Snubber circuit question


I'm planning on using some S208T02 solid state relays on a project (a solar water heater controller). The project will use 2 of these solid state relays, 1 to drive a 60w pump, and another to open a solenoid valve (so they are both inductive loads).

It recommends using a snubber circuit in the datasheet. It does not elaborate beyond that, so I've been doing some research. Since the output of this SSR is a triac, I'll be using an RC snubber, which is a series-connected resistor and capacitor placed in parallel with the load. I'm going to start with a 0.22uf capacitor and a 47ohm resistor to start with.

Now my question is with the capacitor.. I have a pretty good basic knowledge of electronic circuits, but have very limited practical experience. Would my hunch be correct that the capacitor has to be non-polarized? Would this limit it to a ceramic capacitor? Also, with 120VAC, I'm assuming any voltage rating over 120 would work?

Does anyone have any other tips regarding snubber circuitry in conjunction with a triac?


Also, would I be better off just using a regular micro-relay (driven by a transistor) instead of a solid state relay?

Yes you will need a non-polarized capacitor. Typically you will see snubbers use film caps. They have high voltage ratings and large capacitances. When selecting a film give yourself some headroom. For 120V don't select a cap rated for 125V. Go for 150V or higher.

A little known fact about (most) ceramics is that they lose significant amounts of capacitance when a voltage is applied to them. So while at 0V the cap is 0.22uF, once a bias or AC signal is applied the C drops.

For 120V don't select a cap rated for 125V. Go for 150V or higher.

You need to go higher then that. Caps have to be rated for the peak voltage they will be subject to, not the RMS voltage. 120vac service has a peak value of 170 volts. I think a common voltage rating for non-polarized caps used in 120vac service is 277 volts.


I know this is really after the fact, but I am running into the same thing. I was planning to also use a S208T02 and wasn't sure how to pick the components for the snubber, as I am pretty new at the practical side of this (really my first bigger project). So, any inside on what components you picked in the end would be helpful. On the other hand, have you considered using a AQG22105 from Panasonic? Spec says it already has a snubber included.


these PDF links might be very helpful