1 www.fairchildsemi.com FAN1851A Ground Fault Interrupter Features Description Improved performance over industry equivalents The FAN1851A is a controller for AC outlet ground fault Tight fault current range (Typ 100A) interrupters. These devices detect hazardous grounding con- Temperature compensated fault current characteristics ditions (example: a pool of water or an electrical equipment No external trimming required connected to opposite phases of the AC line) in consumer Direct interface to SCR and industrial environments. The output of the IC triggers an Supply voltage derived from AC line26V shunt external SCR, which in turn opens a relay circuit breaker to Adjustable sensitivity prevent a harmful or lethal shock. Grounded neutral fault detection Meets UL943 standards Full advantage of the U.S. UL943 timing specification is 450A quiescent current taken to ensure maximum immunity to false triggering due Ideal for 120V or 220V systems to line noise. A special feature in the circuitry rapidly resets Package options: 8L DIP and 8L SOIC the integrating timing capacitor in the event that noise pulses introduce unwanted charging currents. Also, a flip-flop is included that ensures firing of even a slow circuit breaker relay on either of the two half-cycles of the line voltage when external full wave rectification is used. The application circuit can be configured to detect both normal faults (hot wire to ground) and grounded neutral faults. Block Diagram Timing Sensitivity Sense Amplifier +V Capacitor Set Resistor Output S I TH I D3 2 I for I > 0 TH F I = Q2 3I for I = 0 TH F SCR Trigger I F Q3 Latch Q1 D1 Q5 +V A1 S Q4 D2 I F 10V Ground Inverting Input Non-Inverting Input REV. 2.0.1 6/17/05FAN1851A PRODUCT SPECIFICATION Pin Assignments SCR Trigger 1 8 +V S Input 2 7 C T + Input 3 6 R SET Ground 4 5 Amp Out Functional Description The voltage at the supply pin is clamped to +26V by the where I is the value of current set by the external R TH SET internal shunt regulator D3. This shunt regulator also resistor. If fault signals are present at the input of A1 (which generates an artificial ground voltage for the noninverting is held at virtual ground, +10V), one of the two current input of A1 (shown as a +10V source). A1, Q1, and Q2 mirrors in the feedback path of A1 (Q4 and Q5) will become together act as a current mirror for fault current signals active, depending on which half-cycle the fault occurs. (which are derived from an external transformer). When a This action will raise the voltage at V , switching I to a S 1 fault signal is present, the mirrored current charges the value equal to I , and reducing the discharge rate of C to TH T external timing capacitor until its voltage exceeds the latch better allow fault currents to charge it. trigger threshold (typically 17.5V). When this threshold is exceeded, the latch engages and Q3 turns off, allowing I to Notice that I discharges C during both half-cycles of the 2 TH T drive the SCR connected to theSCR Trigge pin. line, while I only charges C during the half-cycle in which F T I exits the- Inpu pin (since Q1 will only carry fault cur- F Extra Circuitry in the feedback path of A1 works with the rent in one direction). Thus, during one half-cycle, I -I F TH switched current source I to remove any charge on C charges C , while during the other half-cycle I discharges 1 T T TH induced by noise in the transformer. If no fault current is it. present, then I discharges C with a current equal to 3 I , 1 T TH Definition of Terms Normal Fault: Grounded Neutral Fault: An unintentional electrical path, R , between the load termi- An unintentional electrical path between the load terminal of B nal of the hot line and the ground, as shown by the dashed the neutral line and the ground, as shown by the dashed lines lines in Figure1. in Figure 2. Hot Hot Hot Hot Line GFI R GFI R R LOAD LOAD Line B Neutral Neutral Neutral Neutral R IN R R G G Figure 1. Normal Fault Figure 2. Grounded Neutral Fault 2 REV. 2.0.1 6/17/05