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Diff between Triac, SCR, Optoisolator w/ triac, and SSR

Discussion in 'General Electronics Chat' started by jmb4370, Sep 16, 2008.

  1. jmb4370

    jmb4370 New Member

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    I know there are often many ways to get to the same result with components, but...

    Hopefully these don't seem like really silly questions:

    If I understand some of this, using an Optoisolator to feed a Triac can indeed then control an AC or DC powered load, and keep the load isolated from the logic control side of the Optoisolator. Does this setup function just the same as a SSR (Solid State Relay), or does the SSR still have to feed a Triac? ...when would the SCR be used instead? ...does the Optoisolator/triac combo or the SSR really function jsut as the electromechanical relays do in that when energized, a current can flow through the switched contacts, and when not energized, the circuit is open, and not current will flow?

    When is it that a SCR would be used instead of a Triac to switch a load or other low voltage circuit that is carry data and not a current load as in a lamp or motor?

    Here is an Optocoupler:
    http://www.fairchildsemi.com/ds/MO/MOC3010-M.pdf

    Here is a SSR (Solid State Relay):
    http://document.sharpsma.com/files/pr31ma11_e.pdf

    Thanks for any help in further explaining some of this...

    Michael
     
  2. gerty

    gerty Member

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    A scr is typically used to switch a dc load, but can be used on ac. A ssr is typically used for a ac load, but some can be had for a dc load. Clear as mud ain't it. A scr will also keep conducting as long as the load is continious, in other words as long as the circuit is drawing current. A ssr is usually dc controlled and will stop conducting when the control voltage is removed, regardless of load condition.
     
  3. jmb4370

    jmb4370 New Member

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    Thanks gerty!

    Can the SSR (as in above data sheet link), being fed from the output of a 4017 (decade counter), directly switch a DC low voltage circuit?

    When might the Optocoupler (MOC3010) to Triac then be used, or what is different of this combo to a SSR?

    Michael
     
    Last edited: Sep 16, 2008
  4. dave

    Dave New Member

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  5. gerty

    gerty Member

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    Yes the ssr can be fed by a 4017, but you'll need a different type output for switching dc. The triac is designed for ac and inside is two inverse parallel scrs, all of this means that if it turns on it might not turn off. You can use a dc output ssr or a opto-isolator for switching dc.
    The moc3010/triac combo is essentially the same as a ssr (in your link) but capable of higher currents. Again , the triac is for ac..
     
  6. crutschow

    crutschow Well-Known Member Most Helpful Member

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    A SSR for AC use usually consists of an opto isolator driving a Triac.

    A SSR for DC use usually consists of an opto isolator driving a power MOSFET (although some can also be used in AC applications).

    SCR's conduct current in only one direction and Triacs conduct in either direction, thus Triacs are usually used for standard power line AC applications. Both an SCR and a Triac will stay conducting once triggered until the current flow is reduced to zero, which is the reason they are usually used only in AC circuits, where the current goes to zero twice each cycle, or pulse type circuits where the current returns to zero after the pulse.
     
  7. jmb4370

    jmb4370 New Member

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    Ok, starting to understand, but...

    Perhaps some additional info might help, although I didn't post in the Projects forum as I was trying to get the understanding of the different components myself...

    Here is a circuit from 2-10-08 (MrDeb) for a 10 LED relay sequencer, that uses an MOC3010 Optoisolator into a Triac because there is 110 volt line voltage (I didn't know how to copy the thumbnail from the other thread, which is why I just put the link to the thumbnail):

    http://www.electro-tech-online.com/attachments/counter-gif.17092/

    I am actually trying to combine this circuit with another from 3-20-08 (SYE)

    http://www.electro-tech-online.com/attachments/4017-gif.18285/

    where the trigger of the 4017 could be activated either by the 555 timer, or a momentary pushbutton switch so the sequence the LED's could be automatically or manually sequenced.

    For what I am trying get to, not only will a LED light in sequence, but I wanted use a SSR or optoisolater to switch on a low current DC circuit for each different LED that lit up. Now I am not sure how to change the output(s) to use the MOSFET and switch another circuit.

    The de-bounce section of the manual trigger seems to indicate a Hex inverting Schmitt Trigger (74HC14) which looks like a 14 pin chip that has 6 inverting buffers... Is it available as a single unit (as opposed to 6 in one chip), and possibly called something else?
     
  8. gerty

    gerty Member

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    Are leds the only thing you plan to drive from the 4017?. If so you really don't need the ssr or opto. The 4017 can drive a led directly.
     
