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RV4141A

Low Power Two-Wire Ground Fault Interrupter Controller

Fairchild Semiconductor 

Circuit Operation

(Refer to Figure 1 and Figure 3.)

The precision op amp connected to pins 1 through 3

senses the fault current flowing in the secondary of the

sense transformer, converting it to a voltage at pin 1.

The ratio of secondary current to output voltage is

directly proportional to feedback resistor, RSET.

RSET converts the sense transformer secondary current

to a voltage at pin 1. Due to the virtual ground created at

the sense amplifier input by its negative feedback loop,

the sense transformer's burden is equal to the value of

RIN. From the transformer's point of view, the ideal value

for RIN is 0Ω. This causes it to operate as a true current

transformer with minimal error. However, making RIN

equal to zero creates a large offset voltage at pin 1 due

to the sense amplifier's very high DC gain. RIN should be

selected as high as possible, consistent with preserving

the transformer's operation as a true current mode

transformer. A typical value for RIN is between 200 and

1000Ω.

As seen in Equation (1), maximizing RIN minimizes the

DC offset error at the sense amplifier output. The DC

offset voltage at pin 1 contributes directly to the trip

current error. The offset voltage at pin 1 is:

VOS  RSET /(RIN  RSEC )

(1)

where:

VOS = Input offset voltage of sense amplifier;

RSET = Feedback resistor;

RIN = Input resistor;

RSEC = Transformer secondary winding resistance.

The sense amplifier has a specified maximum offset

voltage of 200μV to minimize trip current errors. Two

comparators connected to the sense amplifier output are

configured as a window detector, whose references are

-6.5V and +6.5V, referred to pin 3. When the sense

transformer secondary RMS current exceeds 4.6/RSET,

the output of the window detector starts the delay circuit.

If the secondary current exceeds the predetermined trip

current for longer than the delay time, a current pulse

appears at pin 7, triggering the SCR.

The SCR anode is directly connected to a solenoid or

relay coil. The SCR can be tripped only when its anode

is more positive than its cathode.

Supply Current Requirements

The RV4141A is powered directly from the line through

a series-limiting resistor called RLINE; its value is

between 24k and 91k

The controller IC has a built-in dioderectifier, eliminating

the need for external power diodes.The recommended

value for RLINEis 24kto 47k for110V systems and

47kto 91kfor 220V systems. WhenRLINEis

47kthe shunt regulator current is limited to3.6mA.

The recommended maximum peak line currentthrough

RLINE is 10mA.

GFCI Application

(Refer to Figure 3)

The GFCI detects a ground fault by sensing a difference

in current in the line and neutral wires. The difference in

current is assumed to be a fault current creating a

potentially hazardous path from line to ground. Since the

line and neutral wires pass through the center of the

sense transformer, only the differential primary current is

transferred to the secondary. Assuming the turns ratio is

1:1000, the secondary current is 1/1000th the fault

current. The RV4141A’s sense amplifier converts the

secondary current to a voltage compared with either of

the two window detector reference voltages. If the fault

current exceeds the design value for the duration of the

programmed time delay, the RV4141A sends a current

pulse to the gate of the SCR.

Detecting ground-to-neutral faults is more difficult. RB

represents a normal ground fault resistance. RN is the

wire resistance of the electrical circuit between load/

neutral and earth ground. RG represents the ground-to-

neutral fault condition. According to UL 943, the GFCI

must trip when RN = 0.4Ω, RG = 1.6Ω, and the normal

ground fault is 6mA.

Assuming the ground fault to be 5mA, 1mA, and 4mA

goes through RG and RN, respectively, causing an

effective 1mA fault current. This current is detected by

the sense transformer and amplified by the sense

amplifier. The ground / neutral and sense transformers

are mutually coupled by RG, RN, and the neutral wire

ground loop, producing a positive feedback loop around

the sense amplifier. The newly created feedback loop

causes the sense amplifier to oscillate at a frequency

determined by ground/neutral transformer secondary

inductance and C4, which occurs at 8KHz.

C2 is used to program the time required for the fault to

be present before the SCR is triggered. Refer to

Equation (2) for calculating the value of C2. Its typical

value is 12nF for a 2ms delay. RSET is used to set the

fault current at which the GFCI trips. When used with a

1:1000 sense transformer, its typical value is 1MΩ for a

GFCI designed to trip at 5mA.

RIN should be the highest value possible that ensures a

predictable secondary current from the sense

transformer. If RIN is set too high, normal production

variations in the transformer permeability causes unit-to-

unit variations in the secondary current. If it is too low, a

large offset voltage error at pin 1 is present. This error

voltage in turn creates a trip current error proportional to

the input offset voltage of the sense amplifier. As an

example, if RIN is 500Ω, RSET is 1M, RSEC is 45 and

the VOS of the sense amplifier is its maximum of 200μV;

the trip current error is ±5.6%.

© 2003 Fairchild Semiconductor Corporation

RV4141A • Rev. 1.0.8

5

www.fairchildsemi.com

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