Relay protection circuit

I came across a relay circuit that uses two diodes and a capacitor. I know the diode across the coil (D1) is a flyback diode and that capacitor (C1) is a filter capacitor. What is diode D2 for?

schematic

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Answers 3

  • There are many circuits that need to cause a relay to remain active for a short time after power is removed or set to zero. So, when the supply (Vin) drops to 0 volts, the charge on the capacitor will provide an energy source for the relay to keep it activated for a few more tens of milliseconds to several seconds.

    What is diode D2 for?

    If D2 was replaced with a short and the power supply (Vin) dropped to 0 volts then the capacitor would be immediately discharged and not provide a turn-off delay.

    I know the diode across the coil (D1) is a flyback diode and that capacitor (C1) is a filter capacitor.

    Do you really know that C1 is a filter capacitor or did you just make that assumption?


  • Diode \$D_2\$ and capacitor \$C_1\$ form a half-wave rectifier, as the high value of the capacitance of the capacitor suggests: probably the relay is controlled by a pulsed/AC signal. Thus their function is not to smooth switching spikes, but to provide a sufficiently stable DC voltage to the coil.


  • "I was planning on using this prebuilt circuit.." tells what happened.
    That diode prevents reverse connection. Without D2, D1 has to take all the power, if the circuit gets connected reverse. The manufacture does not want smoked products returned.


    Edited
    Along with the reason above, It can be a rectifier when AC is applied.


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