Phone charger instead of 2xAA batteries

[Damo6]

My daughter has one of those IKEA LED light strings over her bed powered by 2x AA batteries. After about 2 nights the lights are noticeably dim (depending on how long after she goes to bed I manage to sneak in and turn it off for the rest of the night).

Got me wondering, as I have a few mobile phone and other power adaptors around the house, could I just wire up one of these and free myself from buying AA batteries all the time?

Apart from the voltage being a nominal 3v, what should I be looking at to match 2x AA batteries?

Thanks
D

 

[MarkieSparkie]

Hi Damo, Welcome to the forum. How many LEDs are there in the string?

 

[Damo6]

Thanks for the welcome.

There are 30 LEDs on the string.

Damo

 

[Marvo]

Phone chargers might not be very clean power especially if there's no battery in the circuit and LED's can be unreliable if the voltage and waveform isn't clean. Iwould suggest instead of a charger maybe use an LED driver supply. They shouldn't be expensive for a small one if you shop around but it would help if you knew the current required to run the string.
You could of course use rechargable batteries which would be a lot simpler.

 

[Damo6]

Thanks Marvo,

The existing set-up doesn't have a driver or any electronic circuitry. It's simply a string of 30 LEDs and a plastic box that contains 2 x AA batteries and a switch. Voltage will fall as the batteries depletes.

My simple thought was to substitute an old phone charger for the batteries.

2 x AA batteries would give you;
Voltage = 3v Nominal
Amps = ?

From there I would look through the drawers for a suitable phone charger and hopefully find one suitable.

Damo

 

[Richard Burns]

I think that this could work but as Marvo says the voltage control may not be so specific as with batteries and may reduce the life of the LEDs.
Normal LED strings with mains connected LED drivers work on a 12V supply and usually draw 350mA.
I would be surprised if your string would draw more than 500mA if the batteries last two days.
You may have difficulty finding a phone charger that supplies 3V as they are usually around 5/6V.
Overdoing the voltage will quickly burn out the LEDs. The current drawn will be determined by the LED string and so long as the charger can supply greater than this current then it will be OK.
It will be important to get the polarity correct so make sure you know which is positive and which negative on the charger and connect appropriately, however do not be surprised if they do not last long.

 

[Damo6]

Hmmm. I'm begining to see the picture now, thanks. Wonder if I was to get another string of this IKEA lights and join them up, so I'd have 60 LEDs in series. The 5/6V phone charger voltage would now be a closer match (?) and daughter would be happier with more lights. Anyone any idea how many mA two AA batteries would give, or is this a silly questions?

 

[DPG]

3V regulated DC plug-top power supplies are available (probably on ebay). These will give a nice steady output regardless (almost anyway) of current drawn.
Use a multimeter set to mA or Amps range to check what current the LEDs are drawing from the batteries and make sure you pick a power supply that can give out this figure + a little bit extra. Daz

 

[WDMDL]

You should be aware that LEDs behave in a very similar way to a Zener diode.
At a small amount below their nominal voltage, they will draw a tiny fraction of their nominal current.
At a small amount above their nominal voltage, they will draw many times their nominal current.

There should always be some limiting element in the circuit - the original setup is quite likely relying on the internal resistance of the batteries to limit the current!

All the LEDs are connected in parallel on these things, so try and aim for a current of 5-10mA per LED.

If you could find a 3.3V power supply, you could probably connect it through a low value resistor, say 2.2 Ohm to provide current limiting.

As suggested you could use two strings of LEDs connected in series to a higher voltage charger.
Something handy to know is that the Nokia switch-mode chargers are a higher voltage than the nominal 5.7 printed on them, but they do have a current limiting profile built in, so the voltage falls under load.
That's why if you've ever tried a cheap USB charger lead for a Nokia, they don't work. The phone senses an overcurrent and refuses to charge!

Simon.