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M54125L

Earth Leakage Current Detector

Renesas Electronics 

M54125L

Leakage Detection Function

When a leakage current Ig occurs on the primary side of ZCT (a zero-phase current transformer), a leakage signal

voltage VILK appears in the secondary-side output, and is input to the ILK input pin, with the bias pin VB as reference.

During the half-cycle in which VILK is negative, in the interval in which the voltage is equal to or below the DC trip

voltage VLKT, the capacitor CMM connected to the pin MM is charged.

In this case, if the pin MM voltage VMM does not reache the MM positive-direction threshold voltage, when the

charging interval ends the capacitor CMM is discharged with a current larger than the charging current. If the pin voltage

VMM reaches the MM positive-direction threshold voltage, at this time the output OUT is "ON" (the state in which

output current is flowing out), and at the same time a small current is discharged from the capacitor CMM. When VMM

falls to the MM negative-direction threshold voltage, the output OUT is returned to "OFF" (the state in which output

current flows in).

That is, the leakage current for which the dead time tMM coincides with the interval during which the amplitude of the

input voltage VILK is equal to or greater than the DC trip voltage VILK becomes the sense current of the ground-fault

breaker.

The output OUT is "ON" for the length of time tOUT, and the output current turns on the thyristor, effectively causing

the breaker to operate.

Abnormal Voltage Detection Function

Normally, the rectified voltage of an AC power supply with uniform amplitude is voltage-divided and input to the

abnormal voltage input pin IBL as the voltage VIBL. In this case, if the neutral line N is interrupted, depending on the

load states of the voltage lines A and B, the input voltage VIBL may become a rectified voltage with amplitude

alternating between high and low.

At this time, the capacitor CWF connected to the pin WF discharges during the interval in which the amplitude of VIBL is

equal to or greater than the abnormal voltage trip voltage VBLT. (When the discharge interval ends, the charging interval

begins.) When the WF pin voltage VWF falls below the WF threshold voltage, the capacitor CTD of pin TD begins to

charge, and after the delay time tTD has elapsed, when the TD pin voltage VTD reaches the TD threshold voltage, the

output OUT is turned "ON", effectively causing the breaker to operate.

When single-shot noise is superposed on the input voltage VIBL causing it to rise above VBLT, the original state is

restored after approximately time tWF, thus preventing erroneous operation.

Current flows in this abnormal voltage detection unit to enable operation only when an external resistor RX is connected

to pin RX.

Rev.1.0, Sep.16.2003, page 4 of 10

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