Overvoltage output protection circuit

Add crystal relays and control circuits to the amplifier outputs to provide overvoltage protection.

In test and measurement applications, it is necessary to provide overvoltage protection for amplifier output terminals, energy supplies, and similar components. The traditional way to accomplish this task is to add series resistance from the output node to the power rail or other threshold voltage along with clamping diodes (Reference 1 and Figure 1). The resistance significantly reduces the output current capability and voltage output fluctuation with low resistance loads. Another method is to use a fuse or other current limiting device that has a higher energy absorption capability than a diode. The circuit in Figure 2 is a bipolar current source. When the voltage drop across the source resistor R6 increases to a value greater than that of the depletion MOSFET

When the threshold voltage of Q1 and Q2 is reached, the current is limited by diodes (Reference 2). The disadvantage of this method is the high energy dissipation in the series devices during overload conditions.

When an overload voltage appears at the output terminals, it is reasonable to separate the amplifier output node from the output terminals for a period of time. Experienced engineers have used this series separation, relying on electromechanical relays in audio power amplifiers, but for a different reason: loudspeaker protection.

SSR

(Solid-state relays) including optoeleCTRonIC, photovoltaic, OptoMOS, and PhotoMOS devices, meet load separation tasks at moderate current levels due to the galvanic isolation between the control and load pins (Reference 3).

The series protection circuit in Figure 3 uses a series high-voltage SSR to separate the amplifier output terminals. When the output voltage increases above the positive reference voltage limit or drops below the negative reference voltage limit, the AND logic device IC5 causes the IC2 or IC3 comparator to change its output state and turn off the SSR.

IC4. Figure 4 shows a simple circuit to implement this method.

The circuit in Figure 4 requires only a pair of external components to implement output overvoltage protection using an SSR. The added overvoltage turns off the resistor in IC2, interrupting the flow of current through the control LED of IC3. Relay IC3 is disconnected, protecting the amplifier and clamping diode. Whether or not there is internal current protection, the circuit is Clare, Matsushita

Electronic Works and Panasonic

SSR test. Power supply is ±15V; R10, R11 and R12 set the trip point to ±16V. Omitting R11 shifts the trip point to ±14.5V. With the protection circuit in operation, the SSR with 0.5V overvoltage protection has a trip delay of 100 to 200µs, and the delay time is slightly reduced for higher overvoltages. Note that the current peak is higher than with the low resistance SSR through the clamping diode