Building the Adapter
amp. This current is well within the rating of the
powerFETs, won't put a strain on the power supply, and
doesn't require heavy wire on the coil. At full scale, with
100 amps in a one-turn sense winding, there will be . 5
amp in the main winding, so use a power transformer with
at least a . 5 amp rating.
NUTS & VOLTS
I built my adapter (Figure 3) by mounting the
components on perfboard and soldering the connections
on the back, using the component leads and/or bus wire.
As seen in Figure 3, the adapter was built in a 5. 25 x 3 x
2.125 inch aluminum box (RadioShack p/n 270-238).
The power transformer (RadioShack p/n 273-1366) is
mounted in the center of the box. This leaves the ends for
mounting two perfboards on 1/2-inch spacers.
The oscillator section is on the right side of Figure 3.
The “ZERO” adjust pot stands up on the board in the
lower corner. There is a small hole drilled in the box to
allow adjustment of the pot. Leads to the external meter
exit to the right through a rubber grommet. An 1/8-inch
stereo phone jack (RadioShack p/n 274-249) is mounted
on the wall behind the oscillator. Its terminals are bent
over to avoid touching anything on the board.
The rectifier/regulator section is at the left side. The
voltage regulators are in TO-220 packages, attached to
the box for heatsinking. The + 5 V regulator has its metal
tab grounded, so it's screwed directly to the box.
The - 5 V regulator's metal tab is not at ground, so
insulating hardware is used between the regulator
and the box. A T1 size LED is used as a power
indicator. It is mounted on the perfboard, but its
body sticks partially through an 1/8-inch hole in
the box when the perfboard is screwed in place.
The heart of the design is the toroid coil.
Recall that well-defined trip points on the hysteresis
curve are crucial to good performance. This means
a core with high permeability and good “
squareness” is preferred, so the impedance change from
unsaturated to saturated is large and well-defined.
Tape-wound toroids and high-permeability ferrite
toroids are obvious choices. Both types will work.
Figure 4 shows some different coil designs.
The left coil has a 1-inch, tape-wound core
(Magnetics p/n 51061-1A). The other three have
5,000 permeability ferrite cores. The large coil on
the right has a 3.375-inch core. This is about the
biggest ferrite core available; look for these at
hamfests, where they are intended for use in
high-power antenna baluns. Each of the two coils
in the center has a .87-inch ferrite core. One uses
MMG/Krystinel K82 material and the other uses
Philips/Ferroxcube 3E2A material. I expect, but
haven't verified, that almost any toroid of 5000-
permeability ferrite material would work, as well.
Results were just as good with a lower permeability
material, such as 2,700; I experimented with
Philips/Ferroxcube 3C8 in a .87-inch toroid. The
coil shown at the bottom center of Figure 4 was
made with a one-turn sense winding of 10 AWG
connected to big lugs for those cases where you
want to connect to a wire that already has big lugs,
1/4W, 5% resistors
1 W, 5% resistor
LM311N voltage comparator
LM7805 (+ 5 V), LM7905 (- 5 V), TO-220
IRF9Z20 (P-channel), IRFZ20 (N-channel)
1N4001 (qty. 14)
Red, T-1 ( 3 mm)
330 Ω (qty. 3), 1.5K (qty. 2), 2.2K, 3K, 3.3K
0.1 µF, 50 V (qty. 2)
100 µF, 16 V (qty. 2), 220 µF, 25 V (qty. 2),
1,000 µF, 10 V
120 V: 25.2 VCT, . 45 A (RadioShack #273-
FT82-75 (Ocean State Electronics) or
FT87-75 (Surplus Sales of Nebraska)
0.100” spacing perfboard, 5. 25 x 3 x 2.125 metal enclosure
(RadioShack 270-238), rubber grommets, fuse ( 5 x 20 mm, 1 amp) and
fuse holder, toggle switch, banana plugs, 1/8-inch stereo phone plugs
(and jack), 8-pin DIP socket for LM311N, TO-220 mounting insulator
kit, 26 AWG magnet wire, 24 AWG bus wire, 24 AWG hook-up wire,
PC board standoffs (1/2-inch long), line cord.