Bit of help with an LED project please?

Thread Starter

dameo182

Joined Nov 12, 2022
35
Hi, I'm stuck on a little project I'm making, for anyone good with electronics it's simple but I'm a fabricator and have no clue what I'm doing.

So I bought a 5v usb led strip that will be placed into a small picture frame to light up the picture, a 3metre strip is 18watts, with 90 separate light sources. I only need to use 17 of the 90, I've removed the usb plug as I plan to connect it to a lipo battery with a size of 4000mah with an on off switch. My issue is that the battery is 3.7v and the strip is 5v. Ive found loads of videos or info on how to find the right resistor value for led strips but they all focus on the battery voltage being higher than the led voltage, I can't find anything to show what to do when the battery voltage is the same as the LEDs or lower. Do I even need to add a resistor before the strip if the supply voltage is 3.7? Ive tested them without a resistor and they work fine but I'm concerned they might not be safe like that.

I've also bought a little charging board for the battery and a boost converter board. Would boosting the battery voltage to 5v help? Or is it not needed in this situation as the lights work already? Any help would be appreciated, thanks.
 

djsfantasi

Joined Apr 11, 2010
8,676
First, LED strips are manufactured to work on a set voltage. You’re unlikely to get them to work on 3.7V. You could purchase a boost power supply but it will shorten the battery life. Or you could get a voltage regulator to run them on a higher voltage.

Since the strips are manufactured to work on a set voltage, they have resistors built-in on the strip.

If you change your mind on the power supply, let me know what you’ve decided on, and I can make a recommendation.

Also, most strips have a series|parallel circuit with several LEDs in series. Most strips that I know of have 3 LEDs in series which means you couldn’t have 17 LEDs. They’d have to be a multiple of 3. Eighteen would be the closest. Seventeen LEDs in such a strip would only light 15 of them… If you could post a close up if both sides, showing 4-5 LEDs, that would help.
 

Irving

Joined Jan 30, 2016
3,190
@djsfantasi These are single LED + resistor paralled, typically 20 or 30 per metre.

1668289905325.png

@dameo182 They will work on 3.7v, though you will get some brightness variation, and, worst case, one or two may not light up. The forward voltage of those LED is typically 3v so the resistor is sized for 40mA per LED; on 3.7v they'll be running at around 15mA each. That said, no harm will come from running them on a LiPo even at its fully charged 4.2v. If, as you say, they're working OK, I'd not bother with the boost converter.
 

BobTPH

Joined Jun 5, 2013
6,082
They will work on 3.7V, just not at full brightness. Try it and and see if it is acceptable. I’ve run 5V strips on a single lipo cell and they were fine.
 

Audioguru again

Joined Oct 21, 2019
5,420
The LEDs appear yellowish then they are white which need about 3.3V. The black 121 things are 120 ohm resistors.
With 5.0V each LED draws 14mA and are fairly bright.
A Li-Po battery cell is 4.2V when fully charged then the LED current will be 7.5mA which is a little dim.
When the battery drops to half-charged it is 3.7V then the LED current will be 3.3mA which is fairly dim.
When the battery drops to 3.2V then it is almost dead and the LED will be off.

The battery charge will last about 4000/(7.5mA x 17)=31.4 hours, slowly dimming all the time.
 

Thread Starter

dameo182

Joined Nov 12, 2022
35
Thanks for the replies so far, this is the LED tape and parts I bought for the project, I'm limited to the battery size as it has to fit within 9mm of space, so the battery is 6mm thick, the picture frame is for my dad to hold a picture of my mum who died 3 weeks ago, so I don't want to give him something that won't work properly for him, so I really do appreciate the help.
 

Attachments

djsfantasi

Joined Apr 11, 2010
8,676
@djsfantasi These are single LED + resistor paralled, typically 20 or 30 per metre.

View attachment 280531


@dameo182 They will work on 3.7v, though you will get some brightness variation, and, worst case, one or two may not light up. The forward voltage of those LED is typically 3v so the resistor is sized for 40mA per LED; on 3.7v they'll be running at around 15mA each. That said, no harm will come from running them on a LiPo even at its fully charged 4.2v. If, as you say, they're working OK, I'd not bother with the boost converter.
…And forget about the multiples of three that I mentioned. Seventeen LEDs will be fine.
 

