RV4145A Datasheet, PDF (4/11 Pages) Fairchild Semiconductor – Low Power Ground Fault Interrupter

PRODUCT SPECIFICATION
RV4145A
Principles of Operation
The 26V shunt regulator voltage generated by the string of
zener diodes is divided into three reference voltages: 3/4 VS,
1/2 VS, and 1/4 VS. VREF is at 1/2VS and is used as a refer-
ence to create an artifical ground of +13V at the op amp non-
inverting input.
Figure 1 shows a three-wire 120V AC outlet GFI application
using an RV4145A. Fault signals from the sense transformer
are AC coupled into the input and are amplified according to
the following equation:
V7 = RSENSE × ISENSE/N
Where V7 is the RMS voltage at pin 7 relative to pin 3,
RSENSE is the value of the feedback resistor connected from
pin 7 to pin 1, ISENSE is the fault current in amps RMS and
N is the turns ratio of the transformer. When V7 exceeds plus
or minus 7.2V relative to pin 3 the SCR Trigger output will
go high and fire the external SCR.
The formula for V7 is approximate because it does not
include the sense transformer characteristics.
Grounded neutral fault detection is accomplished when a
short or fault closes a magnetic path between the sense trans-
former and the grounded neutral transformer. The resultant
AC coupling closes a positive feedback path around the op
amp, and therefore the op amp oscillates. When the peaks of
the oscillation voltage exceed the SCR trigger comparator
thresholds, the SCR output will go high.
Shunt Regulator
RLINE limits the current into the shunt regulator; 220V
applications will require substituting a 47kΩ 2W resistor. In
addition to supplying power to the IC, the shunt regulator
creates internal reference voltages (see above).
Operational Amplifier
RSENSE is a feedback resistor that sets gain and therefore
sensitivity to normal faults. To adjust RSENSE, follow this
procedure: apply the desired fault current (a difference in
current of 5mA is the UL 943 standard). Adjust RSENSE
upward until the SCR activates. A fixed resistor can be used
for RSENSE, since the resultant ±15% variation in sensitivity
will meet UL’s 943 4-6mA specification window.
The roll-off frequency is greater than the grounded neutral
fault oscillation frequency, in order to preserve loop gain for
oscillation (which is determined by the inductance of the
200:1 transformer and C4).
The senstivity to grounded neutral faults is adjusted by
changing the frequency of oscillation. Increasing the fre-
quency reduces the sensitivity by reducing the loop gain of
the positive feedback circuit. As frequency increases, the
signal becomes attenuated and the loop gain decreases. With
the values shown the circuit will detect a grounded neutral
fault having resistance of 2Ω or less.
The input to the op amp are protected from overvoltage by
back-toback diodes.
SCR Driver
The SCR used must have a high dV/dt rating to ensure that
line noise (generated by noisy appliances such as a drill
motor) does not falsely trigger the SCR. Also, the SCR must
have a gate drive requirement of less than 200µA. CF is a
noise filter capacitor that prevents narrow pulses from firing
the SCR.
The relay solenoid used should have a 3ms or less response
time in order to meet the UL 943 timing requirement.
Sense Transformers and Cores
The sense and grounded neutral transformer cores are usu-
ally 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, Camden, NJ 08101,
[phone], and Magnetics, 900 E. Butler Road,
P.O. Box 391, Butler, PA 16003, [phone] are full line
suppliers of ring cores and transformers designed specifi-
cally for GFI applications.
Two-Wire Application Circuit
Figure 2 shows the diagram of a 2-wire 120V AC outlet GFI
circuit using an RV4145A. This circuit is not designed to
detect grounded neutral faults. Thus, the grounded neutral
transformer and capacitors C3 and C4 of Figure 1 are not
used.
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REV. 1.0.3 3/6/02