Hanging LED Christmas Lights - Cutting out battery pack, powering by 5v Power Adapter

[richgit]

Hi all, first post!

I built a hanging Christmas light display above our dining table a few days ago with 15 x micro-led glass balls of varying sizes and quantities of LED's. Each light had its own 3 x AA battery pack which I cut off, and simply connected all 15 x positive and 15 x negative wires to a connector block, and then to a 240v AC to 5v DC power supply, buried in the ceiling to run off my regular wall light switch.

The ratings of the light sizes are as follows:

Small - 15 LED, 0.225w x 6 = 1.35w total
Medium - 30 LED, 0.45w x 4 = 1.8w total
Large - 40 LED, 0.6w x 3 = 1.8w total
Stars - 15 LED, [0.06w/3v - 0.9w] x 2 = 1.8w total (I'm confused by this rating, see last picture)

So if I'm correct, all together I need a power supply with 6.75w (1.35amp) minimum. 3 x AA batteries = 4.5V so I assumed 5V would be OK and not blow them.

I started with a multi-voltage adapter I have, which is rated at 2.25A @ 5v, however only 3 of the lights lit, but were flickering - https://cpc.farnell.com/ansmann/aps...-supply-27w-5-15v/dp/PW02783?CMP=TREML007-005

I assumed it wasn't providing enough power, so then swapped with my Anker Powerport 10 USB charger, which can provide 5v @ 2.5A maximum per USB output. Hey presto! All lights on and working great. Well, for 2 days. Turned them on this morning, and they're all flashing on and off rapidly.

Have I fried the lights or the USB power bank? Or have I fundamentally misunderstood how to connect these together (appreciate bunging all +ve and -ve into a connector block is a bit cowboy). Should I be separating each size of light and feeding to their own independent USB output on the powerbank? Is combining the power requirements of each different size of light into this parallel setup wrong? Any help is much appreciated.

 

[sagor1]

Multi voltage adapters are junk, they have poor regulation and often the voltages are out of specifications unless you are exactly at the rated load level.
Sounds like the USB bank worked, but then failed. Most likely the bank got overheated as it is not really intended for steady 24/7 running at full current. Chargers/banks expect the current to get less as the battery charges up. A few hours may be ok, but not steady all the time.
Get yourself a proper 5V supply that is rated at least twice the required current. Lots of those on places like Ebay or Amazon. 5v should be ok for powering those lights, rated around 3A to 5A.

 

[Nigel Goodwin]

I would suggest there's probably no current limiting on the cheap LED lights, the poor design relies on the high internal resistance of the batteries feeding it - hence they don't work from mains PSU's.

Add a series resistor to each chain to limit the current, and this should sort things out. The values will depend on the required current, and LED specification. For the 40L try 10 ohm, 30L try 15 ohm, and 20L try 22 ohm.

 

[richgit]

Thanks gents. I have a 4a, 5v power adapter arriving tomorrow, and have a selection of resistors lined up to wire in, in case that still doesn't work.

Nigel - would I have to put a resistor in line with every light, or could I put it in line with a few of the same type in parallel?

 

[Nigel Goodwin]

Thanks gents. I have a 4a, 5v power adapter arriving tomorrow, and have a selection of resistors lined up to wire in, in case that still doesn't work.

Nigel - would I have to put a resistor in line with every light, or could I put it in line with a few of the same type in parallel?

With each string of lights is probably best, try and see how it works.

 

[richgit]

Thanks all, I purchased a 5v, 4A power supply which has been working reliably for the past week. The wire after the PSU to the connector block which then feeds all 15 light fitments does run warm, I assume this is in turn building up some resistance and allowing it to all work. When I dismantle it all in the next week or so I'll try running some in-line resistors as per Nigel's recommendation.