Electronic – Power supply for multiple parallel LED strings

ledpower supply

My sister is building a display sign, and wants to have several "marquee letters" lit up on it. Like this one:

Letter with 9 LEDs

She bought 9 letters like this, and they all run off two AA batteries, and internally they're just a string of 7 to 9 LEDs hooked up in parallel with no resistors, just direct to the battery compartment running 3 volts.

So, she wants me to remove the battery compartments and wire them all together (74 LEDs in total) so they can all be powered off one power supply, and I feel like this is a little outside of my electronics knowledge. It would basically be 74 LEDs in parallel, and I don't have any concrete specs except they light with 3 volts across them.

In my past dealings with LEDs, it was always understood that there should be some current limiting resistor at play to protect them. There's none there now, should there be?

Also, if I try to recreate their current setup would I get a 3 volt power supply? Would it be wiser to use something larger and if so how should it be wired? Much of what I find on google on the subject is a bunch of info about choosing power supplies for 12-volt LED strips, but this is a bit different.

Could one of you be so kind as to help me understand? I hope I'm overthinking a simple circuit, I just need a little help sorting it out.

Would it make more sense to pull out the existing LEDs and insert a string of LEDs that's designed to plug into an outlet?

Thanks.

Best Answer

Get a single 6V wallwart and assume each letter is drawing 140-180 mA of current, so you find one that can source enough current for 9 letters. Put a 5W resistor that can drop 6V down to ~3 V and give you ~1.6A (a real big 2 ohm should work for that, or even better a couple of 5W 5ohms in parallel), noting that you're gonna dissipate a full 5W in that resistor) in front of the whole thing. If relying on one big current regulation resistor bugs you because the not perfectly matched parallel loads will vary a bit in brightness, tailor your resistor size to the number of LEDs, noting that each LED wants up to 20mA. Same principle.

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