Thursday, October 15, 2020

Capacitive transformerless power supply schematic for PIR motion sensor NV-1111.35

In the previous article I have explained how a PIR motion sensor works. There you can also find the schematic for the NV-1111.35 PIR sensor which is based on a popular three stage op-amp topology. Understanding this can help you understand other pyroelectric sensors as well.

This sensor is powered by a capacitive transformerless power supply with an output of 8V and 7mA of current.

Capacitive power supply for pyroelectric (PIR) sensors

The board includes a power 24V relay that can switch a load up to 1200W. The relay is controlled by the sensor's main board.

Capacitive power supply for pyroelectric (PIR) sensors
A rectifier diode and the output 8V Zener were burned which evaporated the copper traces. The board was repaired and components replaced

Schematic of a capacitive transformerless power supply for NV-1111.35 PIR outdoor sensor switch

Schematic of a PIR sensor capacitive power supply

WARNING: this type of power supply is meant only for use cases where no humans or animals can touch the board. This is because it doesn't have a galvanic isolation. If the Zener diode fails or the wrong part is touched will lead to electrocution with mains voltage.

The schematic is self explanatory. To understand how a capacitive transformerless power supply works read this very interesting PDF which also talks about resistive power supply, half and full bridge types and which one can be used to drive SCRs and TRIACs:

Transformerless Power Supply Design

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