Switching an LED strip to mains power from battery

Thread Starter

Nicholas Mason

Joined Aug 18, 2019
25
Hi All,

Preface: In case you didn't see the previous one, totally new to all this, so probably asking dumb questions, feel free to explain it like I'm 5.

Objective: I have a strip of LEDs fitted inside a box which backlight a name panel on the front (think something similiar to a contestant nameboard on a quiz show). Simply I would like this to light and stay lit (as they will be on the vast majority of the time). However, currently they are wired in to a battery power source which uses 3x AAA (1.5V) batteries, which it depletes in a matter of hours. So need an alternative that can literally keep the lights on (That was funnier in my head). The LED strip is about 114cm long, an LED every ~3.5cm, I would guess they are waterproof 3528 diodes, but wouldn't know, as I didn't buy them.

Approach: Seems so far, easiest solution would be to replace the battery and wire to a mains power source (from the UK -so 240V). I'm guessing I'll need a step down transformer, but don't know much more than that (I assume the LED strip is 12V, so 20:1 ratio). Also ideally a switch in line would be nice, as the plug socket is beneath some desks and would be really handy to just be able to lean over to turn it on and off

So...
1) Is this the best way of achieving the objective?
2) How do I go about switching the battery pack for 240V mains and a switch, what components will I likely need, and where can I find them? What are the steps?
3) How do I do this without burning all the LEDs in the strip and them getting red hot, and avoiding dangerous exposed bits if possible? If it helps, I have some clip on connectors for LED strips that might help for this, but that still brings me back to question 2.

Cheers for any help
(Again, can supply pictures of the box, LED strip, battery pack etc. if it would help, just let me know)
 

ebeowulf17

Joined Aug 12, 2014
3,274
Hi All,

Preface: In case you didn't see the previous one, totally new to all this, so probably asking dumb questions, feel free to explain it like I'm 5.

Objective: I have a strip of LEDs fitted inside a box which backlight a name panel on the front (think something similiar to a contestant nameboard on a quiz show). Simply I would like this to light and stay lit (as they will be on the vast majority of the time). However, currently they are wired in to a battery power source which uses 3x AAA (1.5V) batteries, which it depletes in a matter of hours. So need an alternative that can literally keep the lights on (That was funnier in my head). The LED strip is about 114cm long, an LED every ~3.5cm, I would guess they are waterproof 3528 diodes, but wouldn't know, as I didn't buy them.

Approach: Seems so far, easiest solution would be to replace the battery and wire to a mains power source (from the UK -so 240V). I'm guessing I'll need a step down transformer, but don't know much more than that (I assume the LED strip is 12V, so 20:1 ratio). Also ideally a switch in line would be nice, as the plug socket is beneath some desks and would be really handy to just be able to lean over to turn it on and off

So...
1) Is this the best way of achieving the objective?
2) How do I go about switching the battery pack for 240V mains and a switch, what components will I likely need, and where can I find them? What are the steps?
3) How do I do this without burning all the LEDs in the strip and them getting red hot, and avoiding dangerous exposed bits if possible? If it helps, I have some clip on connectors for LED strips that might help for this, but that still brings me back to question 2.

Cheers for any help
(Again, can supply pictures of the box, LED strip, battery pack etc. if it would help, just let me know)
A few pictures might help. A little bit of testing with a multimeter would be even better.

You say it uses a battery power source with 3 AAA batteries (4.5V total nominal voltage) and then later speculate that the LED strip requires 12V. If that's the case, the power source must have some sort of switching boost circuit in it. Detailed pictures of the power source (especially any labels, model number, power ratings, etc) would be helpful.

Good pics of the strips would be helpful too. Most LED strips have resistors built into them, which greatly simplifies the rest of the design. We just need to make sure this strip is the kind I'm picturing in my head.

If it's a 12V system, then you just need to buy an isolated 240VAC to 12VDC power supply and a switch, both rated for high enough current. With 30-ish LEDs, and assuming 20mA or less each, you could probably use a 1-1.5A supply (it's always good to over-spec things a little bit.)

