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Entry type: FAQ Entry ID: 29010775, Entry date: 04/11/2008

Operation of solid state contactors and relays 3RF2 at lower voltage in main circuit

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For physical reasons, solid state switching devices can only be energized and kept alive if a minimum voltage is applied.
Therefore the value for the minimum voltages is relatively high prescheduled.

Since there are often applications with lower voltages for heating purposes, questions are recurring for the permissibility resp. possible restrictions.
This is to clear up those questions.

The solid state devices 3RF2 are available for different main voltage levels.
As example, for single pole applications with N conductor connected in a 230 / 400 V network the versions of solid state relays 3RF21..-.A..2 with operating range 20 to 253 V are deployed.

Abovementioned devices are optimized to operate with resistive loads (power factor cos phi = 1) and feature a voltage-zero-crossing detection.
This means, they Das heißt, sie always switch on in the zero-crossing as the optimum moment.
Physically conditioned, just as for alimentation of the control, there is always a minimum voltage required to switch on the power semiconductor.
To achieve correct functionality and full control of the complete sinus wave there are relatively high minimum voltages specified.

Regardless, the devices can be applicated with lower voltages, for instance heater circuits running 12 V only.
Indeed some limitations of the current flow have to be accepted in these cases.

On account of the voltage-zero-crossing detection, optimized for operating range 20 to 253 V, and the minimum voltage required for switching on, the power semiconductor is not fully controlled through when lower voltages are applied.
On every sinus half wave there is "missing" a section of the curve to a greater or lesser extent.
As a result, with resistive loads the full power is not provided anymore.

Following graphics show the typical, ideal shapes of voltage (blue curve) and current (red curve) when a solid-state switching device is controlling a resistive load.


Fig.1 Application of resistive load with "normal" voltage

When operating solid-state switching devices at voltages in the specified operating range there is practically no reduction in current flow to be notified.
Full power is provided to the load.


Fig.2 Application of resistive load with "lower" voltage

Fig. 2 shows obviously that the full power can not be provided to the load anymore.
However, this service condition is permitted for the solid-state switching devices 3RF2 and can practically act with minimum voltages of above 4 V effective.

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