SSR vs Coil relay

Discussion in 'General Electronics Chat' started by Dritech, Mar 29, 2012.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Hi all,

    Does anyone know what is the average power consumption of a coil relay and a SSR which both have an output of 230-240VAC 50A ??

    What are the advantages of using SSR when compared to coil relay??

    Thanks in advance
     
  2. JimG

    Member

    Dec 7, 2009
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    0
    If you are asking how much power is dropped across the load side of the SSR, I can answer that part of the question. Generally, there is around a 1V drop across the load terminals (maximum is usually spec'd at 1.6V but I think 1V is about average).

    So if you are switching 10A, then there is around 10W consumed by the SSR.

    The power consumed on the control side is very small, just enough to make an LED do its thing.

    Jim
     
  3. Stuntman

    Active Member

    Mar 28, 2011
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    To add to what Jim said, there is differences that you may want to consider.

    SSR's are not all around "better" than mechanical relays. In a nutshell, the primary advantages are:

    A.) They can turn on with very low supply current/voltage and no inductive kickback associated with a mechanical relay coil.

    B.) More importantly, they have no moving parts. This allows them to be turned on and off rapidly, without the delays and "bounce time" of mechanical relays, and generally have a much longer lifespan (usually measured in hours on instead of # of cycles that a relay would spec)


    There are some small downsides. Things like leakage current and on-resistance (hence why large currents need heatsinks).

    You might give this a read:

    http://www.clare.com/home/pdfs.nsf/www/an-145.pdf/$file/an-145.pdf

    For a 50A application, you will have to do some datasheet browsing.
     
  4. JimG

    Member

    Dec 7, 2009
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    Yup, what he said :)

    I overlooked the 50A part in the OP. That'll require a pretty serious heatsink if you go the SSR route. SSR data sheets have current derating curves to help you size a heatsink based on load and ambient temperature.

    Jim
     
  5. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks for your replies, and what is the average current consumed by a mechanical relay input (i.e. to power the coil) ? (with the relay output rating being 230vac 50A)
     
  6. Stuntman

    Active Member

    Mar 28, 2011
    181
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    That depends greatly on what voltage you have to drive the coil. If you only have 5V DC, it will be much more current than if you have 12V DC to play with.

    Here is a data sheet for a relay Sparkfun sells (30A... don't get excited ;)) Look at the coil data. You can see there are different coils for different voltages...

    http://www.sparkfun.com/datasheets/Components/T9A_DS.pdf
     
  7. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Talking mechanical relays, IMO for 50A, you're across the fuzzy line between where "relay" stops and "contactor" starts. Contactor is going to draw a lot more coil current than a relay because it needs to really pull in the contacts hard to make good contact. contact contact contact.
     
  8. #12

    Expert

    Nov 30, 2010
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    True that word contactor. In my job, I have seen the contacts worn so far that there was nothing left but the arm that used to hold it...after about 200,000 cycles of applying 3x to 5x their rated cary currrent during the "start" surges. One coil I tested was worth about half a henry in inductive kick. It wasn't really a half henry coil, but the magnetic circuit changes when the activation voltage stops. The solenoid moves and changes the amount of iron in the core area.
     
    strantor likes this.
  9. Stuntman

    Active Member

    Mar 28, 2011
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    I will mention (since 50A may be difficult to achieve with a single device) that I looked into building my own AC control system with a TRIAC for a heater control. Most devices I looked at were rated at 40A, which means a >50A may be available.

    I did a quick search and found a couple. Just a thought. Triacs will be a bit more difficult to implement, but may meet you needs.
     
  10. strantor

    AAC Fanatic!

    Oct 3, 2010
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    an SSR usually is a triac if I'm not mistaken.
     
  11. Stuntman

    Active Member

    Mar 28, 2011
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    Yes, but I seemed to find, bang/buck, I could get more current through dedicated Triac's than the standard commercial SSR's. This is why I went his route for a heating element control system. However, I also had motives in using my circuitry to control power, not just turn on and off.
     
    strantor likes this.
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