Thread Starter

dameo182

Joined Nov 12, 2022
35
And this is what I wrote down while messing about with the multimeter, how much of it is accurate I'm not sure, although I am sure I will have done something wrong somewhere
 

Attachments

Thread Starter

dameo182

Joined Nov 12, 2022
35
Also I plan to use LED strips in other circumstances so if anyone could show me what I need to do with the multimeter to find the values, then show me how to use those values to find the size of the resistor needed and the size of the power supply etc. Everyone seems to know how many miliamps the LEDs draw and I literally have no clue how to work that out from the given 18watts per 3metre length the packaging states. It would be good to know how to work it all out since most things I would use the lights for would be projects where the strips would have to be cut down to variable lengths
 

djsfantasi

Joined Apr 11, 2010
8,676
Did you use a current limiting resistor before the strip?
It should not be needed. The little black squares on the strip ARE current limiting resistors. YOU don’t need to add them; the manufacturer has already supplied them.

You MIGHT need one IF your using a higher voltage than 5V. Like 12V or 9V.
 

Boggart

Joined Jan 31, 2022
67
Did you use a current limiting resistor before the strip?
No, the resistors are on the strip already, the strip has appropriate resistors for 5V, so anything at or less than 5V will work, down to the Vf of the LEDs, which is around 3V. You are probably overthinking this, just connect the strips to the battery and see if they are bright enough, if not, then upgrade to a beefier strip with 5050 LEDs (3 LED chips per LED) or use a boost converter as others have suggested.
 

Thread Starter

dameo182

Joined Nov 12, 2022
35
Yeah I am probably overthinking it, everything I've watched or read about this seems to suggest I needed to add a resistor, so I'm glad I made this post and got some proper advice, it was stressing me out and stopping me from trusting the thing not to set fire to itself once made. The lights do work connected directly to the battery but maybe I will add the boost converter just to make it brighter if need be. Thanks guys, I'll upload a pic of the finished frame in a few weeks so you can see what you were helping with, thanks again guys
 

Ya’akov

Joined Jan 27, 2019
6,856
Welcome to AAC. A couple of suggestions for you.

1. Run the LEDs at well under the rating of the manufacturer to increase the usable life of the strip. The inexpensive strips usually have ratings that push the capacity of the LEDs to deal with heat, and they will suffer failures.

In this vein, plan for replacing them. Buy spares to have on had, or even to store inside the project for eventual use. Design your project o replacement is easy (best) or at least practical (minimum). In general, treat LED strips as consumables.

If you can‘t make replacement practical, protect yourself by buying high quality strips (will not be as cheap) and run them at below their ratings. This should allow them to be used for a very long time.

Also, do not depend on the adhesive supplied on the strips for mounting in inaccessible locations or where there will be vibration, heat considerably higher than room temperature or cold considerably lower. The truth is some of the strips will have decent adhesive but most do not, and it is hit or miss. (3M branding on the liner is no guarantee. I am convinced that 90% of 3M products coming from China are nothing more than junk with the 3M logo, not genuine).

2. Consider COB (Chip On Board) style light strips. These are relatively new but provide much more even illumination. They are not immune to the considerations of 1. but they have an effectively continuous light. I have been very impressed with them. These are an example that I have tested with good results. Note they are not (easily) cut, and are not intended to be. In that way they are less flexible than the traditional strips.
 

Thread Starter

dameo182

Joined Nov 12, 2022
35
Welcome to AAC. A couple of suggestions for you.

1. Run the LEDs at well under the rating of the manufacturer to increase the usable life of the strip. The inexpensive strips usually have ratings that push the capacity of the LEDs to deal with heat, and they will suffer failures.

In this vein, plan for replacing them. Buy spares to have on had, or even to store inside the project for eventual use. Design your project o replacement is easy (best) or at least practical (minimum). In general, treat LED strips as consumables.

If you can‘t make replacement practical, protect yourself by buying high quality strips (will not be as cheap) and run them at below their ratings. This should allow them to be used for a very long time.

Also, do not depend on the adhesive supplied on the strips for mounting in inaccessible locations or where there will be vibration, heat considerably higher than room temperature or cold considerably lower. The truth is some of the strips will have decent adhesive but most do not, and it is hit or miss. (3M branding on the liner is no guarantee. I am convinced that 90% of 3M products coming from China are nothing more than junk with the 3M logo, not genuine).

