Ac-relay

When zero voltage switching is not necessary, methods for providing this function are demonstrated. The simplest version of a solid-state relay is an optoisolator, specifically the triac driver H11. However, the ability of the H11 to drive a load on a 60-Hz line is significantly restricted by its power dissipation and the dynamic characteristics of the detector. These limitations confine applications to 30-50 mA resistive loads on 120 Vac, with slightly higher values at lower voltages. These specifications are suitable for neon lamp drives, pilot and indicator incandescent bulbs, and low voltage control circuits, such as those used in furnace and bell systems, provided the current is adequate. However, less forgiving loads necessitate the use of a discrete triac. The H11 triac trigger optocoupler can potentially enable a straightforward power switching circuit that utilizes only the triac, a resistor, and the optocoupler. This arrangement is sensitive to high rates of voltage change (dV/dt) and noise in standard power-line voltages, which necessitates the configuration illustrated in Fig. 67-14B, where the triac snubber serves as a filter for the line voltage to the optocoupler. Since the snubber is typically not employed for resistive loads, the cost-effectiveness of the circuit is somewhat diminished. Despite this drawback, the savings in labor, board space, and inventory of components make this circuit optimized for isolated logic control of power-line switching. In scenarios where transient voltages on the power line are common, measures should be taken to protect the H11 from breakover triggering.

The described circuit utilizes an optoisolator, specifically the H11 triac driver, to facilitate solid-state relay functionality without the necessity for zero voltage switching. The H11 operates effectively within its limitations, driving resistive loads in the range of 30-50 mA on a 120 Vac line. This makes it suitable for applications involving neon lamps and indicator bulbs, as well as low voltage control circuits. The circuit configuration must account for the power dissipation characteristics of the H11, as excessive load currents can lead to overheating and potential failure.

To enhance the reliability of the circuit, especially in environments with significant electrical noise or rapid voltage fluctuations, the inclusion of a snubber circuit is recommended. The snubber, typically composed of a resistor and capacitor in series, acts to dampen voltage spikes and reduce the likelihood of false triggering of the optocoupler. This filtering effect is crucial for maintaining stable operation in the presence of high dV/dt conditions.

While the use of a snubber may increase the complexity and cost of the circuit, the overall benefits in terms of reliability and performance in power-line switching applications justify its inclusion. Additionally, the simplicity of the circuit design, which only requires a triac, resistor, and optocoupler, contributes to reduced assembly time and lower inventory costs.

In applications where transient voltages are a concern, it is essential to implement protective measures for the H11 triac driver. This may include the use of voltage clamps or transient voltage suppression devices to safeguard against breakover triggering, ensuring the longevity and reliability of the circuit in demanding electrical environments. Overall, this approach provides a cost-effective solution for isolated logic control of power-line switching while addressing the challenges posed by dynamic electrical conditions.When zero voltage switching is not required, methods of providing this function are illustrated. The lowest parts count version of a solid-state relay is an optoisolator, the triac driver HlU. Unfortunately, the ability of the Hll} to drive a load on a 60-Hz line is severely limited by its power dissipation and the dynamic characteristics of the detector. These factors limit applications to 30 -50 mA resistive loads on 120 Vac, and slightly higher values at lower voltages.

These values are compatible with neon lamp drive, pilot, and indicator incandescent bulbs; low voltage control circuits, such as furnace and bell circuits, ifdVI dt are sufficient; but less than benign loads require a discrete triac. The Hll]l triac trigger optocoupler potentially allows a simple power switching circuit utilizing only the triac, a resistor, and the optocoupler.

This configuration will be sensitive to high values of dV/dt and noise on normal power-line voltages, leading to the need for the configuration shown in Fig. 67-14B, where the triac snubber acts as a filter for line voltage to the optocoupler. Since the snubber is not usually used for resistive loads, the cost effectiveness of the circuit is compromised somewhat.

Even with this disadvantage, the labor, board space, and inventory of parts savings of this circuit prove it cost-optimized for isolated logic control of power-line switching. In applications where transient voltages on the power line are prevalent, provisions should be made to protect the Hll}l from breakover triggering.

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