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Filter capacitor for relay

Dakk33

New Member
Hello all,

So I've made a circuit that actuates a solenoid using a relay. The relay is getting a 5V signal from a microcontroller. I've been advised to add some capacitors to filter out any noise for the signal on the relay. Does anyone have any input on what I should be looking for in terms of capacitance and how I should add the capacitor in the circuit (in series with the signal, or in parallel across the relay?)

Attached is a drawing of the circuit.
 

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rjenkinsgb

Well-Known Member
Add a flyback diode across the relay coil, the same as you have with the solenoid.

Capacitors would be a good idea near the relay connections, from 5V to the 0V that the drive transistors connects to, and across the 24V supply for the solenoid.
 

dr pepper

Well-Known Member
Most Helpful Member
A flyback diode is usually fine for this as Rj said.
However you could do it with a RC across the coil, the R would be chosen so that the current through it is the same as the relay coil. and the C is chosen so that it stores most of the energy as the relay coil does, then the kickback energy from the relay is damped, you see this a bit in industry on AC coils, obviously a diode cannot be used on AC.
 

eTech

Active Member
Hello all,

So I've made a circuit that actuates a solenoid using a relay. The relay is getting a 5V signal from a microcontroller. I've been advised to add some capacitors to filter out any noise for the signal on the relay. Does anyone have any input on what I should be looking for in terms of capacitance and how I should add the capacitor in the circuit (in series with the signal, or in parallel across the relay?)

Attached is a drawing of the circuit.
o_OThe JCZ-11F relay is a 12V relay according to the datasheet. It won't work with 5V to its coil.:confused:

eT
 

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