www.fairchildsemi.com
RV4141A
Low Power Ground Fault Interrupter
Description
The RV4141A is a low power controller for AC receptacle
ground fault circuit interrupters. These devices detect
hazardous current paths to ground and ground to neutral
faults. The circuit interrupter then disconnects the load from
the line before a harmful or lethal shock occurs.
Internally, the RV4141A contains a diode rectifier, shunt
regulator, precision sense amplifier, current reference, time
delay circuit, and SCR driver.
Two sense transformers, SCR, solenoid, three resistors and
four capacitors complete the design of the basic circuit inter-
rupter. The simple layout and minimum component count
insure ease of application and long term reliability.
Features not found in other GFCI controllers include a low
offset voltage sense amplifier eliminating the need for a
coupling capacitor between the sense transformer and sense
amplifier, and an internal rectifier to eliminate high voltage
rectifying diodes.
The RV4141A is powered only during the positive half
period of the line voltage, but can sense current faults inde-
pendent of its phase relative to the line voltage. The gate of
the SCR is driven only during the positive half cycle of the
line voltage.
Features
•
•
•
•
•
•
•
•
•
•
Powered from the AC line
Built-in rectifier
Direct interface to SCR
500 µA quiescent current
Precision sense amplifier
Adjustable time delay
Minimum external components
Meets UL 943 requirements
For use with 110V or 220V systems
Available in 8 pin DIP or SOIC package
Block Diagram
RV4141A
Amp Out
–
+
+
V
FB
–
+
+
–
V
REF
Delay
–
4.7K
+V
S
SCR
Cap
Gnd
Line
65-4141-01
REV. 1.0.1 7/8/03
PRODUCT SPECIFICATION
RV4141A
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
SET
.
R
SET
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 R
IN
. From the
transformer's point of view, the ideal value for R
IN
is 0
Ω
.
This will cause it to operate as a true current transformer
with minimal error. However, making R
IN
equal to zero cre-
ates a large offset voltage at Pin 1 due to the sense amplifier's
very high DC gain. R
IN
should be selected as high as possi-
ble consistent with preserving the transformer's operation as
a true current mode transformer. A typical value for R
IN
is
between 200 and 1000
Ω
.
As seen by the equation below, maximizing R
IN
minimizes
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:
V
OS
x R
SET
/(R
IN
+ R
SEC
)
Where:
V
OS
= Input offset voltage of sense amplifier
R
SET
= Feedback resistor
R
IN
= Input resistor
R
SEC
= Transformer secondary winding resistance
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 -6.5
volts and +6.5 volts referred to Pin 3. When the sense trans-
former secondary RMS current exceeds 4.6/R
SET
the output
of the window detector starts the delay circuit. If the second-
ary current exceeds the predetermined trip current for longer
than the delay time a current pulse appears at Pin 7, trigger-
ing 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.
The recommended value for R
LINE
is 24 k
Ω
to 47 k
Ω
for
110V systems and 47 k
Ω
to 91 k
Ω
for 220V systems. When
R
LINE
is 47 k
Ω
the shunt regulator current is limited to
3.6 mA. The recommended maximum peak line current
through R
LINE
is 10 mA.
GFCI Application
(Refer to Figure 1)
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 iine 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
B
repre-
sents a normal ground fault resistance, R
N
is the wire resis-
tance of the electrical circuit between load/ neutral and earth
ground. R
G
represents the ground to neutral fault condition.
According to UL 943, the GFCI must trip when R
N
= 0.4
Ω
,
R
G
= 1.6
Ω
and the normal ground fault is 6 mA.
Assuming the ground fault to be 5 mA, 1 mA and 4 mA will
go through R
G
and R
N
, respectively, causing an effective 1
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
G
, R
N
and the neutral wire ground loop, producing a posi-
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
SET
is used to set the fault current at which the GFCI trips.
When used with a 1:1000 sense transformer, its typical value
is 1 M
Ω
for a GFCI designed to trip at 5 mA.
R
IN
should be the highest value possible which insures a
predictable secondary current from the sense transformer.
If R
IN
is set too high, normal production variations in the
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
IN
is 500
Ω
,
Supply Current Requirements
The RV4141A is powered directly from the line through a
series limiting resistor called R
LINE
, its value is between
24 k
Ω
and 91 k
Ω
. The controller IC has a built-in diode
rectifier eliminating the need for external power diodes.
4
REV. 1.0.1 7/8/03
RV4141A
PRODUCT SPECIFICATION
R
SET
is 1 M
Ω
, R
SEC
is 45
Ω
and the V
OS
of the sense ampli-
fier is its maximum of 200 µV, the trip current error is
±5.6%.
The SCR anode is directly connected to a solenoid or relay
coil. It can be tripped only when its anode is more positive
than its cathode. It must have a high dV/dt rating to ensure
that line noise (generated by electrically noisy appliances)
does not falsely trigger it. Also the SCR must have a gate
drive requirement less than 200 µA. C3 is a noise filter that
prevents high frequency line pulses from triggering the SCR.
The relay solenoid used should have a response time of 3 ms
or less to meet the UL 943 timing requirement.
Calculating The Values Of R
SET
and C2
Determine the nominal ground fault trip current requirement.
This will be typically 5 mA in North America (117V AC)
and 22 mA in the UK and Europe (220V AC). Determine the
minimum delay time required to prevent nuisance tripping.
This will typically be 1 to 2 ms. The value of C2 required to
provide the desired delay time is:
C2 = 6 x T
where:
C2 is in nF
T is the desired delay time in ms.
The value of R
SET
to meet the nominal ground fault trip cur-
rent specification is:
4.6
×
N
R
SET
= ---------------------------------------------------------------
I
FAULT
×
COS 180
(
T
⁄
P
)
where:
R
SET
is in k
Ω
T is the time delay in ms
P is the period of the line frequency in ms
I
FAULT
is the desired ground fault trip current in mA RMS
N is the number of sense transformer secondary turns.
This formula assumes an ideal sense transformer is used.
The calculated value of R
SET
may have to be changed up to
30% to when using a non-ideal transformer.
Sense Transformers and Cores
The sense and ground/neutral transformer cores are usually
fabricated using high permeability laminated steel rings.
Their single turn primary is created by passing the line and
neutral wires through the center of its core. The secondary is
usually from 200 to 1500 turns.
Magnetic Metals Corporation
1900 Hayes Ave.
Camden, NJ 08105
(856) 964-7842
Is a full-line suppliers of ring cores and transformers
designed specifically for GFCI and related applications.
R
TEST
Mov
Sense Transformer
1:1000
5 Ring Steel Core
Phase
Line
Neutral
15K
Press to
Test
Normally Closed
Latching Contacts
Grounded Neutral
1:200
Load
Solenoid
C4
R
N
0.4
R
B
20K
R
IN
470
1000 pF
C1
10 nF
C2
1
8
7
2
3
4
12 nF
C
F
+
1µF 35V
1N4004
Q1
TAG
X0103DA
C3
10 nF
R
G
1.6
Fault
Resistance
Not Part of
Application
R
SET
1.1 Meg
R
LINE
24K
1W
RV4141A
6
5
GFCI
65-4141A-03
Note:
1. Portions of this schematic are subject to U.S. patents 3,878,435 and Re. 30,678.
Figure 1. GFI Application Circuit
REV. 1.0.1 7/8/03
5
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