DC Current Transformer
but won't fit through the small hole in the toroid.
The main winding of each coil was terminated with an
eighth-inch stereo phone plug. This allows different ones to
be easily attached to the adapter. The coil winding is
connected to the “tip” and “ring” terminals, since neither
end of the coil is grounded. The grounded “sleeve”
terminal is left unconnected.
Calibrating the Adapter
The number of turns on the coil and the value of R1
are directly linked. For example, if the resistance of R1 is
1% high, you compensate by increasing the number of
turns on the coil by 1%. Once the adapter is built, wind the
coil, but leave enough extra wire to be able to add several
more turns, if needed.
For calibration, you need an accurate DC source in the
range of 20 to 50 amp-turns. Wind a sense coil on the
toroid, using hook-up wire that fits the amount of current
you have to calibrate with. For example, 1 amp in 20 turns
or 2 amps in 10 turns both give the same 20 amp-turns
bias to the coil's core. With 20 amp-turns in the sense
winding, the external voltmeter across R1 should read 200
mV. If the voltage is 1% too low, reduce the number of
turns on the main winding by 1%. Add turns to the main
winding if the external voltmeter reading is too high. Each
turn added or removed affects the reading by about 0.5%,
so you should be able to calibrate the readings within 0.5%.
Figure 5 shows a plot of data from one of the coils. It
shows good linearity over the entire range. Once the turns
are adjusted during calibration, many different cores can
be used with the same adapter.
A note of caution: when you use sense windings with
more than a few turns, the coupling of the sense winding
to the main winding can cause ringing. This can make the
oscillator have multiple transition bursts at one switching
point and ruin the calibration. Whenever you use sense
windings with many turns, add a second external coil in
series with the sense winding to suppress the ringing. A
good choice for the external coil is a winding with the same
number of turns as the sense winding, but on a separate
core. You can check for this waveform with a scope at the
Using the Adapter
Using the adapter is straightforword; you put the wire
carrying the DC current to be measured through the
center hole of the toroid and read the value on an external
voltmeter. There are, however, things you should be aware of.
Circle #144 on the Reader Service Card.
Circle #120 on the Reader Service Card.