KA2803B_11 Datasheet, PDF (2/11 Pages) Fairchild Semiconductor – Earth Leakage Detector

Block Diagram
Application Circuit
Figure 1. Block Diagram
Figure 2. Full-Wave Application Circuit
Figure 3. Half-Wave Application Circuit
Application Information
(Refer to full-wave application circuit in Figure 2)
Figure 2 shows the KA2803B connected in a typical
leakage current detector system. The power is applied
to the V_CC terminal (Pin 8) directly from the power line.
The resistor R_S and capacitor C_S are chosen so that Pin
8 voltage is at least 12V. The value of C_S is
recommended above 1µF.
If the leakage current is at the load, it is detected by the
zero current transformer (ZCT). The output voltage
signal of ZCT is amplified by the differential amplifier of
the KA2803B internal circuit and appears as a half-cycle
sine wave signal referred to input signal at the output of
the amplifier. The amplifier closed-loop gain is fixed
about 1000 times with internal feedback resistor to
compensate for zero current transformer (ZCT)
variations. The resistor R_L should be selected so that
the breaker satisfies the required sensing current. The
protection resistor R_P is not usually used when high
current is injected at the breaker; this resistor should be
used to protect the earth leakage detector IC
(KA2803B). The range of R_P is from several hundred
to several k.
Capacitor C₁ is for the noise canceller and a standard
value of C₁ is 0.047µF. Capacitor C2 is also a noise
canceller capacitance, but it is not usually used.
When high noise is present, a 0.047µF capacitor may be
connected between Pins 6 and 7. The amplified signal
finally appears at the Pin 7 with pulse signal through the
internal latch circuit of the KA2803B. This signal drives
the gate of the external SCR, which energizes the trip
coil, which opens the circuit breaker. The trip time of the
breaker is determined by capacitor C₃ and the
mechanism breaker. This capacitor should be selected
under 1µF to satisfy the required trip time. The full-wave
bridge supplies power to the KA2803B during both the
positive and negative half cycles of the line voltage. This
allows the hot and neutral lines to be interchanged.
© 2000 Fairchild Semiconductor Corporation
KA2803B • Rev. 1.0.7
2
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