Most difficulties arising from the application of AC solid state relays are due to an incomplete analysis of the operating conditions that specific loads impose upon the relays.
Loads of constant value resistance are probably the simplest application of AC solid state relays. Observing the steady - state current and blocking voltage specifications will normally result in a trouble free application. The rate-of-rise of current (di / dt) in a purely resistive load is limited only by the line impedances and the turn-on characteristics of the output thyristor. It is possible, particularly in high current applications, to exceed the di / dt to within the relays rating. The addition of some series inductance especially in high duty cycle applications, may some times be necessary to limit the di / dt to within the relay rating. The use of relay with zero-voltage switching is an effective way of keeping the di / dt within the rating of the output thyristor. With zero-voltage switching is an effective way of keeping the di / dt within the rating of the output thyristor with zero-voltage switching relay turn-on occurs at a point near the zero crossing of the voltage and, therefore, it is very difficult to have a high di / dt through the relay.
Incandescent lamp loads, though basically resistive, present some special problems. Because the cold resistance of a tungsten fi lament is only 10% or less of the hot resistance, a large inrush current can occur. The duration of the inrush current can range from one half cycle to several cycles, depending on the thermal time constant of the fi lament. It is essential to verify that this inrush current is within the surge rating of relay. Because of the unusually low fi lament resistance at the time of turn-on potential problems with di / dt may be more severe with lamp loads. A zero-voltage switching relay is particularly desirable with tungsten fi laments because of the ability to reduce the di / dt stress impesed on the relay and to increase lamp life. Certain types of lamps can momentarily apply near short circuit conditions on the relay at the moment of burnout. This occurs if a mechanically failed fi lament falls back across itself or the input lines, in such a manner as to result in a greatly reduced impedance, or if a low impedance gaseous discharge path exists, as it does in some lamps at burnout. The lamp characteristics at the moment of burnout should be carefully investigated and adequate precautionary measures taken to assure reliable operation, fast acting semiconductor fuses, of some series impedance line can be used to limit fault current to within capability of the relay.
Capacitive loads are not extremely common but they are encountered in applications such as switching capacitor discharge banks or capacitor input power supplies. Caution must be used with low impedance capacitive loads to verify that the di / dt capabilities of the relay are not exceeded. The di / dt of a discharged capacitive load without external limited impedance can approach infi nity. The valuable means of limiting di / dt with capacitive loads. Particular attention should be given to the safety margin on the relay blocking voltage rating, and voltage transients must be limited when switching capacitive loads. False operation at near peak line voltage into a discharged capacitive load can result in very large and potentially damaging di / dt values. The addition of series line impedance or absolute voltage clamping may be necessary to limit di / dt and protect the relay against the inevitable, occasional large voltage transient on the line.