# Help needed Converting battery units

Discussion in 'The Family Room' started by videoman3857, Jan 13, 2017.

1. ### videoman3857Full Time Elf

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My wife purchased a heap of Village Buildings for her Christmas Winter Wonderland display that are battery operated. Some 3 x 1.5 volt battery and some 2 x 1.5 volt.
To make it easier to set up and turn on every night, I want to modify these units and use something like a 5 volt 60 watt switching power supply.

My thoughts are to run a figure 8 lead from the power supply, around the display and bring it back to the power supply. - making a ring main as it were (i'm an irrigator, not an electrician/electricaql engineer)
I want to cut off each battery pack/controller at each house, splice on a new piece of cable and tap into the ring main, connecting each house using a simple plug and socket system.
I am assuming that I will need a resistor at each house to reduce the current (?) to stop the LED's from burning out.
If my understanding is correct. The lead voltage of the LED is 3 volts, the LED current 0.018 and my input voltage is 5 volts.
I am assuming the number of leds - which varies from 4 to 8 in each building does not matter.
(5-3)/0.018 = 111.111' (Following the laws of hydraulics) I would round up to the next resistor which I think would is 150 ohms ?

Are my assumptions and calculations correct and what wattage resistor do i need to use?
The size of .25W would suit nicely into my construction needs, but if I need a higher wattage...then so be it.

Peter

2. ### SmartAlecLightsIm a SmartAlec what can i say!Community Project Designer

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ok.. my mother does the same..
i just let her waste batterys each year
(im a nice son.. i give her the crap lets an battery packs out of lights i put pixels in)

now to the problem..
well its not realy a problem.. ebay offers voltage stepdowns..
you can get them adjustable or not..
i have used them to stepdown the power from 5v to 3.3 or round abouts. work great little heat output.
max is like 80ma, so should work for a string.

otherwise you can muck around with resistors etc..

3. ### David_AVDBite my shiny metal ass!Community Project DesignerGenerous Elf

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I would grab some cheap "buck regulator" boards off eBay. Last lot I got were about \$7 for 10 delivered.

They have an adjustable output via a tiny trimpot. From memory they can go down to around 1.2V.

Using resistors can be tricky. Check the battery packs to see if they have a small resistor inline with the switch.

4. ### i13Senior Elf

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Surely you'd be able to get a power supply to do this. Let's use the 4.5V light sets as an example. If they were originally supplied with a 4.5V battery pack then it should be safe to continue supplying them with 4.5V. I haven't tested it but a 5V power supply should be able to turn down to 4.5V. You can also do current measurements because I suspect the LEDs are running below their minimum brightness (lower current) to extend the battery life. Look carefully at how the light sets are wired because series sections of LEDs share the same current and therefore lower the current measurement for the entire light set.

5. OP

### videoman3857Full Time Elf

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