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1.5v circuit to replace AA battery & recharge

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  • 10-12-2007 5:08pm
    #1
    10-10-20
    Registered Users Posts: 6,742 ✭✭✭


    I have a temperature module from Maplin which runs on a single 1.5v battery.
    I am integrating this into a hot water cylinder controller of my own design which will run from 3.3v.
    I want to power the Maplin module from the 3.3v and provide for a standby NiMh battery to retain the temperature settings when the 3.3v circuit is turned off (twice daily), but I am having problems designing the 1.5v supply.

    My setup presently is as follows. I have a red LED protected by a series resistor of 220 ohms. The voltage drop across the red LED is 1.7v, sufficient for the Maplin module, but too high to place across a rechargeable battery to keep a standby charge on it. The regulation of this supply is good as the resistor limits the current to 15mA and the voltage is stable at 1.7v.
    The current pull from the module isn't high - 1x AA battery works for 3 years!

    Does anybody have a design in mind to provide 1.4 to 1.6v at approx 50mA from 3.3v? I'd prefer low component count or a discreet component over a transistor based solution.


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    #2
    DublinDilbert
    Registered Users Posts: 5,367 ✭✭✭DublinDilbert


    You can use diodes to do some crude simple regulation.....

    I've attached 2 circuits which would do this...

    Top Circuit:-
    As the quiescent current of the 1.5V source is so low, you can drop the voltage difference across a serires resistor, and use a couple of diodes to "clamp" the voltage.


    Bottom Circuit:-
    if you use standard silicon rectifing diodes each one will drop about 0.6V, hence 3 in series would drop 1.8. As the current draw of the 1.5V charger is so low each diode will probably drop 0.75V, hence you might only need 2 diodes in series...


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