If the strips don't have resistors, or if the battery pack is simply a battery pack, without any voltage boosting circuitry, then we'll need to get a better idea what the actual voltage and current requirements are.

In any case, I wouldn't recommend building your supply with discrete components. Mains power is dangerous, and you should build up experience working on safer circuits before diving in to high voltage stuff. If you buy an off the shelf, isolated supply, you can accomplish what you need to safely and fairly easily.
 

Tonyr1084

Joined Sep 24, 2015
4,889
Do you have an old phone charger laying around ? ? ? I usually can find half a dozen in my arsenal. They're 5 volts (close to your 4.5 volt batteries). Some produce 200 mA of power, some as much as 700 mA of power. I don't know if that would be sufficient to drive your LED's because you haven't given us enough information on what you have. A schematic of how many LED's there are and how they're wired, any resistors that may be present would also be very helpful to decipher what solution you need. Being that this is a "Sign" of sorts - I'm going to guess that most cell phone chargers will not be sufficient.

If you went with a larger PS, such as a 12 volt wall wart, you'd have to add additional circuitry to manage the higher voltage. LED's are current driven devices, so with a higher voltage you'll need higher resistances. The most important things are knowing how it's wired and what value parts are present. Without the numbers and a diagram the best we could go would be to guess at a solution.
 

Bernard

Joined Aug 7, 2008
5,606
The LED string sounds familiar; mine is 30 LEDs, 5 cm spacing, power 3 AA's in a box with a switch & timer. If same, the LED's are all in parallel. But better to wate for pictures.
To add a switch, it would be best to pick an AC adapter with a power cord.
 

MisterBill2

Joined Jan 23, 2018
6,685
If it runs on three AAA batteries and if it was not terribly expensive then it does not have a power converter, it has LEDs and possibly a resistor. So certainly a small 5 volt wallwart type supply, probably with a series resistor to limit the current a bit, and you will be all set. But be very careful of polarity because many LEDs are totally non-forgiving of reverse voltage of very much.
 
Last edited:

MisterBill2

Joined Jan 23, 2018
6,685
Addition for post #4, 30 LED's, total drain = 50 mA, series R = 36 ohms, LED V = 2.8 V.
We still need real facts.
What we do know is that #1, the package runs on three AAA cells, providing a maximum of 4.5 volts, and #2, it runs the battery down in a few hours, which shows that they are not high power LEDs. So now we can know that a 5 volt supply with a resistor to limit the current will work.I suggest starting with a 100 ohm resistor and observing the brightness. We can also guess that possibly there are sets of 2 in series, although I doubt it.
 

Thread Starter

Nicholas Mason

Joined Aug 18, 2019
25
Hi Guys,
Apologies for the delay in replying. Also I love the admirable effort in trying to figure out the solution from a lack of information based of small clues - it's pretty awesome.

Let's see if we can get some helpful additional information. (Pictures of Battery Pack, Connection from strip to wiring into battery pack, whole box, length of the LED strip, and what it looks like on with a fresh set of batteries - and thus the effect I'm hoping to reproduce but more consistently)

20190821_190106.jpg 20190821_185949.jpg 20190821_190014.jpg 20190821_190956.jpg jlksPj32.jpg
There are seemingly no model numbers or ratings or labels on anything. I had a good look. Maybe there's something on the back of the strip but i peeled a bit back and found nothing - didn't really want to strip the whole lot off because I'm pretty sure it won't go back well.

If there's anything else going on in the battery pack it's in that top section. Since there's a switch on it, there must be something in there, but it's sealed plastic, so not sure how to find out.

The outer dimensions of the box are 36cm x 11.5cm (the thickness of the wood is about 2.5cm), and there's an 'overlapping' portion on the lower edge of about 0.5*the length of the box (So essentially: 2.5 lengths + 2 width, which is where the figure 114cm came from - approximating the width as 12cm at the time)

I can't for the life of me find our multimeter, but will supply some readings when I can. Though between the adhesive strip on the back and the waterproof cover on the front i'm not sure how I'm going to do that.

