A reasonable approach disconnects the amplifier-output node from the output terminals for the period when the overload voltage exists on output terminals. Engineers for decades have used such serial disconnection by means of eleCTRomechanICal relays in audio power amplifiers but for a different reason: louDSPeaker protection. SSR s (solid-state relays), inc

luding optoelectronic, photovoltaic, OptoMOS, and PhotoMOS devices, suit the task of load disconnection at moderate current levels because of galvanic isolation between the control and the load PINs (Reference 3).

　　The series-protection circuit of Figure 3 disconnects the amplifier-output terminal using a series-connected, high-voltage SSR. Raising the output voltage above the positive-reference-voltage or below the negative-reference-voltage threshold causes either the IC2 or the IC3 comparator to change its output state and turn off SSR IC4 through AND logic element IC5. Figure 4 shows the simple circuit realization of this approach.

　　The circuit in Figure 4 requires only a couple of external components to use an SSR for output-overvoltage protection. Rising overvoltage turns off both transistors in IC2, interrupting current flow through the control LED of IC3. Relay IC3 opens, protecting the amplifier and clamping diodes. The circuit was tested with a handful of Clare, Matsushita Electronic Works, and Panasonic SSRs with and without internal current protection. The power-supply rails are ±15V; R10, R11, and R12 set the triggering levels and are equal to ±16V. Omitting R11 shifts the triggering levels to ±14.5V. The SSR turn-off delay in protection-circuit operation is 100 to 200 µsec for relays with 0.5V overvoltage protection and becomes slightly shorter with higher overvoltage. Note that the peak current through clamping diodes CAN be rather high with low-on-resistance SSRs.

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