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Help with a simple relay switch oo gauge train modelling connecting two circuits together

Discussion in 'General Electronics Discussion' started by Timbeckett, May 10, 2014.

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  1. Timbeckett

    Timbeckett

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    May 10, 2014
    Ok essentially I do a bit of electronic work in my spare time, currently I operate a dcc oo gauge train layout at home which I run my electric dc steam engines on. Okay so put simply I want to use what's called a seuthe smoke unit or even build my own from scratch. The problem is that the smoke unit is essentially just a wire filament that heats up oil to produce smoke, but the unit draws too many amps to be used by the decoders output in one of my engines. So I need a relay in order to switch on a larger circuit drawing more amps.


    Here are the numbers to work with

    The decoder puts out roughly 15v dc current and has a max rating of 100ma draw on it. Usually these wires are used for lights etc. these wires will be used to switch the relay on and off.

    The smoke unit will be powered directly from the track instead of directly connected to the decoder and draws 400 ma when connected. It also has a positive and negative wire.


    So my question is this,

    I don't know which relay switch I need to connect this up? Also the relay needs to be relatively small to fit in a locomotive.
    When the relay is powered it will be constantly on or constantly off I need one that doesn't work by pulses of current it needs to be as I said when powered it makes the circuit, and when not powered it breaks the circuit.

    So which relay do i need to do this, also I live in the uk so if you could give me a link to website so I can purchase it that would be extremely helpful
     
  2. Electric Al

    Electric Al

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    7
    Jan 16, 2014
    Log into ModelTrainForum.com

    There you will find a solution to your problem !
     
  3. Timbeckett

    Timbeckett

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    May 10, 2014
    I have but not been of any help so far please if anyone could help me it would be appreciates .thanks
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,499
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    Jan 21, 2010
    This should work. It's a bit expensive, but 15V is an unusual coil voltage. You also might get away with a 12V or 18V relay as they would probably be OK to operate at 15V, but you would have to check the specs.

    Here is a 12V relay that might work. They don't actually specify a range of coil voltages though.
     
    Last edited: May 10, 2014
  5. Timbeckett

    Timbeckett

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    May 10, 2014
  6. duke37

    duke37

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    Jan 9, 2011
    You could add a resistor in series with the relay coil if there is a danger of overheating the coil. The coil takes 16.6mA so you can calculate the necessary resistance from the voltage drop you require to get down to 12v

    I would not use a relay but a MOSFET, Maplin shows an IRF540 in an old catalogue.
    A resistor of about 1k should feed the gate and a 10V or so zener placed between source and gate to protect it from voltage pulses.
     
  7. Timbeckett

    Timbeckett

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    May 10, 2014
    Thanks for the help duke but I really would prefer to use a relay in this circuit, so will the one at maplins work which I have suggested? Also the positive and ground of the switching circuit and the positive and ground of the high amp circuit have to remain separate from one another, ie they cannot share the same ground if that makes sense? So two wires plus and minus to activate the relay and a separate plus and minus for the high amp circuit.
    Thanks again,

    If this one isn't suitable do you know if maplin sells ones that work?


    Thank you for bearing with me I am only a beginner at best.
     
  8. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    That relay should work at your source voltage.
    I do not know what the decoder will stand but relay coils can produce high voltages which can be damaging. To protect against these, a diode should be connected across the coil. Any small power diode will do, the common series is 1N4001 to 1N4007 The diode needs to be connected the right way round, the banded end should go towards the positive supply.
     
  9. Timbeckett

    Timbeckett

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    May 10, 2014
    So which wire should I connect to which pins ? And I don't understand where I am supposed to connect the diode ? Do I connect it across the two wires on the decoder ? Or do i connect it Across the two wires on the higher amp circuit? Thanks again you rock for helping me with this
     
  10. Timbeckett

    Timbeckett

    7
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    May 10, 2014
    Also I just looked under the comments section on the map line website and discovered this about the relay

    It took me an awfully long time (and burnt fingers due to overheating components) to work out that these relays have an integral back-emf protection diode built in. It would have been nice if the published information had mentioned that.

    Does that now mean I no longer need a diode connected to it?
     
  11. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    The built-in diode will do the job but the polarity must be right. Nothing should get hot, if it does, the polarity is probably wrong. Check the resistance of the coil with a meter using both polarities. There should be a measurement of coil resistance (700 ohm) in one direction and much less in the other direction.
     
  12. Timbeckett

    Timbeckett

    7
    0
    May 10, 2014

    also just found that maplin doesn't have this one in stock or any that meets my requirements,

    did some searches and i have found this relay online and it seems to be the right one i think?

    http://www.ebay.co.uk/itm/PCB-24V-S...arts_Vehicles_CarParts_SM&hash=item2eb6b19d1f

    do you think that this would work and do the job with my setup i want to make.? thanks tim;
     
  13. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    A 24V relay will not work reliably on your much lower voltage.

    Rather than me reading the thread from the beginning please define
    1. The voltage source to drive the relay.
    2. The load voltage and current
    £. The contact configuration.
     
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