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Quick question about AC/DC adapters

Discussion in 'General Electronics Discussion' started by hwd, Oct 7, 2014.

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  1. hwd


    Oct 7, 2014
    Hi all,

    It has been many years since my GCSE electronics and so I apologise if this is a very stupid question...

    I want to replace 3 AA batteries in a torch, with a plugged in supply, and I am having trouble finding a suitable adapter. I know that I need a 4.5V supply (3 x AA) and have read that the current is typically 900mA from AA batteries.

    I have found a 4.5V 1200mA adapter. Is this going to destroy the LED bulb / circuitry or am I being a muppet?
    I have also found a 5V , 900mA adapter. I assume this is worse..?

    Thanks for your help

  2. Gryd3


    Jun 25, 2014
    When finding a supply, the most important thing to match is the voltage.
    The second thing to deal with is current... this value should be higher than what you need. Current is not pushed into things. So even if you get a 4.5V 150000Amp supply, your torch will only pull what it needs from that 150000 ;)

    There's a catch 22 though... some cheap torches rely on the battery's internal resistance to limit the current through the LEDs... using a battery of a different chemistry, or a different supply could cause problems. If possible take a look inside the torch to see if there is a small circuit board.
    One of two things is required to ensure this will work properly.
    A) Look for a resistor
    B) Look for a circuit board with a few components.
  3. JimW


    Oct 22, 2010
    Maybe is always a safe answer. Batteries (Alkaline) are 1.5v and then drop from there as they age. They can supply up to the batteries rated max current. Some lights use the internal resistance of the battery to limit the current, or help limit the current, into the LED. This keeps the LED from burning out.

    An AC to DC converter can be designed a variety of ways. A great many of them specify the output voltage at the maximum rated current. So your 4.5V 1200ma adapter might be 5.5 volts if you only draw 600 ma. So depending on the design of both the torch and the voltage adapter, you could easily burn out the LED. Or it could work just fine.

    A safe way out would be to put a 10 ohm resistor in series with the voltage to the LED. And then take a few measurements. And maybe see how bright the LED is. If you had a few 10 ohm resistors, you could start with one, then add a 2nd in parallel, then maybe a third. Taking measurements as you go.

    KrisBlueNZ and Gryd3 like this.
  4. Gryd3


    Jun 25, 2014
    Good call on the voltage being higher Jim. This is common with cheaper 'transformer' AD-DC adaptors. Switching transformers should be a lot more stable.
  5. Arouse1973

    Arouse1973 Adam

    Dec 18, 2013
    I need to point out that the internal resistance of a fresh AA battery will be less than 300 mOhm. The voltage drop will be quite small maybe only 100 mV not enough to limit the current to a safe level. So you can not rely on the internal resistance of a fresh battery to limit the current significantly.
    Gryd3 likes this.
  6. hwd


    Oct 7, 2014
    Cheers guys!
    So it sounds like I should pick up a 4.5V adapter, with a current as close to 900 as I can get and a bunch of resistors, then dig out the old voltmeter/ammeter and get cracking... This is going to be more fun that I thought
  7. Gryd3


    Jun 25, 2014
    Resistors are always good to have, and you are free to get something with a little more mA. Just don't under cut that value.
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I'm not sure where the 900mA has come from. I suspect that it has come from 900mAh. mAh does not mean mA. You need to find out what the actual current draw is. A 3W LED will draw less than 900mA, a 5W LED more than 900mA. So if you can determine the power this will provide a good estimate
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