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LM1851

GROUND FAULT INTERRUPTER

Unisonic Technologies 

UTC LM1851

LINEAR INTEGRATED CIRCUIT

APPLICATION CIRCUITS

In practice, the actual value of C1 will have to be modified to include the effects of the neutral loop upon the net

charging current. The effect of neutral loop induced currents is difficult to quartile, but typically they sum with normal

fault currents, thus allowing a larger value of C1.

For UL943 requirements, 0.015μF has been found to be the best compromise between timing and noise.

For those GFI standards not requiring grounded neutral detection, a still larger value capacitor can be used and

better noise immunity obtained. The larger capacitor can be accommodated because RN and RG are not present,

allowing the full fault current, I, to enter the GFI.

In Figure 2, grounded neutral detection is accomplished by feeding the neutral coil with 120 Hz energy

continuously and allowing some of the energy to couple into the sense transformer during conditions of neutral fault.

TYPICAL APPLICATION

LOAD

HOT

NEUTRAL

MOV

CIRCUIT

BREAKER

GND/NEUTRAL COIL

200:1

0.01/400V

HIGH µ COIL

SENSE COIL

1000:1

LINE

10 μF

+ TANT

SCR

15K/2W

0.01/400V

0.01

* Adjust RSET for desired sensitivity

7 TIMING

CAP

1 SCR

TRIGGER

5 OP AMP

Ct

OUTPUT

0.0158

VCC

2

-IN

3

+IN

RSET 6

GND 4

RSET*

+ 1.0μF

TANT

0.0033

200 pF

FIGURE 2. 120Hz Neutral Transformer Approach

UTC UNISONIC TECHNOLOGIES CO., LTD. 7

QW-R122-007,A

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