IL4145A

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 reference to create an artifical ground of +13V

at the op amp noninverting input.

Figure 1 shows a three-wire 120V AC outlet GFI

application using an IL4145AN. Fault signals from the

sense transformer are AC coupled into the input and are

amplified according to the following equation:

V7 = RSENSE x 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

transformer 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 47k

Ω 2W

resistor. In addition to supplying power to the IC, the

s

h

u

n

t

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

frequency 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 have a gate drive requirement of less than

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

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.

Two-Wire Application Circuit

Figure 2 shows the diagram of a 2-wire 120V AC outlet

GFI circuit using an IL4145AN. 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.