  9. jmb4370

    jmb4370 New Member

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    For what I am trying get to, not only will a LED light in sequence, but I wanted use a SSR or optoisolater to switch on a low current DC circuit for each different LED that lit up. Now I am not sure how to change the output(s) to use the MOSFET and switch another circuit.

    Both the LED will light directly from the 4017, and also close the switch to allow another low voltage DC circuit to be connected as well.
     
  10. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi there,


    I am not sure if you realize this or not, so i'll mention this...

    A triac or scr and many ssr's do not turn 'OFF' unless the current
    going through them falls to zero. In other words, if you use them
    to switch DC current you wont be able to turn them off unless
    you force the current to go to zero one way or another. Thus,
    you can not simply send a signal to turn 'on' and turn 'off' and
    expect the load to follow that and turn on and off as you planned.
    Yes, it will turn 'on', but it will not turn 'off'.
    This of course would be acceptable if you needed to turn a load
    on and never turn it off until it's main power was turned off, but
    otherwise it would not be acceptable.

    Using a transistor, you can turn it on AND off, so you wont have
    this problem. Using an opto isolator to drive the transistor you
    can achieve total isolation for each circuit too. Choose an
    opto with a high CTR (Current Transfer Ratio) so you dont have
    to drive it too hard.
    If you need details im sure someone could post a circuit.
     
  11. jmb4370

    jmb4370 New Member

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    If the optoisolator is a "transistor output" does this mean it has the transistor within the 6 pin DIP package, or does this mean that optoisolater needs to be connected to a transistor for further processing?
     
  12. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi there,

    This means the transistor is built into the opto package and turns on when the light
    from the internal LED hits a photodiode connected to the base.
    If you look up some opto's on the web you can find a data sheet on most of
    them and on the data sheet it will have a little schematic showing the transistor.
     
  13. jmb4370

    jmb4370 New Member

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    MrAl: I have seen that, thanks! I noticed that many circuits using the opto with the triac output then feed into a higher power triac from switching AC loads which is why I though the opto with the transistor output needed to feed into another transistor as well.

    Using an opto with a darlington output to get a higher gain (because of the second internal transistor), is the same as an opto with transistor output feeding into a second transistor, is this pretty much correct?
     
  14. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,

    Yes, that's about right. The second transistor provides more gain.

    I would think that if you intend to drive some substantial current you would
    need at least one more transistor that can handle the current even if you
    use a darlington output opto, unless of course you can find one with a
    high current output (i dont know of any offhand though). The opto outputs
    are probably good for lower currents, but again check a few data sheets.
    Keep in mind you need to carefully consider the CTR too in order to get
    your opto to switch correctly, and depending on speed you might need
    a base resistor for the opto's internal transistor(s).

    Another thing that is good to keep in mind is the life expectancy of the
    opto's internal LED. At rated current it might not last more than 5 years
    but at lower current like 1/2 that it could last much much longer. See
    if you can find anything about this on the data sheets.
    As the LED ages it gets dimmer and dimmer, so the CTR constantly goes
    down with age, and aging is affected greatly by the LED drive current
    level. If driven too hard after a while the circuit will stop switching,
    or switch only at certain temperatures.
    I think 1/2 rated current is a good design goal, but of course lower is better.
     
    Last edited: Sep 18, 2008
  15. jmb4370

    jmb4370 New Member

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    MrAl:

    The proverbial light bulb is starting to blink above my head, thanks! I think I have enough information to do just a bit of damage now, and over the next several days, I will try some of this to see if I understand any of it! I am hoping I can get a functioning circuit on the breadboard soon, so when the other parts come in (like the decade counter 4017) I will already have something that works, and can then expand the project. Thanks for the info so far!
     
  16. vinjoc

    vinjoc New Member

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    Hi All,

    I am trying to develop very Simple circuit using a very Simple Circuit for Switching 3KW load on secondary of " MOC 30x1 " series OptoTriacs. But evry time I am unable to Control the Seconadry side of the Optotriac. Either It remains SWITCHED On as soon as I connect it to AC-230 V or never Switches On at all. I am having Digitally controlled PSU on Primary side, so that I can go on slowly Increasing Voltage and simultaneously Monitor Current being consumed. Please help.
     
  17. johnyradio

    johnyradio Member

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    Why would you want the switch to shut off every cycle? At 60 hz, it's switching at 120 hz. Why?

    Is the idea that each time current goes to 0, the switch will detect if the control voltage is 0, giving the switch a chance to close?

    thx!
     

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