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Discussion in 'Electronic Basics' started by Abstract Dissonance, Jan 9, 2006.

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  1. For my project I have to switch analog signals in certain combinations. I
    was going to use reed relays since I prefer to have the least amount of
    degredation of the signals from the circuit(which consists basicaly of turn
    on and off the signals in different combinations). I have several questions
    about relays though that I need to be aware.

    1. Do relays retain there state with no power? At first I thought that one
    had to switch on the relay by continuously supplying power(i.e. the
    "contacts" wouldn't "snap" into an on or off state and say there without
    power)... but I'm not sure about this since I've seen some "on" current and
    "off" current in the data sheets of some relays. If it is true that one can
    just pulse the contacts with the appropriate current to turn them off and
    turn them on this would mean that they would require no power when not being
    switched?(which is what I would like for my project since it minimizes the
    power used since I don't switch very often(few times a min at most)).

    2. How do analog switches that use semiconductors compare to relays? I can
    get 4x's the number of switches at about the same price as one relay(and I
    will be using about 30 relays). Obviously the analog switches have to be
    continuously powered which is a drawback but the main thing I'm worried
    about is how they will affect the signal. Unfortunately I can't seem to
    find any data on that aspect for the relays or analog switches.

    3. I will be using some simple microcontroller to control the relays but I
    figure that they probably can't supply enough current to the realys to
    switch them... I guess I have to read the data sheets and figure out if they
    can or not and if they can't I will have to use a transistor to control the

    4. Whats the difference between voltage and currents sensing relays? I
    thought all relays needed current to switch? Hence is it just a matter of
    how much(i.e., "current sensing" requires a much larger current and voltage
    sensing requires a little?)?

    5. The only relays I can find seem to come 1 per IC. Is it possible to find
    ones that are like spdt(but they alternate so one switches off when the
    other switches on)? or even more like how one can get quad analog switches
    but instead get quad reed relays(or whatever).

    and the last question

    6. Where is a good site to buy components from? Right now I've been looking
    at Jameco and Newark but Jameco is pretty expensive and as few products but
    is easy to navigate and search yet Newark is cheaper and has tons of stuff
    but can be very confusing at times on what they are selling and where its

    Thanks for any help,
  2. ryan weihl

    ryan weihl Guest

    you can do that with a bi-stable relay. pulse it on or off.
    you should use relay drivers. 8 drivers in a package.

    relays are specified by voltage. for 5v systems you need a
    4.5v relay.
    are we talking USA?

  3. Rich Webb

    Rich Webb Guest

    "Latching" relays will, others will not.
    Insufficient information. Your best bet is to look at the datasheets to
    see if their behavior is okay in your specific application.

    Also, in addition to parts that are labeled as "solid state relays"
    check for solid state switches like the 4066 family. Might (or might
    not) be a fit.
    One option is to use a ULN2803 or similar to "boost" the capacity of the
    uC output ports. It's basically an array of Darlingtons with a clamping
    diode on each. If you do this then you'll need to account for drop
    across the bipolars, of course.
    AFAIK these terms are related to "protective" relays, not to the garden
    variety switching relays you're looking for.
    In general there's one coil per package so all contacts are thrown at
    the same time. Omron, for example, has some 4 pole, double throw relays
    in one package but one coil operates all four.
    Ya, Newark could use nicer "drill down" and filtering capabilities.

    Also in the majors are Digikey, Mouser, and Allied; check them as well.
    However, there are smaller vendors that carry odd lots and surplus items
    who might have what you're looking for at better prices. They won't
    carry an entire range of sizes/types but may be selling the remaining
    418 relays that got designed out of the 2006 model Acme Rocket Powered
    Skateboard for a great price (but once those are gone, they may never
    have that part again). There are others but the ones I usually check (in
    no particular order)
  4. Dan Hollands

    Dan Hollands Guest

    Most relays require continuous power to stay energized but there are some
    types - usually called latching relays that just need a pulse to switch and
    no power to remain in the last state.

    Whether analog switches will work in your application depends on the signal
    being switched. The work well for low power low voltage signals

    You will need some type of driver circuit between the micro and the relay

    It is possible to get relay with spdt contacts - sometimes called form C
    It is also possible to get relays with multiple contacts

    The type of relay you want is rated by coil voltage. such as 5v coil or 12v
    coil. The data sheet will give the must operate and must drop out voltage

    Another source is - again lots of choices

    To find out more about relays choose one that you think might do the job and
    get the detailed data sheet from the manufacturers web site.

    Dan Hollands
    1120 S Creek Dr
    Webster NY 14580
  5. Ok, Thanks for all the replies. Very helpful. I think now I might be able to
    actually do the project... its so freaken hard to move from the concept to
    the real thing ;/

  6. Jasen Betts

    Jasen Betts Guest

    It can be done. With reed relays one way is to put good magnet near
    the coil such that the reed almost switches then a forwards pulse
    on the coil will switch the relay on and a reverse pulse will switch
    it off.

    also latching relays are available that do the same thing
    you'll have to get that info from the data sheets a relay typically has an
    on resistance less than 0.01 ohms whereas the switch chips are moore like
    a few cheap transistors can take care of that. probably about 15c each retail.
    yes I think so. AUIU a current sensing relay has a low resistance and
    connects in series with some oother device and switches when it is
    I've noot seen any like that.
    I've heard good things about digikey.
  7. Rich Grise

    Rich Grise Guest

    You need to start with simpler toys. :) We all did. Remember the dry cell,
    one wire, and compass experiment? Another way kewl thing to do is wind a
    coil of #24 telephone trunk wire, about 2' long, about 1 1/2" diameter,
    about 5-8 turns per inch, air core, and let it dangle from one end, and
    pass about an amp through it. ;-)

    Have Fun!
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