Figure 1 depicts a circuit that detects the opening of a miniature circuit breaker or high-rupture-capability fuse in a high-reliability telecommunications power supply. The circuit generates an alarm when a failure changes the impedance of an electromagnetic sensor. Traditional fault-detection circuits sense the voltage difference developed across an open fuse, leakage current flowing through a fused circuit, or closure of an auxiliary (volts-free) contact by an actuator fuse. All three methods suffer from disadvantages: Voltage-difference circuits can introduce unacceptable delays as long as 30 minutes because the system's batteries sustain the bus voltage. Leakage-current sensors rely on the presence of a load that may not be present under certain conditions. Adding auxiliary miniature-circuit-breaker support circuits or special high-rupture-capability indicator fuses and their connectors can significantly increase system cost.

Figure 1. |
This sensor circuit operates from a single 5 V power supply. |

Capacitor C_{4} and the secondary inductance, L_{2}, of transformer T_{1} resonate at approximately 42 kHz, a frequency that minimizes noise production in the audio, R_{F}, and psophometric noise bands. Operational amplifier IC_{1} and associated components form an ac-coupled positive-feedback amplifier with a gain of 20. Under normal operation, an intact fuse or closed circuit breaker completes a low-impedance path through T_{1}’s single-turn primary (sense) winding. Transformer action presents a low impedance at the junction of C_{2}, C_{4}, and R_{5} and reduces the loop gain around IC_{1} to an amount insufficient to sustain oscillation.

When a fault occurs and interrupts current through T_{1}’s primary winding, its secondary impedance increases, allowing full loop gain and permitting IC_{1} to oscillate at 42 kHz, which L_{2} and C_{4} determine. Under fault conditions, T_{1}’s turns ratio injects less than 10 mV of wideband conducted noise into the dc bus. Capacitor C_{3} couples the oscillating signal to IC_{2}, a gain-of-3 amplifier, which in turn drives a peak detector formed by D_{3} and C_{5}. Transistor Q_{1} saturates and provides a logic-low signal to an external alarm.

Figure 2 shows a typical application for sensing backup-battery-circuit failure.

Figure 2. |
The system wiring diagram shows transformer T_{1}’s primary winding. Low-voltage-disconnectunits LVD _{1} and LVD_{2} isolate the 48 V battery or the customer’s load for maintenance. |

To design transformer T_{1}, you calculate the required impedance and turns ratio. Equation 1 describes the basic transformer relationship:

(1) |

where Z_{1} is the impedance of the primary winding, Z_{2} is the impedance of the secondary winding N_{1} is the number of primary turns, and N_{2} is the number of secondary turns.

Under normal operation with current flowing in the primary winding, the secondary impedance comprises the low primary-side impedance plus T_{1}’s leakage reactance. When no current flows in the primary winding, the number of turns in the secondary and the toroidal core A_{L} (inductance per turn) determine the secondary winding L_{2}’s inductance and number of turns per Equation 2:

(2) |

where N_{2} is the number of turns around the toroidal core.

Ferrite-core manufacturers publish inductance-per-turn data that simplifies alteration of T_{1}’s design, but if that data is unavailable, you can use Equation 3 to calculate the inductance.

(3) |

where µ_{e}, the effective permeability, equals the magnetic constant, 4π×10^{–7} Hm^{–1}, L is the path length, and A is the cross-sectional area in millimeters squared.

Select a core that presents a high value of inductance to ensure that the difference between an open and a closed primary circuit causes a large change in relative secondary-winding impedance. Also, select a core material that doesn't saturate at full primary current.

Note that the core's central area must provide clearance for the battery cable (primary winding) and secondary winding. This application uses a Philips 3C85 toroidal ferrite core (part no. TN 16/9.6/6.3-3C85) with a secondary winding comprising five turns of 0.2-mm^{2} insulated copper wire. (Philips, however, has discontinued the 3C85 ferrite core. Ferroxcube's type 3C90 ferrite may serve as a replacement. Figure 3 shows the completed transformer.

Figure 3. |
The primary winding (battery cable) passes through transformer T_{1}’s center. |