---

If it's a 12V system, then you just need to buy an isolated 240VAC to 12VDC power supply and a switch, both rated for high enough current. With 30-ish LEDs, and assuming 20mA or less each, you could probably use a 1-1.5A supply (it's always good to over-spec things a little bit.)

If the strips don't have resistors, or if the battery pack is simply a battery pack, without any voltage boosting circuitry, then we'll need to get a better idea what the actual voltage and current requirements are.

In any case, I wouldn't recommend building your supply with discrete components. Mains power is dangerous, and you should build up experience working on safer circuits before diving in to high voltage stuff. If you buy an off the shelf, isolated supply, you can accomplish what you need to safely and fairly easily.
Completely agree re: mains stuff. I would prefer discrete components anyway, just for simplicity. Whilst I'm still not sure of the voltage, hopefully those pictures will elucidate things a little.

Do you have an old phone charger laying around ? ? ?
A schematic of how many LED's there are and how they're wired, any resistors that may be present would also be very helpful to decipher what solution you need. Being that this is a "Sign" of sorts - I'm going to guess that most cell phone chargers will not be sufficient.
Yeh, I have a wealth of phone chargers in various drawers. I genuinely have no idea of the specifics like the wiring, resistors etc. but it might be visible in the photos above. I make it exactly 30 LEDs.

The LED string sounds familiar; mine is 30 LEDs, 5 cm spacing, power 3 AA's in a box with a switch & timer. If same, the LED's are all in parallel. But better to wate for pictures.
To add a switch, it would be best to pick an AC adapter with a power cord.
You could be right, the string is 30 LEDs exactly, and obviously doesn't quite fit the perimeter exactly, because about 6 or 7 LEDs loop round again, making me think the strip came as a string of 30 to start with. There is no timer on the battery pack, picture is above, but the only thing on the back is a plastic slide on/off switch. Also I think 5cm spacing would be a bit too much, otherwise the strip would be 150cm long, but mine is only about 114cm.

If it runs on three AAA batteries and if it was not terribly expensive then it does not have a power converter, it has LEDs and possibly a resistor. So certainly a small 5 volt wallwart type supply, probably with a series resistor to limit the current a bit, and you will be all set. But be very careful of polarity because many LEDs are totally non-forgiving of reverse voltage of very much.
It was bought off of Etsy, as a whole unit, so discerning it's "true" cost is a little difficult. The whole thing cost £20 inc. postage. I've seen conflicting opinions on the phone charger thing, so curious about this, sounds like a simple solution if it works, but wouldn't I have to wire up the LEDs to a USB plug? If this is the way to go, how do I make sure I don't bugger up the polarity?

What we do know is that #1, the package runs on three AAA cells, providing a maximum of 4.5 volts, and #2, it runs the battery down in a few hours, which shows that they are not high power LEDs. So now we can know that a 5 volt supply with a resistor to limit the current will work.I suggest starting with a 100 ohm resistor and observing the brightness. We can also guess that possibly there are sets of 2 in series, although I doubt it.
The drain, at least in light given off seems almost like a negative exponential - it last longer when dimmer. So if the 5 volt phone charger is the way forward, how do I, erm, actually execute this? Because as far as I've played with any of these, USB cable goes in and my phone charges.

Also, just for clarity, I'm guessing 'Wall wart' = mains based charger of some description?
 

ebeowulf17

Joined Aug 12, 2014
3,274
Hi Guys,
Apologies for the delay in replying. Also I love the admirable effort in trying to figure out the solution from a lack of information based of small clues - it's pretty awesome.

