using a non rechargeable battery in RTC circuit

Discussion in 'General Electronics Chat' started by kipper2k, Nov 14, 2014.

  1. kipper2k

    Thread Starter New Member

    Oct 29, 2014
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    Hi All,

    I am trying to find a simple way to provide battery backup for a RTC circuit (for a OKI MSM62X42BRS RTC chip) The normal primary power supply will be 5v and i am hoping to use a 3v non rechargeable Lithium CR1220 battery(which should be just enough). Am i correct in thinking that i can use a 3v Zener diode to protect the battery when 5v is available and allow 3v to supply the IC when 5v is not available ? Any help would be greatly appreciated.

    Thanks
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    1,790
    A zener diode is not really a protection device since it can only dissipate a few tens of milliamperes. You should investigate battery changeover or battery switching circuits. They usually involve a Schottky diode for the main supply and a low impedance switch for the battery. The idea is to avoid sacrificing any of the battery voltage on even the minimal drop of a Schottky diode.
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Meh... the zener is not going to help.

    Even if you arrange some switch to just power the RTC off whatever battery the output pins of this chip need to all be tristated to make sure that the RTC doesn't supply power to the rest of the circuit thru the data lines.

    By the time you are done you'll be wishing you would have started with a chip designed to do this in the first place.
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Do you have space for 3 x AAA cells, or 3 button cells or 2 x CR1220, instead of the single CR1220? If so, a simple diode-OR arrangement could handle the switch-over seamlessly.
     
  5. kipper2k

    Thread Starter New Member

    Oct 29, 2014
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    Hi Alec,

    i could use a double CR1220 cell, The RTC clock arrangement is part of a 1mb chipmem board i have built so it is actually sitting on a small board in an enclosure. I jwould like to have the option of using a RTC circuit. Is the Diode-OR arrangement simply opposing diodes ?
     
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Diode OR is two diodes with anodes tied together tied to the RTC power pin, one cathode goes to the 5V and the other to the battery, so that whatever supply is higher will power the RTC. Use schlocky diodes for the least drop for better signal noise immunity.
     
  7. kipper2k

    Thread Starter New Member

    Oct 29, 2014
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    Thats what i thought. I did find some diodes with a 0.1 - 0.3 voltage drop across them so that would probably be about the best available. According to the data sheet of the OKI RTC IC the time recording supply voltage is 2 - 5v so i am assuming a 3.0 battery should be sufficient if the voltage drop across the diode is minimal
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Re the above, Ernie's got his anodes and cathodes transposed ;)
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    Feed the chip from both the battery and the running supply via small diodes.
     
  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Probably. I have to draw it out to get em right all the time. I should have said "put the sides with the colored line together..."
     
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