DC Current Transformer
winding, which cancels the effect of the DC current in the
Remember, it is the product of current multiplied by
the number of turns that is balanced. A small current in a
large number of turns can be used to cancel a large current
in a small number of turns. All of the current in the main
winding also flows through R1, so measuring the average
DC voltage across R1 tells us the average DC current in
the main winding. If we know the number of turns in both
windings, we can find the DC current in the sense winding.
As an example, imagine that a wire carrying 100
amps DC is put through the center of the toroid. This
forms a one-turn sense winding. The added magnetizing
force is 100 amps through one turn, which equals 100
amp-turns. Suppose the main winding has 200 turns. To
cancel 100 amp-turns in the sense winding, the DC current
in the main winding must be (100 amp-turns)/(200 turns)
= . 5 amp DC (in addition to the AC oscillator current). If R1
is 2 Ω, . 5 amp produces a 1.0-volt average. The scale factor
is 100 amps per volt; i.e., there is 1.0 VDC across R1 with
100 amps DC in a one-turn sense winding. Figure 2 shows
how this is accomplished in an actual operating circuit.
The simple comparator detailed in the section above
has been implemented using an LM311 integrated circuit,
followed by an inverting buffer to supply the coil with current. Since the buffer inverts the signal, the +IN and -IN
terminals of the comparator are swapped. The buffer
consists of complimentary emitter-followers that drive
complementary power MOSFETs in an inverting configuration. The emitter-followers present a high impedance to
the comparator, but source enough current to rapidly
charge the gate capacitance of the powerFETs. A string of
four series diodes offsets the voltage to the gate of each
powerFET so that the powerFETs can't turn on at the same
time (which would put a short across the power supply).
The power supply provides ± 5 V to operate the circuit.
Three-terminal voltage regulators are used, which have
built-in current and thermal protection, in case of a fault.
When the coil core saturates, the current drawn suddenly
increases, so large electrolytic capacitors are used on
the ± 5 V lines to stabilize the voltage during the current
spikes. With large capacitors on the regulator outputs, a
protection diode is connected across each regulator to
keep reverse current out of the chip when the AC power is
A value of 1.0 volt was selected to represent 100
amperes, since it makes the meter reading easy to interpret.
The feedback resistors, R2/R3, are selected so that the
comparator trip voltage is ±2 V. This was arbitrarily picked,
being twice the maximum 1.0-volt average DC voltage
expected across R1. It is well within the common-mode
range of the comparator, but much larger than the worst-case input offset voltage of the comparator. A network is
included with a potentiometer which allows compensation
of the comparator offset voltage; it is labeled “ZERO” on
the schematic, since it is used to make the meter read
zero, with no current being measured.
Since R1 is 2 Ω, the current at the trip voltage is 1