Let's see if we can get some helpful additional information. (Pictures of Battery Pack, Connection from strip to wiring into battery pack, whole box, length of the LED strip, and what it looks like on with a fresh set of batteries - and thus the effect I'm hoping to reproduce but more consistently)

View attachment 184450 View attachment 184451 View attachment 184452 View attachment 184456 View attachment 184458
There are seemingly no model numbers or ratings or labels on anything. I had a good look. Maybe there's something on the back of the strip but i peeled a bit back and found nothing - didn't really want to strip the whole lot off because I'm pretty sure it won't go back well.

If there's anything else going on in the battery pack it's in that top section. Since there's a switch on it, there must be something in there, but it's sealed plastic, so not sure how to find out.

The outer dimensions of the box are 36cm x 11.5cm (the thickness of the wood is about 2.5cm), and there's an 'overlapping' portion on the lower edge of about 0.5*the length of the box (So essentially: 2.5 lengths + 2 width, which is where the figure 114cm came from - approximating the width as 12cm at the time)

I can't for the life of me find our multimeter, but will supply some readings when I can. Though between the adhesive strip on the back and the waterproof cover on the front i'm not sure how I'm going to do that.

---



Completely agree re: mains stuff. I would prefer discrete components anyway, just for simplicity. Whilst I'm still not sure of the voltage, hopefully those pictures will elucidate things a little.



Yeh, I have a wealth of phone chargers in various drawers. I genuinely have no idea of the specifics like the wiring, resistors etc. but it might be visible in the photos above. I make it exactly 30 LEDs.



You could be right, the string is 30 LEDs exactly, and obviously doesn't quite fit the perimeter exactly, because about 6 or 7 LEDs loop round again, making me think the strip came as a string of 30 to start with. There is no timer on the battery pack, picture is above, but the only thing on the back is a plastic slide on/off switch. Also I think 5cm spacing would be a bit too much, otherwise the strip would be 150cm long, but mine is only about 114cm.



It was bought off of Etsy, as a whole unit, so discerning it's "true" cost is a little difficult. The whole thing cost £20 inc. postage. I've seen conflicting opinions on the phone charger thing, so curious about this, sounds like a simple solution if it works, but wouldn't I have to wire up the LEDs to a USB plug? If this is the way to go, how do I make sure I don't bugger up the polarity?



The drain, at least in light given off seems almost like a negative exponential - it last longer when dimmer. So if the 5 volt phone charger is the way forward, how do I, erm, actually execute this? Because as far as I've played with any of these, USB cable goes in and my phone charges.

Also, just for clarity, I'm guessing 'Wall wart' = mains based charger of some description?
Well, the good news is that you've definitely got the "usual" setup as far as LED strips go, and it's got the necessary current limiting resistors built-in, so you're not dealing with any complex control circuitry (constant current sources, etc.)

I'm guessing the battery box is literally just that, no boost circuit, and so the LED strip is getting whatever voltage the batteries deliver. Once you find your multimeter, it will be easy to confirm. If you're going to replace the battery pack with some other power supply, you'll eventually need to cut and strip those wires, so you can carefully strip back some insulation off of the wires whenever you want, and measure the voltage between the two wires. Then you know for sure what voltage it takes to replicate what you're currently seeing.

I'm guessing it'll be between 4.8 and 3.0V ( a very rough estimate of the total usable voltage range of AAA batteries from fresh to dead.) Assuming it's in that range, you could try a regular phone charger. They're usually set for 5.0V, which will be a little higher voltage than 3 batteries deliver, and will make the LEDs run brighter (and hotter, with potentially shorter life expectancy.) You could also look for a 4-4.5V wall wart if you wanted a closer match.

Regardless, step one is confirming the voltage that is being fed to the LED strip. Then step two is buying a suitable wall wart power supply, or hacking one that you've already got.

As for the subtleties of execution, try to separate and mark the two wires before cutting them. If you can strip some insulation back to get voltage readings without actually completely cutting the wire, that's great too - if not, just cut it, strip it, and twist the end back together (still with their original polarity) so you can get it running and get voltage measurements. Measure voltage with your meter and determine which side is positive (when the red wire on your meter is on the positive supply wire, your voltage reading will be positive, and vice versa.) Once you've marked which wire needs to be positive, you can cut the wires completely. Then fire up your wall wart (not connected to LEDs yet) and determine the polarity of its wires, marking them accordingly. It took me a lot of words to describe, but you'll find that it's easy to do in practice.

