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convert a mains LED clock to battery

Discussion in 'Power Electronics' started by Gmufter, Aug 23, 2012.

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  1. Gmufter


    Aug 23, 2012

    I have a small project I am doing and could use a little advice. I want to convert a LED clock that currently runs on 230v mains to run on battery. I have a infant son who is sleeping in our room at the moment, when he stirs at night I would like to see what time it is, currently illuminating the phone to check the time is too bright and often wakes him fully. As there is not enough sockets in the bed room I thought to convert a simple cheap red LED clock to run on battery.
    I opened the clock and the ac mains cable goes through what I can only assume is a step down unit, it appears to be a coil of wire?, heavy enough anyway. I ran a volt metre across the input where it is soldered onto the circuit board and it reads as 6.3v (if I am doing that right)
    So what to do to next to be able to have it run on a 9v battery for example.
    There is a battery connection for a back-up for the clock, but it appears that this only keeps the time without the clock display being illuminated. If I set the time, un plug it for 30 mins, then replug it in, the time has moved on 30 mins but the LED display has not been on.
    The only other information I can gather is from a sticker on the case that says power "Power source: AC 230V - 50hz
    DC9V battery back up for clock.
    power consumption 3W/hour
    standby consumption 1.7W/hour."
    I appreciate any help that can be given.
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    You could probably run the clock from a battery, but at 3W any battery will drain rather quickly if the LEDs are active.
    You could use the 9 V battery as provided to ensure timekeeping.
    From this battery make a connection throug a momentary switch (pushbutton) to the point where you measured 6.3 V. You can insert a 2.7 V Zener diode into this connection to reduce the voltage.
    Now the backup battery should keep the time while the display will light up only at the push of the pushbutton.

    You may want to use a rechargeable battery - I have experienced very short lifetimes of these batteries in several clocks even if used only for backup purposes. But you'll have to move the clock near an outlet so it can be mains powered while you change the batttery. Otherwise you will have to set the time after changing the battery.

    It is possibly easier to buy an LCD alarm clock which has a light that's activated by pushing a button.

    One note at the side: The "step down unit" you mention could be a transformer (likely) or a switch mode power supply. In any case be careful when tinkering with the primary side. There is dangerous high potential there.
  3. GreenGiant


    Feb 9, 2012
    if it does truly run at 6.3 volts then you are going to need some pretty large batteries, 3W from 6.3 volts is a little under half an amp, so in order to make it last all night you will need (assuming night=8 hours) a 4Ah battery... even the idle of 1.7W you would need a little over 2Ah battery...

    probably going to cost a pretty penny, so my recomendation would be to get something like this cheap and basic, but enough to keep the time and make it visible when you need it (just one example you can find hundreds more)

    To put it in perspective the cheapest 4Ah battery I could find was just over 10 bucks, and it was rated at 3.7 volts, so you would need 2 of them (20 bucks) and you would need a way to charge them as well (probably another 10-20 bucks)

    Just trying to get you the cheapest option here man
  4. Gmufter


    Aug 23, 2012
    Thank you both for your replies, unfortunate I cannot (or would not be practical) to do what I had hoped to do and shall look else where for a solution to the problem. Another purchase, perhaps on of the recommendations.
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