Electronic Design

Broken Bulb Safety Trip

A classic safety requirement involves disconnecting the ac mains supply from a light bulb should the glass envelope become broken. Although the filament can draw a much higher than normal current upon exposure to air prior to burning out, this can’t be relied on to blow a fuse or trip a miniature circuit breaker (MCB).

With the solution shown, on the other hand, the current through the bulb is sensed by means of the lowvalue resistor R19 (see the figure). That resistor is then compared with a reference derived from the mains voltage so that fluctuations in the ac mains are compensated out. R9 to R11 act as a voltage divider for the sense voltage developed across R19, and R3-R5,R7,and R8 act similarly for the mains voltage reference (rectified by D3). D5 and D4 provide rectification and protection.

Usually, the output of op-amp N1a is a series of pulses at the mains frequency that keeps C7 discharged via R12 and D6. However, if the bulb current falls substantially below the reference level for more than a few seconds, the voltage on C7 rises until the comparator N1b toggles, activating relay RL1. This, in turn, draws a large pulse of current through the PTC resistor R18, causing the MCB S1 to trip. Thus, the bulbholder is rendered “dead.” Note that S1 also will trip if the bulb is removed, rendering the contacts “safe.” A triac may be used in place of the relay.

The current-limiter disk R18 was chosen for reasons of compactness. Its resistance remains low for just long enough to trip S1, whereas even a large wirewound resistor will often fuse during the surge. Initially, this circuit was developed to offer safety for a 60-W tungsten bulb, but in the arrangement illustrated, a phase-angle detector has been added. As a result, the lamp may take the form of a fluorescent tube.

With a conventional ballast, consisting of a series inductor and a parallel capacitor, one has to be able to tell the difference between “normal” current (which is roughly in phase with the mains voltage) and the “no tube” current (which can be appreciable, but leads the voltage by 90° due to the large compensating capacitor). A leading ac reference from C1 is fed into N1a via R6, making the latter behave as a phase-sensitive detector. The output pulses at R12,D6 spend much more time high than low if the tube is removed or broken (and the in-phase current consequently disappears).

This can be understood by seeing what happens in the absence of the tube: Appreciable current is drawn through the compensating capacitor, but the signal produced at N1a’s inverting input is overcome by the signal from C1 at the noninverting input. Thus, the output goes high. C1 additionally serves as a capacitive-divider or charge-pump to create the 12-V rail to power the circuit. To allow for random firing of the discharge upon initial switch-on, R13,C7 provides a delay of four seconds. A photoelectric switch S2 can be added to provide dusk-to-dawn lighting control.

Note that for the circuit as drawn, the MCB is tripped through overcurrent when RL1 closes. However, if an MCB with an additional high-resistance coil (“voltage coil”) is available, this coil can be employed for tripping. Hence, R18 can be omitted. Unfortunately, such MCBs aren’t common. As an overall safety device, the novel system described here has an advantage over an earth-leakage circuit breaker alone, since it trips with a broken lamp even if no earthleakage has occurred. Thus, the remnants of the lamp are “dead,” not “live” until touched!

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