rv4141 Fairchild Semiconductor, rv4141 Datasheet - Page 4

• Manufacturer Part Number: rv4141
• Description: Low Power Ground Fault Interrupt Controller
• Manufacturer: Fairchild Semiconductor

PRODUCT SPECIFICATION
Circuit Operation
(Refer to Block Diagram and Figure 1)
The precision op amp connected to Pins 1 through 3 senses
the fault current flowing in the secondary of the sense trans-
former, converting it to a voltage at Pin 1. The ratio of sec-
ondary current to output voltage is directly proportional to
feedback resistor, R
R
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 R
transformer's point of view, the ideal value for R
This will cause it to operate as a true current transformer
with minimal error. However, making R
ates a large offset voltage at Pin 1 due to the sense amplifier's
very high DC gain. R
ble consistent with preserving the transformer's operation as
a true current mode transformer. A typical value for R
between 200 and 1000 .
As seen by the equation below, maximizing R
the DC offset error at the sense amplifiers output. The DC
offset voltage at Pin 1 contributes directly to the trip current
error. The offset voltage at Pin 1 is:
Where:
V
R
R
R
The sense amplifier has a specified maximum offset voltage
of 200 µV 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/R
window detector starts the delay circuit. If the secondary cur-
rent 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 R
24 k and 91 k . The controller IC has a built-in diode
rectifier eliminating the need for external power diodes.
4
SET
SET
IN
SEC
OS
V
= Input resistor
= Input offset voltage of sense amplifier
converts the sense transformer secondary current to a
OS
= Feedback resistor
= Transformer secondary winding resistance
x R
SET
/(R
SET
IN
IN
+ R
.
should be selected as high as possi-
SEC
LINE
)
, its value is between
SET
IN
equal to zero cre-
the output of the
IN
IN
IN
. From the
minimizes
is 0 .
IN
is
The recommended value for R
110V systems and 47 k to 91 k for 220V systems. When
R
3.6 mA. The recommended maximum peak line current
through R
GFCI Application
The GFCI detects a ground fault by sensing a difference cur-
rent in the line and neutral wires. The difference 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 dif-
ferential 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 which is com-
pared with either of the two window detector reference volt-
ages. If the fault current exceeds the design value for the
duration of the programmed time delay, the RV4141A will
send a current pulse to the gate of the SCR.
Detecting ground to neutral faults is more difficult. R
sents a normal ground fault resistance, R
tance of the electrical circuit between load/ neutral and earth
ground. R
According to UL 943, the GFCI must trip when R
R
Assuming the ground fault to be 5 mA, 1 mA and 4 mA will
go through R
mA fault current. This current is detected by the sense trans-
former and amplified by the sense amplifier. The ground/
neutral and sense transformers are now mutually coupled by
R
tive 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. Typically it occurs at 8 KHz.
C2 is used to program the time required for the fault to be
present before the SCR is triggered. Refer to the equation
below for calculating the value of C2. Its typical value is
12 nF for a 2 ms delay.
R
When used with a 1:1000 sense transformer, its typical value
is 1 M for a GFCI designed to trip at 5 mA.
R
predictable secondary current from the sense transformer.
If R
transformer permeability will cause unit to unit variations in
the secondary current. If it is too low, a large offset voltage
error at Pin 1 will be present. This error voltage in turn cre-
ates a trip current error proportional to the input offset volt-
age of the sense amplifier. As an example, if R
LINE
G
G
SET
IN
, R
= 1.6 and the normal ground fault is 6 mA.
IN
should be the highest value possible which ensures a
N
is used to set the fault current at which the GFCI trips.
is set too high, normal production variations in the
is 47 k the shunt regulator current is limited to
and the neutral wire ground loop, producing a posi-
G
LINE
represents the ground to neutral fault condition.
G
and R
is 10 mA.
N
, respectively, causing an effective 1
(Refer to Figure 1)
LINE
is 24 k to 47 k for
N
REV. 1.0.6 6/30/05
is the wire resis-
IN
is 500 ,
N
RV4141A
= 0.4 ,
B
repre-