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In the schematic below there is a relay coil driven by a 24V DC battery. In parallel to the coil there is a flyback diode and also a ceramic capacitor.

Schematic

What is the purpose of the capacitor in this circuit?

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    \$\begingroup\$ Thanks for the drawing. The question would have been better received if you added it the first time. \$\endgroup\$
    – pipe
    Oct 11, 2017 at 11:53

2 Answers 2

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That capacitor is not really needed but it does help in reducing high-frequency emissions and switching pulses which could disturb other circuits working on the same supply (that 24 V line).

Especially when a relay coil is switched off a very sharp pulse is generated (due to the magnetic flux in the relay core wanting to induce a current in the coil). The flyback diode takes care of most of this but it is possible that it is not fast enough to catch the sharpest edges of the pulse. Then the capacitor helps to smooth these edges out.

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The capacitor is used as an absorber. The diode cannot respond fast enough and the back emf generated by the coil when current to it is switched off can affect other circuits. The capacitor in effect increases the time for the back emf to grow and gives the diode more time to effectively clamp the voltage.

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