Electronic – the use of the capacitor in this relay circuit


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.


What is the purpose of the capacitor in this circuit?

Best Answer

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.