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It is designed to run off of 12VDC, I would like it to run off 24VDC

Discussion in 'General Electronics Discussion' started by Y2KEDDIE, Sep 8, 2014.

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  1. Y2KEDDIE

    Y2KEDDIE

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    Sep 23, 2012
    ringdetect.GIF I built this niffty Telephone Ring detector with Relay Output from Velleman.

    It's designed to run off 12 VDC. I would like to run it off +24 VDC.

    Aside from changing the working voltage of the two capacitors, could I just double the resistor values or R3 & R4, and add a resistor in series with the relay coil (DC coil resistance value) ?

    Thanks
     
    Last edited by a moderator: Sep 8, 2014
  2. Arouse1973

    Arouse1973 Adam

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    Why do you want to do this? You waste energy doing this.
    Thanks
    Adam
     
  3. Gryd3

    Gryd3

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    R3 and 4 form voltage divider responsible for T1. Doubling them both will result in roughly the same ratio of voltage drop across R3 and 4. This will result in twice the voltage being delivered to T1. You need to increase the value of R4 to 230Ω if you want to keep the same voltage on T1. This may not even be required though, as the higher voltage will still result in saturating the transistor and I believe ~3V is fine to shove into a transistor's base.
     
  4. davenn

    davenn Moderator

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    changed pdf to gif image that's easier to access :)

    Dave
     
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  5. Arouse1973

    Arouse1973 Adam

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    Thanks Dave I couldnt open it earlier.
    cheers
    Adam
     
  6. Arouse1973

    Arouse1973 Adam

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    You only need to change the voltage to the relay if its a 12V type. Adding a resistor will do this but it will waste a lot of energy.
    Adam
     
  7. Colin Mitchell

    Colin Mitchell

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    Just change D2 for a 12v zener
     
  8. Gryd3

    Gryd3

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    The point of D2 is to protect from reverse voltage conditions. Swapping it out would allow for the diode to conduct in both directions. (one with a 0.7V drop, and the other with a 12V drop) would it not?
    An additional diode would be required, not a substitute.
     
  9. Externet

    Externet

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    Aug 24, 2009
    Install a 7812 regulator in series to D2.
     
  10. Y2KEDDIE

    Y2KEDDIE

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    I guess the 7812 would be the simplest solution. I was hoping I could do it simply with adding resistors.

    Adam,
    Everything in my facility is powered by 24 VDC. Typically I use dc to dc converters to change 24vdc to other voltages needed. I cannot find a 24VDC telephone ring detector. Power conservation is not an issue as the ring detector requires 100mA and my source is good for 1200A/hr.

    I guess the 7812 would be a simple solution. I was hoping I could do it simply with adding resistors.

    I believe a zener may work, Ill give it a try. I may have been over thinking the solution.

    Thanks
     
  11. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Wow! What a lot of discussion over a simple question!
    Yes!

    You don't even need to change R3 and R4. They're just there to limit the current to the base of the PNP and ensure it turns OFF cleanly when the optocoupler stops conducting.

    Just add a resistor in series with the relay coil of the same resistance as the coil. Yes this will waste energy. The resistor will dissipate the same amount of power as the relay coil. Make sure you use a resistor rated for at least twice that much power dissipation. You may need to mount it off the board if it gets hot. Use a low coil power relay if possible.
     
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  12. Arouse1973

    Arouse1973 Adam

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    The Zener is a good Idea. But you lose the wally diode.
    Adam
     
    Last edited: Sep 9, 2014
  13. Y2KEDDIE

    Y2KEDDIE

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    I guess what your saying is I can change the relay to a higher voltage (higher resistance). I thought I would need to change R3 &R4 to higher values (same R3/R4 ratio) to maintain the same current through the divider, (less power loss?)
     
  14. Y2KEDDIE

    Y2KEDDIE

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    I'm thinking R3, R4 are in series with the opto coupler transistor limiting the current through the transistor when t is turned on.

    Thank you for your comments.
    Eddie
     
  15. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Right. You can just use a 24V relay with no series resistor.

    Yes, R3 limits the base current. You can leave it at 15k (1.5 mA isn't going to harm the transistor) or increase it to 33k if you like. It does also form a voltage divider with R4 when the optocoupler is OFF (and C3, if connected, is nearly fully charged) but the ratio is not critical at all. Don't increase R4.
     
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  16. KJ6EAD

    KJ6EAD

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    The value of a kit like that is mostly in the PCB, not the components so why not change the relay to a 24V version. Kris suggested this while I was typing.

    Velleman sells this one (VR15M241C):

    http://www.velleman.eu/products/view/?country=be&lang=en&id=17776

    It's just the 24V, contact form 1C version of a JZC-22F which you can find from many suppliers.
     
    Last edited: Sep 10, 2014
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  17. Y2KEDDIE

    Y2KEDDIE

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    Sep 23, 2012
    I didn't think it would be that easy, but I'm not complaining. I still need to increase the working voltage on the two capacitors, sometimes my battery system voltage peaks around 27VDC.

    Thanks guys,
     
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