Consider the Resistor-Capacitor Value in pass filter design


Only a very narrow electrical input power range can trigger a solid state relay, so an electric noise shield at the relay’s input end is necessary to prevent any errors or failure situations. A Resistor-Capacitor (RC) Network is one simple and effective way to absorb this pulse noise.

Although it is an ordinary RC low pass filter, the RC value should be carefully selected.

For example, the datasheet for IXYS CPC1017N states that this solid state relay requires an LED startup current of about 1mA, up to 50mA (DC) or 1A ( for a 10ms pulse). If applied under the control of a 5V pulse, the minimum value of ‘R’ is 5 ohms. To prevent sudden high voltage input, the datasheet suggests using a larger ‘R’ value. However, too large an ‘R’ value will reduce the input current and affect your SSR’s triggering performance. Similarly if the value of ‘C’ is too large, the discharge time is too long and also negatively affects the SSR trigger time.

Another simple method is to use Digi-Key online Resistor-Capacitor (RC) Time Constant Calculator to estimiate the resistor and capacitor values needed for your particular SSR.


Thanks for this post. It took me 11 years back in my school where I studied RC charging circuits. I still remember that time. :heart_eyes: School Physics