RV4145AM datasheet(4/10 Pages) ARTSCHIP

4 / 10 page

RV4145A

Low Power Ground Fault Interrupter

www.artschip.com

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 1/2 VS and is used as reference

to create an artifical ground of +13V at the op amp

non-iverting 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 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 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 frequency, in order to preserve loop gain for

oscillation (which is determined by the inductance of the 200:1

transformer and C4).

The sensitivity 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 2Ω or less.

The input to the op amp are protected from overvoltage by

back-to back 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 200 µA. 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 UL943 timing requirement.

Sense Transformers and Cores

The sense and grounded neutral 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 line and

neutral wires through the center of its core. The secondary is

usually from 200 to 1500 turns.

Magnetic Metals Corporation, Camden, NJ08101, (609)

964-7842, and Magnetics, 900E. Butler Road, P,O. Box 391,

Butler, PA 16003, (412) 282-8282 are full line suppliers of ring

cores and transformers designed specifically for GFI

applications.

Two-Wire Application Circuit

Figure 2 shows the diagram of a 2-wire: 20V 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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