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Transistor as a relay vs mechanical relay

Discussion in 'Electronic Basics' started by [email protected], May 21, 2005.

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  1. Guest

    Just a very basic question:

    Can anyone share with me what's the benefit of using transistor as an
    electrical relay compared to mechanical relay when the incoming signal
    is from a computer?

    Is it related to pricing, input voltage/current, power rating and etc?

    Pls help.

    Thanks.

    Albert
     
  2. Rich Grise

    Rich Grise Guest


    No moving parts.

    Cheers!
    Rich
     
  3. Chris

    Chris Guest

    Yup.

    A basic question, indeed. Let's count the ways.

    * Tranistors switch in microseconds, relays in milliseconds.

    * Transistors switch directly on without contact bounce, relay
    contacts smack and bounce together for a millisecond or so, causing
    intermittent on/off for that time until the contacts settle in.

    * Relays are mechanical devices with moving parts -- they will wear
    out. A typical relay is rated for somewhere between 10 thousand and 10
    million operations before failure. And relay contacts usually wear out
    first, if they're swicthing at anywhere near rated load. Actually,
    they're supposed to.

    * Relay contact bounce can cause EMI which can cause problems.
    Although you generally have to be more careful with transistors when
    switching inductive loads, there isn't a spark being created with
    switching like with relays. That EMI can cause the computer to spit
    up.

    * A typical high current transistor will usually cost much less than a
    relay rated to switch equivalent current.

    * A transistor will typically require much less power to operate than
    a relay coil, especially if you use a darlington transistor or a
    MOSFET.

    I guess there are more reasons, but these are the ones that come to
    mind first.

    Good luck
    Chris
     
  4. Harold Ryan

    Harold Ryan Guest

    It's none of the items mentioned. The signals from the PC may be on a
    separate ground than the other electronic circuitry that is receiving the
    signal. In this case, isolation is required by either by a mechanical relay
    or an opto coupler. Just a transistor is not adequate.

    Harold
     
  5. Read the the intro, pages 1-6, for this book:
    http://www.amazon.com/gp/reader/0521478790/ref=sib_dp_pt/103-2146462-2957458#reader-link
     
  6. [snip]
    A Darlington transistor requires a V drop of twice that of the single
    junction transistor. The C-E V drop of a single junction can be a tenth
    of a volt, but a darlington has to be at least 2 diode drops or about
    1.2V to function. So the power wasted by a darlington is much greater
    than with a single junction transistor.

    If you want to minimize this, use a regular power transistor, and drive
    it with another transistor connected common collector or emitter
    follower. This basically means do _not_ connect the collectors together
    in a darlington config.
     
  7. "Watson A.Name - "Watt Sun, the Dark Remover""
    That is not a useful statement of the facts.
    True (assuming you mean a single BJT). Or it can be more,
    or less. A BJT in hard saturation can have 50 mV C-E drop.
    Actually, the input BJT can and often does saturate to the
    kind of drop stated above. It keeps the output BJT out
    of saturation since the input C-E drop is in series with the
    base of the output BJT. This results in typical darlington
    C-E drops of 0.8 to 1.0 V. There is nothing about this
    situation that makes "2 diode drops" significant.
    Assuming you mean a single bipolar junction transistor,
    the truth of your claim depends largely on what supply
    the base current is taken from. Where the input BJT
    forced Beta is X, then for a bias supply greater than
    X times the additional drop of darlington, your claim
    is strictly false, (meaning "much greater" is no greater).
    That may be a good strategy if a low voltage bias
    supply is used and if the extra parts count is worth
    the power savings. But the OP should be aware
    that Darlington transistors have been used in many
    places by people familiar with the alternatives.

    These days, stringing together BJTs as you suggest
    is rare. If the output drop is important, a single
    MOSFET is generally favored.
     
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