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I am trying to figure out how to run a device that requires 1 AA battery (1.5v?) Using a mains adapter.

All the adapters seem to be 3v minimum.

Can anyone help please?

Product details: https://www.amazon.co.uk/dp/B074HY3MKC/ref=cm_sw_r_cp_apa_i_9UNKFbZA0ET4N?_encoding=UTF8&psc=1

Jay
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  • Use a 3V adapter and a further buck converter? (Or 5V and ...) –  Oct 18 '20 at 20:28
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    How much current does the device use? Or, how long does it last on its AA battery? – bobflux Oct 18 '20 at 20:31
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    Just use an additional 1.5V regulator after the mains adapter. – Aaron Oct 18 '20 at 20:42
  • It's a string of LEDs in a bottle. With a fully charged AA battery, it doesn't last more than 12 hours. – Jay Oct 18 '20 at 20:55
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    Just a string of LEDs? If they're in parallel, which they probably are, you can probably dump the existing driver and power the thing from a 5V phone charger with a resistor... – bobflux Oct 18 '20 at 21:06
  • It doesn't have a driver in the conventional sense. https://imgur.com/a/Xiuzp7O – Jay Oct 18 '20 at 21:22
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    There's an inductor on the PCB picture, which means there's probably a boost converter in there. – bobflux Oct 18 '20 at 22:55
  • What is your electronics experience level? Do you have a DMM? A Scope? Can you solder? I think that bobflux is on the right track. What color are the LEDs? Can you measure the voltage going to the LEDs? – Mattman944 Oct 19 '20 at 00:01
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    @bobflux - a single AA cell (0.9-1.5V) is not enough to light even a red LED, let alone a green, blue or white. There must be a step up converter. – Kevin White Oct 19 '20 at 00:51
  • My electronics experience level is novice. I have a multi-meter and soldering iron. The LEDs are bright white. I could probe the two soldered joints to check voltage. I e got conflicting information from other sources (Amazon), one person says it's supplying 1v and running hot as a result, someone else says it's designed for two or more batteries (3v+) and running hot because voltage is too high. – Jay Oct 19 '20 at 06:38
  • My last comment refers to one of those battery converters used for concept testing. The plug says 4.5v. I need a 1.5v supply. – Jay Oct 19 '20 at 06:49
  • If you measure the output voltage you'll probably find around 3-3.2V if all the LEDs are in parallel. Put some red permanent marker on the positive wire while you're at it... – bobflux Oct 19 '20 at 08:31
  • I didn't get a chance to check the voltage as it died. I suspect that the battery adapter was supplying too much voltage. – Jay Oct 22 '20 at 20:45

2 Answers2

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The LEDs are probably in parallel. One AA-battery has about 2000 mAh capacity, so if it lasts about 10 hours the current draw is about 200 mA.

If you use a 5V adapter, you need to drop about 2V. 2V/200mA = 10 ohms. Since these are very rough values from rough data, you need to experiment. Buy ten 10 ohm 1W resistors. With these you can get a lot of different resistances with various series/parallel combinations.

Start with 30 ohms, lower the resistance until you get the same brightness.

schematic

simulate this circuit – Schematic created using CircuitLab

Edit: you don't normally want to put LEDs in parallel, but for a cheap set of lights it can be acceptable. I ran a simple test with 4 LEDs of the same model and build lot, the brightness match is very good.

Edit2: I bought a set of fairy lights that operate one AAA battery. On this set, the LEDs are definitely wired in parallel, they are all wired like the picture. I can probe the wires at the end of the string, the voltage is about 2.6V. The step-up switcher (most likely a boost regulator) is operating at about 450 kHz. The electronics is sealed, I would have to destroy one to examine it further.

enter image description here

enter image description here

Mattman944
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  • Hello Matt, thank you for your reply. I have provided a link to the product page, is your answer still the same! – Jay Oct 23 '20 at 06:49
  • The product page doesn't provide any useful details. My answer assumes that all the LEDs are in parallel. If you carefully examine the wires you may be able to confirm this. Measuring the voltage may help also, but if they are pulses you may need a scope to get useful information. – Mattman944 Oct 23 '20 at 09:06
  • I thought the images might help, particularly https://images-na.ssl-images-amazon.com/images/I/71H%2BbOzToAL._AC_SL1500_.jpg. – Jay Oct 24 '20 at 12:57
  • I have been wanting to get some fairy lights, so I just purchased some powered by a single 1.5V battery. They should arrive tomorrow. I will see if I can learn anything about their operation. No guarantee that they will operate the same as yours. – Mattman944 Oct 24 '20 at 17:53
  • Thanks Matt, hopefully we can get this sorted. It should be difficult in this day and age. – Jay Oct 26 '20 at 19:34
  • Thanks for the update. Do you have any idea about the efficiency and heat produced by dropping voltage from 5v to 1.5v? – Jay Nov 19 '20 at 17:57
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An LM317 linear regulator would be a good solution, since it is cheap and commonly available. It's been around for years. You can see an example circuit below. Instead of the transformer, T1, and diode bridge, D1-D4, you could just use any AC-DC power supply that generates an output voltage of around 3-9V. A 5V supply would be ideal.

If the load current is high, e.g. > 0.5A, you may need to use a small heatsink.

schematic

Image source: ElecCircuit - Variable power supply using LM317

SamGibson
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mr_js
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    This is a very short answer that misses a lot of needed detail, especially useful for a newcomer, like: that the input voltage would need to be at least 3.5 volts, maybe more, that the LM317 may require heat-sinking if the current is significant – anrieff Oct 19 '20 at 08:31
  • Your Answer needs to contain some explanations to justify it rather than just firing off one line. It should be written clearly enough to teach future readers. Please can you edit it and greatly improve it, adding detailed explanations to make it informative rather than just factual. Thanks. – TonyM Oct 19 '20 at 08:54
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    Added more info. – mr_js Oct 19 '20 at 14:25
  • Going from 24V to 1.5V with a LM317 will cause it to melt through the floor... and no, heat sinks won't save you. I very much doubt that the posted schematic has ever been used in the real world. – Lundin Oct 23 '20 at 06:55
  • @Lundin - did you read the answer properly? If so, you would have read that I recommend using a 5V supply, not a 24V supply. – mr_js Oct 23 '20 at 16:09
  • @mr_js Yes but I'm wondering what the person who made the schematic was thinking. – Lundin Oct 26 '20 at 06:26