# Resistor in RC snubber

What is the purpose of resistor to suppress EMI? Couldn't I just use a suppression capacitor alone between the coil of the relay or contactor? I have tried to use capacitor alone and it did reduce EMI in my project, but I'm just curious of the effect of adding resistor in series with it.

• The high peak currents are not limited without the resistor present. This puts stress on the capacitor. The resistor limits the peak of the current. – Bimpelrekkie Sep 15 '15 at 7:05
• In most snubber circuits the capacitor value is less critical than the resistor value. The resistor is chosen to damp the energy from the inductance of your coil ringing with whatever parasitic capacitance it can find. The choice of capacitor is then large enough that it forms a low impedance in comparison to the snubber resistor at the ringing frequency but you also want to keep it small to minimise the power dissipation. A reasonable rule of thumb is to make the RC corner frequency about 1 fifth of the ring you are trying to damp. – Warren Hill Sep 15 '15 at 14:41

Snubbers are energy-absorbing circuits used to suppress the voltage spikes caused by the circuit's inductance when a switch,electrical or mechanical, opens. and may also cause false triggering of the device (relay or thyristor)

The snubber capacitance has to meet two requirements. First, the energy stored in the snubber capacitor must be greater than the energy in the circuit's inductance.

Secondly, the time constant of the snubber circuit should be small compared to the shortest on time expected, usually 10% of the on time.

consider the case When the transistor switch opens, the snubber capacitor looks like a short to the voltage change, and only the snubber resistor is in the circuit. Choose a resistor value no larger than the characteristic impedance of the circuit so that the inductive current to be snubbed can continue unchanged without a voltage transient when the switch opens. and R is also helps in power dissipation when the switch is open as C alone cannot do it.

hope this helps

• wouldn't the coil helps dissipate the capacitor too? – OnePiece0039 Sep 15 '15 at 15:15

Snubber circuit prevents arcing in relays or false triggering in SCRs, it can reduce EMI as well but this isn't the primary function of snubber. R is for damping the transients, a capacitor only can lead to ringing (oscilation bursts) thus increasing the EMI.
In AC circuit a capacitor also acts as a short, so adding R is the only way to minimize leaking current.

• what causes oscillation bursts? is it when the capacitor is overcharged? so does it mean that instead of reducing EMI, it will actually make it worse? – OnePiece0039 Sep 15 '15 at 15:14
• You didn't mention what kind of switching you have. But speaking for common circuits, the load you want to turn off always has some inductance, even a wire has, ok,..then when you try to open the conntact the inductance in the circuit will induce a voltage spike, this spike will be absorbed by capacitor, then the capacitor will return back the charge into a load (inductance) and so on...until the energy is dissipated in the resitance of the circuit - LCR damped oscilator. If the resitance is enough big, then the sytem is damped enough to prevent oscillations - overdamped system. – Marko Buršič Sep 15 '15 at 15:39
• oh i see now, so resistor helps to reduce the oscillation in the circuit. do I need high resistance or low resistance?. btw, im controling 2-module relay with arduino and the relay controls contactor which is controlling the air conditioning unit. I also wonder if I need to add snubber in parallel with the Air con. – OnePiece0039 Sep 15 '15 at 18:25
• It's not that critical when there is such small load. If I understand you have arduino that conrols a small DC relay, this circuit already has the freewheeling diode in parallel with relay or something...then this small DC relay switches on/off a larger AC relay (contactor) that turns on/ff AC mains voltage to conditioner. Well you don't have to worry about. If you rally like to add some snubber circuit, then the easisest is to add a varistor MOV in parallel to the coil of contactor. – Marko Buršič Sep 16 '15 at 7:21