Cheers!
 

Bernard

Joined Aug 7, 2008
5,606
It looks like the LED strip can be cut between the copper dots so you have much flexability in arranging
strips like 3 parallel strips with the 3 + spots soldered together with red wire & 3 - spots with black wire.
With a 5 V supply I would add a diode ( 1N4002 or any in the series ) in the + wire, anode toward supply, cathode, with band, toward + dot on strip.
This would give a reasonable V drop to match supply with the strip.
 

ebeowulf17

Joined Aug 12, 2014
3,274
It looks like the LED strip can be cut between the copper dots so you have much flexability in arranging
strips like 3 parallel strips with the 3 + spots soldered together with red wire & 3 - spots with black wire.
With a 5 V supply I would add a diode ( 1N4002 or any in the series ) in the + wire, anode toward supply, cathode, with band, toward + dot on strip.
This would give a reasonable V drop to match supply with the strip.
I forgot about the diode trick! That's almost certainly cheaper and easier than sourcing a supply with an unusual voltage.
 

Thread Starter

Nicholas Mason

Joined Aug 18, 2019
25
Okay - that's encouraging. Thank you for all the help and the very specific instructions.

Without the diode, how severe are the effects of supplying it with 5V likely to be? The LEDs didn't get particularly hot if i remember rightly, and I'm not too bothered if they output ~10% more light (Is light intensity proportional to voltage?)

I was thinking of a power supply that just had a slot for a USB cable which is detachable, would I require one with a pair of molded cables?

I have found my multimeter; I haven't really got much to strip the wires with though. No wire strippers or pliers etc. on hand. Could try it with scissors but that might be a 'when it's not 3AM' thing for me.

I shouldn't need to alter the actual LED strip as far as I understand? I'm fine with the arrangement and placement and performance, I'd just need to cut it at the wires that leave the box to join the battery pack.

Any tips for prettying this all up afterwards and keeping it all together or is it just a wrap it in electrical tape job?
 

ebeowulf17

Joined Aug 12, 2014
3,274
Okay - that's encouraging. Thank you for all the help and the very specific instructions.

Without the diode, how severe are the effects of supplying it with 5V likely to be? The LEDs didn't get particularly hot if i remember rightly, and I'm not too bothered if they output ~10% more light (Is light intensity proportional to voltage?)

I was thinking of a power supply that just had a slot for a USB cable which is detachable, would I require one with a pair of molded cables?

I have found my multimeter; I haven't really got much to strip the wires with though. No wire strippers or pliers etc. on hand. Could try it with scissors but that might be a 'when it's not 3AM' thing for me.

I shouldn't need to alter the actual LED strip as far as I understand? I'm fine with the arrangement and placement and performance, I'd just need to cut it at the wires that leave the box to join the battery pack.

Any tips for prettying this all up afterwards and keeping it all together or is it just a wrap it in electrical tape job?
The brightness (and heat) of the LEDs is roughly proportional to current through the LEDs, but the current isn't directly proportional to the voltage you provide the strip.

It's not entirely intuitive at first, but diodes (including LEDs) behave in such a way that they appear to have a relatively predictable voltage drop across them, as opposed to having a predictable fixed resistance. You can do rough circuit analysis by simply subtracting the expected voltage drop across the LEDs (known as its forward voltage) from the supply voltage. The voltage number that remains is what must be dropped across the current-limiting resistors, and ohms law can be applied directly here.

So, as an example, lets suppose you have white LEDs with a Vf (forward voltage) of 3.2V. While the batteries are pretty fresh, they'll be delivering 4.5V, so that leaves 1.3V to drop across the resistors. If the resistors are 100 ohm, divide the 1.3V by 100 and you'll get 0.013A, or 13mA.

