Solid State Relay / TRIAC safe circuit design

Discussion in 'The Projects Forum' started by Ahmed Adel Hosni, Feb 21, 2016.

  1. Ahmed Adel Hosni

    Thread Starter Active Member

    Aug 16, 2010
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    0
    Hello,

    I am confused when to use a SSR like "S202SE2" and when to use a TRIAC like BT137. I am also more concerned with safety designs. For example, the lack of knowledge regarding adding a snubber circuit or not, adding a capacitor in a specific point in the circuit...etc.
    In brief I need to design a "safe" circuit to control AC Lamp up to 200W, and also control heavy load devices up to 10A.

    First design.

    I searched a lot for a well designed TRIAC circuit but couldn't understand HOW the components values were chosen. In this link, the writer designed the cricuit using BT137. [​IMG]

    My question: What values do I need to get from the datasheet which I can use to select R2, R3, R5, C1 and C2 ?

    --------------

    Second design:

    Where I live, the S202SE2 is available ( datasheet ) Why and "how" would I use this and not the above TRIAC ? Do I need any external componets rather than the snubber circuit which is mentioned in the datasheet guide ?

    Thanks.
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    744
    The S202se is an opto triac with zero crossing detection built in, this is used to fire the BT157 at the correct timing of the mains to avoid spikes and rf noise. Use the same value of components as the circuit you posted.
     
  3. Ahmed Adel Hosni

    Thread Starter Active Member

    Aug 16, 2010
    44
    0
    Thanks for your reply but I am getting more confused. Doesn't the S202SE has an internal opto with triac to use it as a single IC, rather than using an optocoupler + a TRIAC like the first design ?
    I didn't understand how will it be used to fire the BT157 ? I thought they are both different design concepts.
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    They are using a smaller triac to fire the big triac, as said before for zero crossing detection.
     
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    It won't, because they are identical design concepts. The S202 replaces everything in your upper schematic except for the snubber and the fuse. It doesn't fire your TRIAC, it replaces it. Internally, it probably has separate TRIAC die, infrared emitter, and a photodiode/diac trigger chip.

    ak
     
  6. Ahmed Adel Hosni

    Thread Starter Active Member

    Aug 16, 2010
    44
    0
    Thanks a lot.

    Can anybody guide me how to read the datasheet of the TRIAC, as I have asked about the price of the s202 and it is very expensive, like 5x or more.

    My question: What values do I need to get from the datasheet which I can use to select R2, R3, R5, C1 and C2 ?
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,791
    1,103
    The S202SE2 can only handle loads up to 8A RMS according to the datasheet, so is unsuitable on its own if you want to drive 10A loads.
    This article gives guidance on snubber design.
     
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