PRODUCT SPECIFICATION

RV4145A

4

REV. 1.0.5 12/7/04

Principles of Operation

The 26V shunt regulator voltage generated by the string of

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:

or minus 7.2V relative to pin 3 the SCR Trigger output will

go high and fire the external SCR.

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

applications will require substituting a 47k

Ω 2W resistor. In

addition to supplying power to the IC, the shunt regulator

creates internal reference voltages (see above).

Operational Amplifier

procedure: apply the desired fault current (a difference in

upward until the SCR activates. A fixed resistor can be used

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 2

Ω 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

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,

(609) 964-7842, and Magnetics, 900 E. Butler Road,

P.O. Box 391, Butler, PA 16003, (412) 282-8282 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.