With a 5V supply, you'd have 1.8V across resistors, yielding 18mA of current. If you add a diode as suggested above, you subtract its Vf, just like for the LEDs. A typical diode Vf is 0.7V, so let's run those numbers:
5V - 3.2Vf for LED - 0.7Vf for diode leaves 1.1V across resistors, and delivers 11mA current through LEDs.

So, LED performance has a predictable relationship with supply voltage, but not the intuitive, directly proportional one you might expect.
 

ebeowulf17

Joined Aug 12, 2014
3,274
As for assembly, there are a million options.

Given that you want to add a switch anyway, you might be able to use that as your junction point between the wires already attached to the box and the new wires from the power supply. If the switch has screw terminals, assembly would be really easy. Terminals for crimp connectors would be straightforward too, but you'd need a crimping tool if you don't already have one (some wire strippers have crimp tools built in - they're generally not the best, but likely sufficient for this job.)

If you need splices mid-wire, electrical tape is ok, but tends to get sticky and messy with age. I use it when needed, but it's sort of a last resort if the wires will be exposed in any way. Heat shrink works nicely for covering joints. Insulated crimp connectors are also good.
 

ebeowulf17

Joined Aug 12, 2014
3,274
A power supply that just has a USB slot is fine if you have a USB cable you don't mind cutting up for this project. A supply with a molded pair of conductors you can easily see and separate would probably be easier to work with, but it's certainly not critical.

Ok, there were lots of questions in that last post - I think I addressed them all, but if I missed something just ask again.
 

Thread Starter

Nicholas Mason

Joined Aug 18, 2019
25
Also, thank you for the explanation ebeowulf, it's much appreciated. Got the wire strippers now, so can potentially get some more information now.
 

ebeowulf17

Joined Aug 12, 2014
3,274
Dumb question - before i start tearing things apart...
Would it just be simpler to buy an off the shelf mains plug meant for running LED strips? Something similiar to https://www.amazon.co.uk/Onforu-Adapter-Daylight-Non-Waterproof-Lighting/dp/B07DZT4SJC/ref=sr_1_3?keywords=led+strip+plug&qid=1566747005&s=lighting&sr=1-3
Probably not, but it depends on what you plan to do with that, and which part of the project seems hardest to you.

If you're thinking of buying that Amazon lighting kit and totally replacing your existing power and lights, that's certainly simple in terms of not having to figure anything out or risk electrical mistakes. However, it's a lot more work physically replacing all the existing LED strips in the box.

If you're thinking of using the power supply and switch from that kit with your existing strips, then all it really gains you is a nice inline switch. You'd still have to identify wire polarity and do several cuts and splices. More importantly, we don't know yet what voltage you need. My best guess from what you've shared so far is that it's direct battery voltage (nominal 4.5, actual range more like 4.8 down to ~3.) If that's the case, a 12V supply would deliver almost 7 times the light and heat desired, most likely destroying the LEDs pretty quickly.

So, unless you plan to replace the strips themselves, step 1 is still measuring your existing voltage so we know for sure what we're dealing with.

P.S. After thinking a bit more about my earlier comments, it occurs to me that the odds are quite low that anyone is making special strips for 4.5V battery packs. You've probably got a standard 5V strip being slightly underpowered. If that's the case, you could replace the battery pack with a 5V USB power supply with no need to worry about overpowering it (no need for extra diode inline.) It would be about 50% brighter, but because human light sensitivity is non-linear, the perceived difference would be much smaller. In other words, if you've got voltage on existing system anywhere between 3-5, feel free to use a 5V supply without sweating the details.
 

MisterBill2

Joined Jan 23, 2018
6,685
It is often "simpler" to purchase what somebody is selling, but the reality is that unless you are burdened with more wealth than you know what to do with, it may not be the best choice, because it is what they want to sell, which may not be what you need. And in almost all instances you are paying more than the (whatever) is worth. If you have a lights system and the only complaint is that it eats batteries, then a power supply of some kid is a solution,
 
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