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FAQ03623 of Solid-state Relays FAQ

FAQ No. FAQ03623

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Solid-state Relays Output Short-circuit Malfunction 1: Why do the Solid-state Relays output always use half-wave operation?


The dielectric strength of one Solid-state Relay channel may have deteriorated as a result of an overvoltage to the Solid-state Relays output terminal caused by a noise surge (including high frequency noise).

Solid-state Relays Internal Circuit with Problematic Section

The I/O insulation element of the affected Solid-state Relays circuit may be a photocoupler or a phototriac coupler, and the load switching (output) element may be a thyristor or a triac.

Example of a Phototriac Coupler with Deteriorated Dielectric Strength on One Channel of Optical Receiver

Positive sideNegative side
Internal phototriac coupler dielectric strengthNormalDetericration of dielectric strength

Note:If the dielectric strength deteriorates on both poles, it will lead to a short-circuit error.

Possible Causes

1.A surge voltage was caused by the load power supply, or other loads connected to the load power supply line.

2.An inverter-controlled power supply was used (including harmonic noise).

Reference Information Relating to Protection against External Noise

As protection against external noise, it is recommended that an overvoltage protection element (e.g., a varistor) is externally connected to the power supply line.

Connection Example 1

Connection Example 2

Note:Check the manufacturer’s information for how to connect the overvoltage protection element.

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