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Relay - SS vs Mechanical

Discussion in 'Hobby Electronics' started by [email protected], Oct 5, 2004.

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  1. Guest

    Hi all,

    Is there any good reason why a Solid State Relay can't be used in
    place of a mechanical one for switching regular appliances on
    240VAC/10A with a DC signal.

    I'm thinking of using one in the Silicon Chip Energy meter and I have
    a suitable 20A device in the parts box but no mechanical equivalent.

    Thanks in advance,

  2. Phil Allison

    Phil Allison Guest

    ** A 20Amp SSR uses a large triac to do the actual AC switching -
    these have minimum current ratings as well as maximum ones. Make sure the
    appliance does not have a low current standby mode that derives its power
    via a small iron core transformer ( eg a microwave oven) . The triac in
    the SSR may not operate this tranny - or worse burn it out by only
    operating during one half cycle so feeding it with a large DC component and
    a small AC one.

    ................ Phil
  3. Bill Bailley

    Bill Bailley Guest

    I don't know anything about the Silicon Chip project, but I have used solid
    state relays to extend the life of some of my older air conditioning units.
    A big problem with the older units is that the thermostat contacts switch
    the compressor directly without benefit of a relay. The thermostat contacts
    last a year or so and then the thermostat has to be replaced. The
    thermostats now just switch a 5 volt DC supply to the SS relay control
    input, and the thermostats appear (after 4 years) to have a much extended
    life. Radio Parts Group has a useful range of SS relays.

    One gotcha. There is always a small leakage current through SS relays. This
    can give the unwary a severe bite, or possibly worse.

  4. Jason James

    Jason James Guest

    Reliability. The SSRs used in control cubicles to switch a bunch of taxiway
    and r/way lighting transformer primaries, used to fail from time to time.
    Then again I'm not sure if there was any inductive-load protection.

    Personaly, in high power situations, a mech relay gives me much more

  5. Phil Allison

    Phil Allison Guest

    "Jason James"

    ** You are not real sure of anything.

    ** What a fool.

    ................ Phil
  6. Jason James

    Jason James Guest

    Well Phil,..I didn't have to work on the things, so it was an observation
    ie, they used to fail,..

    Oh, really, try telling that to the designers of activation cctry for soft
    start HT supplies in transmitters. They all used relays and were virtually
    100% reliable.

  7. Phil Allison

    Phil Allison Guest

    "Jason James"

    ** Proves nothing about all the other SSRs that did not.

    ** Which, even if true, has no connection with the OP's question, my
    comment or reality.

    Relays are mechanical and so wear out.

    SSRs are not mechanical and so do not.

    ............. Phil
  8. Jason James

    Jason James Guest

    Suuuure Phil.

    Relay failure generally is an extremely rare event IME. Heavier contactors
    have been known to fail but fall well short of any rate worthy of mention.

    SSRs in the context of my experience have failed at a rate which was
    significant,..and that is how my answer to his question was framed.

  9. Phil Allison

    Phil Allison Guest

    "Jason James"

    ** All relays *wear out* - contacts have a limited switching life.

    You have trouble with reading as well as thinking.

    ** Your "experience" is non existant.

    ............. Phil
  10. MC

    MC Guest

    True.., but relays (in aggregate), tend to fail in a more
    predictable state, ie. they usually fail to operate when required.
  11. Phil Allison

    Phil Allison Guest


    ** That is bullshit - relays often fail on ( ie contacts welded) or
    else fail to make ( contacts burned) - or- become intermittent (
    tarnished contacts) -or- develop excess contact resistance and
    erheat -or- just plain fail to move when energised due to mechanical
    friction, wear or breakage.

    Vibrate or a relay hard or subject it to a sudden shock and the contacts
    will open in sympathy.

    Chose the wrong relay contact material type for a particular job and it will
    have a short life or even no life.

    Any day you can avoid using one and use a SSR or other electronic switch is
    a good day.

    BTW Do not remove my words from context so that you can misrepresent

    .............. Phil
  12. MC

    MC Guest

    ahhh.. care to tell me which words I removed out of context in my reply.

    There are *many* different failure modes for mechanical relays which are
    highly dependant upon how the device is actually used, but the *most
    common* failure for a relay to *not* function (ie to make or break) the
    contacts when requested.
  13. Phil Allison

    Phil Allison Guest

    "MC" <

    ** See the words from JJ that I put back ??? - that was the context.

    Your post has a different one.

    By snipping you obscured that fact.

    ** And that is supposed to be a point is it ???????

    " When a relay fails it no longer works " ??????

    Piss off.

    ................. Phil
  14. Jason James

    Jason James Guest

    An argument like this has to be put into context. The number of relay
    failures in the ground radio-nav and comm equipment I maintained over a 26
    year period from '74 to 2000, was such a rare event, that the only ones I
    can recall were due to design errors eg the interface circuitry for a
    converted Weston VHF TXRX placed a remote PTT relay which switched supply
    volts, before a large electrolytic filter cap instaed of after. This caused
    frequent relay failure due to switching currents (cap charge current) being
    far beyond the specs for that particular relay.

    The equipment room we had for a flight-service tower, which had 5 consoles,
    contained 100s of relays invarious control cards. Not one of these -50v
    relays which encompassed makes such as Siemans, Heinemann and others,
    failed,...not one.

    Dozens of larger contactors which were in the domain of electrical
    maintenance in the main (but which we had complete observation over) and our
    higher powered HF t stuff including ex- US army 500 pep HF AM Txs from WWII,
    were very reliable, fact the only PTT (contactors which had the most
    duty-cycle) contactors which failed was in one of those ex army units,..and
    it was (at the time) over 30 yrs old. Even then it only failed because after
    10,000s of operations, its 3-phase armature screws which held the contacts
    in place, came undone. It was totally repairable.

    In more complex equipment such as navaids and their control and
    monitoring,..I cannot recall one relay/contactor failure except in the first
    DMEs which were over 20 yrs old at the time,..and even then was confined to
    fault situations which placed an overload on them.

    We did experience a few failures in 240v cct breakers (these were used by
    the tech to turn the main supply on and off as well),..but cct-breakers are
    not quite the same thing.


  15. MC

    MC Guest

    Take a deep breath Phil.
    Take another deep breath.
    Check that you haven't gotten my replies confused with others.
    Ease-up on the abuse.
    Take another deep breath.
  16. Jason James

    Jason James Guest

    Then SSRs have that delightful habit of going short more often than open,..

    The reassuring sound of a contactor releasing when you cut the juice is
    mainly pschological I guess :)

  17. Phil Allison

    Phil Allison Guest

    "Jason James"

    ** It is not an "argument" you tedious fuckwit - it is a simple fact and
    is quoted in the maker's specs.

    ............. Phil
  18. Phil Allison

    Phil Allison Guest

    "MC" <
    ** Piss off moron.

    ** Hold yours indefinitely.

    ** Nope.

    ............ Phil
  19. Phil Allison

    Phil Allison Guest

    "Mike Harding"
    "Phil Allison"

    ** Depends on the overload - surge or continuous ??

    The same inrush surge that will weld a relay permanently - an SCR or
    triac will cope with hundreds of thousands of times.

    Remember that relays have very weak opening forces, unlike a mechanical
    switch, so a tiny weld does them in.

    ............ Phil
  20. Jason James

    Jason James Guest

    The relays which operated on speech in what we called 'audio operated
    relay' modules (used for a number of monitoring and Retx functions in the
    past) literally operated and relaxed millions of times. They were just a
    normal flat cct board unit. Their reliability was phenomonal....

    Anyway Phil,...go back to the spec sheets.

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