LAA127LSTR

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Solid state relays are electronic switching devices that require just a low voltage for controlling a high voltage or high current circuits. Typically used for DC motor speed control, heater or lighting load control and temperature control. The switching is considerably faster than mechanical relays, which helps in energy saving and improved efficiency.

One type of solid state relay is the LAA127LSTR, its application field is widely in heating or ventilation system and is used to switch the heater element on and off in accordance with user-defined temperature set points. The working principle of this kind of relay is based on a semiconductor switching elements. The LAA127LSTR is basically a compact and molded solid state relay type, manufactured with Schotky technology. It can switch up to 40A of AC current and is capable of handling up to 250VAC.

The device incorporates an FET with an insulated gate, which is gated directly with very small input current. When accessing a load, the current flows from supply which creates a magnetic ballast that is stronger than the coil’s magnetic field and generates a high impedance divide. This creates a safe DC control supply to minimize any effect from inductive loads (e.g.: inrush current from motors).

Inside the LAA127LSTR package there are no moving parts, which makes this device both, reliable and maintenance free. The relay utilises an existing PTC thermistor for temperature sensing, a triac for switching and latching, a coil for isolating the control circuit from the load. The thermistor and the coil are the main components of the relay which are responsible for switching on or off the thermal current.

The heat received by the thermistor when conducting current is transferred to a metal plate and the temperature is measured with a PTC thermistor. As the temperature varies, the resistance of the thermistor also varies, resulting in a different amount of current passing through the coil. This creates a magnetic field proportional to the current, which in turn acts on the latching triac which opens or closes the relay thereby switching the heater ON or OFF.

The LAA127LSTR works on the 10-32 VDC signal, which is amplified and rectified by the control circuit. The relay is protected from over-voltage and over-current and is provided with a mechanical protection cover. The dielectric strength of the relay is greater than 4000 VAC. The relay has a built-in zero-crossing detector that delays the switching on of the relay, ensuring that the heating element remains protected against any spikes or ringing in the power supply. It has an ambient temperature range of 0-60 degree C.

In conclusion, the LAA127LSTR solid state relay is a device that provides operational and reliable performance in heating or ventilation systems. Its switching speed is faster than mechanical relays and it utilises a PTC thermistor for temperature sensing, a triac for switching and latching, and a coil for isolating the control circuit from the load. This helps improve the efficiency and makes it maintenance free, as it has got no moving parts.

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