't slow down the meter response noticeably.
I recall that there is some minor non linearity compared with my 3 1/2 digit
RMS AC/DC clamp meter which I can always use if needed for accurate
measurements.
Cheers
Peter (Tesla Downunder)
http://members.iinet-dot-net.au/~pterren

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Tuesday, January 13, 2004 8:59 AM
Subject: Re: Ammeter Question


 > Original poster: "Robert Jones" <alwynj48-at-earthlink-dot-net>
 >
 > Hi, Peter,
 >
 > That may be a quick way of making a current transformer. The usual way a
 > current transformer is configured is by loading it with an ammeter for
 > minimum output voltage hence minimum magnetization current so that the
turns
 > ratio accurately reflects the current ratio. Did you design it to work or
 > just get lucky.  The only limitation I could come up with in your
 > configuration was the voltage across the existing winding must be say less
 > than half its normal operating voltage to avoid saturation.  Because of
the
 > way a current transformer works the capacitor is probably not needed as it
 > will read average magnitude regardless not peak as the presence of the
 > capacitor would suggest.  How linear it would from your calibration point
I
 > don't know but it will give comparative measurements in your circuit..
 >
 > Bob
 >
 >  >
 >  > To make an ammeter, I used the smallest mains transformer I could buy
or
 >  > scrounge. eg old 9 v power pack. Remove the low voltage secondary
windings
 >  > and discard.  Wrap 1 1/2 to 2 turns of heavy wire in place of the
 > secondary.
 >  > When current passes through the heavy winding a voltage gets induced in
 > the
 >  > other winding such as 10 - 50 VAC. depending on the transformer.  This
is
 >  > rectified by a bridge rectifier and smoothed with a small eg 1uF
 > capacitor.
 >  > This can be dropped with resistors to a suitable voltage for whatever
 > spare
 >  > voltmeter you have or scale you are using. Calibrate it with a device
of
 >  > known wattage eg fan heater and there you have it. Stepping up the
voltage
 >  > in this way removes most of the non-linearity created by the fixed
voltage
 >  > drop with the bridge rectifier. I've used this technique to keep mains
 > power
 >  > away from the meters in my control board.
 >  >
 >
http://members.iinet-dot-net.au/~pterren/tesla_coil_latest.htm#The%20variac%20co
 > ntrol%20box
 >  > Cheers
 >  > Peter (Tesla Downunder)
 >
 >
 >
 >