# Wanted: snubber circuit tutor :)

#### devian_t

##### New Member
What do I need to know to figure out what snubber circuit to use?

Using solid state relay to turn on 220VAC incandecent bulb, another circuit for a with relay to turn on a fan 220VAC, another is a sump pump. Im trying to do a hydroponic style set up using picaxe to automate the outputs(fan, light, pump). http://media.digikey.com/pdf/Data Sheets/Sharp PDFs/PR31MA11NTZ.pdf
is my solid state relay and on the recommended circuit on the datasheet it includes a snubber circuit. Help please....

NB:I have no training on electronics, few experience on microcontroller(picaxe&BS2), intermediate skill on soldering.

#### BeanieBots

##### Moderator
Hmm.... snubbers can be a bit of a black art!

Essentially, the requirement is to have enough capacitance to absorb the offending spike together with enough resistance to disipate the spike energy in a timely manner.

The rough guide that I use is that the resistor value should match the DC impedance of the load and the capacitor charge should be equal to about half the inductive charge of the circuit at peak current. However, only experience can really tell when this guide is not good and depends on those things about a circuit which don't appear in the diagram such layout and cable lengths.

This document will explain the maths behind snubbers.
http://www.cde.com/tech/design.pdf

Use the rules as a guideline and then tweak the values with the assistance of 'scope readings.

EDIT:
The bit most people get wrong is the rating of the snubber components used. In particular, watch out for the current rating of the capacitor. Also, consider if a simple diode can be used instead.

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#### drjeseuss

##### Member
Using solid state relay to turn on 220VAC incandecent bulb, another circuit for a with relay to turn on a fan 220VAC, another is a sump pump. Im trying to do a hydroponic style set up using picaxe to automate the outputs(fan, light, pump). http://media.digikey.com/pdf/Data Sheets/Sharp PDFs/PR31MA11NTZ.pdf
is my solid state relay.

Not the topic of your question, but you may want to look more closely at your specs. These particular relays have a max current of 60ma each with a peak of only 1.2A. This may suit your needs, but based on the fans and pumps I've seen, you may not be getting the current you need with these SSRs. Then again you may be wiring these in parallel, etc. Just a heads up in case you hadn't checked into this. I've ended several projects while looking into the blue smoke. Not a good way to see a few weeks of work go.