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Re: progress report (ballast) (calibrating current probes)
Calibrating the probe is fairly easy. You need a good regular ammeter or
voltmeter and a resistive load and your variac. Calibrate the probe
against the ammeter by setting the variac to various settings. In general,
by the way, unless your ferrite is saturating, it will be quite linear.
The problems you get, if any, will be electrostatic leakage and capacitive
coupling (the scope input is high impedance), and droop on a square wave.
----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: progress report (ballast)
> Date: Friday, March 10, 2000 2:59 AM
>
> Original Poster: "bob golding" <yubba-at-clara-dot-net>
>
> Hi All,
> I have been experimenting with one of my inductor cores and the
> windings from an old auto transformer I found at the Scrapyard. I
carefully
> read Richies excellent article on his new ballast inductors, and checked
> the turns ratio to core size to my core . My core area turned out to be 6
> Square Inches to Richies 8.75" . As Richie had 134 turns I worked out I
> would need about 200 to maintain the same ratio. I checked the auto
tranny
> coils and they had 200 turns of 3.5mm square magnet wire. I dragged the
> whole tranny home, all 60 KGs of it, and set about removing the
laminations
> one by one so as not to damage the windings . Also I was reluctant to
take
> it out of the back of the van as I knew I couldn't move it once it was on
> the floor. This only took a day or so. Once I had the coils off I checked
> that they fitted my core . They fitted perfectly.! ah a miracle. I then
> coupled up an ammeter and volt meter to the variac, and tried various
air
> gap sizes while checking the current at 10 volt intervals. I started
with
> no gap ,and got 1.5 amps at 250 volts. I tried larger and larger gaps and
> noted the current up to 10 amps. The lines look really smooth with no
sign
> of saturation. I had to stop at 10 amps as it was 2.00 AM and it was
rather
> loud with all the loose laminations. This evening I tried the ballast out
> with my 2.2 kW radar tranny running a Jacobs ladder, and it works fine
and
> draws 12 amps with everything running cool except my meter leads. I have
> made a current probe by winding 100 turns of 22 AWG around a small
ferrite
> ring with a 1 ohm resistor across it, and looping one turn of the lead
to
> the ammeter though it. This gives a nice 100 millivolt signal to the
scope
> at 12 amps. When I find my 10 x scope probe I will see what the phase
angle
> between the voltage and current looks like with different air gaps. It
all
> seems to be working out fine at the moment. I have put four lengths of
> threaded rod though the angle brackets holding the core together so all
I
> have to do to adjust the air gap is slacken off 4 nuts and fit in
different
> thickness' of poly depending on the current required. I did try the
welder
> under the same conditions but it gets very hot at 12 amps. I might try
using
> the welder after the ballast for fine tuning but It is so easy to just
> change the gap spacing I might not bother. I was worried about
circulating
> currents around the threaded rod but no sign of warming after 10 minutes
of
> Jacob's ladder at 2800 watts. Can't wait to try it out on the coil. Any
help
> on calibrating the current probe welcome. It would be nice if it had a
> linear response between 1.5 and 15 amps. I will write this all up with
> photos and put it up on a web site when I have more time.
>
> cheers
> bob golding
>
>