2. Consider COB (Chip On Board) style light strips. These are relatively new but provide much more even illumination. They are not immune to the considerations of 1. but they have an effectively continuous light. I have been very impressed with them. These are an example that I have tested with good results. Note they are not (easily) cut, and are not intended to be. In that way they are less flexible than the traditional strips.3
When you say run them below their rating do you mean in voltage or current? Its already a lower voltage and I measured a single LED drawing 10ma, the 17 on the strip seemed to take 160ma but I'm really not sure what they should be running at when the specs are worked out (I don't know how to do that).

As for the cob style LEDs I did look at using those to save me coming up with some kind of diffuser for the frame, but I couldn't find any small enough, I have a maximum gap of 5mm for the strip to fit into, but I did like the way they lit up better than the ones I ended up buying.
 

Irving

Joined Jan 30, 2016
3,190
When you say run them below their rating do you mean in voltage or current? Its already a lower voltage and I measured a single LED drawing 10ma, the 17 on the strip seemed to take 160ma but I'm really not sure what they should be running at when the specs are worked out (I don't know how to do that).
18W per 90 LED - 6W per 30LED = 1.2A per 30LED = 40mA per LED @ 5v, assuming LED Vf = 3v then R =50ohm. @3.7v, R drops 0.7v = 0.7/50 = 14mA per LED in theory assuming guess for Vf is right. However, your measurements show 9.4mA per LED, which suggests Vf is nearer 3.25V or R = 75ohm. You could measure the resistors on the strip and Vf when running on 5v, but TBH its all moot. If they're bright enough on 3.7v then go with it.
 

Ya’akov

Joined Jan 27, 2019
6,856
When you say run them below their rating do you mean in voltage or current?
Yes. That is, lower power, which is measured in W (Watts). The formula is W = V * A, so it is a product of both.

By lowering the voltage you lower the current, and the total power is reduced. Heat is proportional to power dissipation, and heat is what kills LEDs (and pretty much every other kind of component).

Its already a lower voltage and I measured a single LED drawing 10ma, the 17 on the strip seemed to take 160ma but I'm really not sure what they should be running at when the specs are worked out (I don't know how to do that).
The area of LED power requirements can be confusing to a beginner, but don’t despair it is actually not very complicated. The reason for the difficulty is the nature of LEDs. They are non-linear devices, that is, their resistance changes with the current that passes through them. That’s why a separate current limiting resistor is required.

A plain LED will reduce in resistance, and so the amount of current it will try to draw, the more current you give it. You can see how this is a bad thing if the power supply can provide enough current to turn let out the magi smoke—the LED will commit self-harm in a moment.

With the correct current limiting resistor in place, there is a guarantee from an ohmic device that the current will not exceed the maximum the LED can handle. A resistor is ohmic because it doesn’t change resistance value based on the current passing through it¹. The LED is non-ohmic because it does. These designations refer to the relationship of each device to Ohm’s law. The ohmic device has a simple one, the non-ohmic device requires taking that changeability into account.

As far as the proper current goes (ultimately, we use the current at the devices forward bias voltage as the test for how much power we will be supplying) the best way to know is to find a datasheet for the device. In the absence of one, the general expectation is that an ordinary LED without a heatsink will be properly powered at 20mA. This doesn’t have to be correct but it is unlikely to be too much.

So, when you lowered the voltage you lowered the current the supplied current limiting resistor would permit to flow. At 10mA you are doing very well so far as longevity goes. If they are bright enough at that power level, that’s great. The goal is to run them as cool as you an while still having sufficient light output.

As for the cob style LEDs I did look at using those to save me coming up with some kind of diffuser for the frame, but I couldn't find any small enough, I have a maximum gap of 5mm for the strip to fit into, but I did like the way they lit up better than the ones I ended up buying.
The strips I linked above are 5mm. I also misspoke a bit. I forgot that I found they had cut marks on them. When I recieved them I thought they didn’t, so cutting them would require finding the proper place through the phosphor, that’s not necessary. The only complication is that the cut marks are on the back, under the adhesive liner, to you have to peel it to find them. Not a fatal flaw, but a bit annoying.
 

djsfantasi

Joined Apr 11, 2010
8,676
The only complication is that the cut marks are on the back, under the adhesive liner, to you have to peel it to find them.
In the picture you included in post #6, the cut marks on the front of the LED strip. They are the black lines across the copper pads.
CA000403-AEB8-44B3-9646-B9614930DE32.jpeg
Also, note that + and - are printed on the strip. That way when you cut the strip, you can use the remainder. Solder wires (or use a connector for LED strips) to the pads and use proper polarity as marked on the strip itself.

The pads are placed between ‘groups’ of LEDs when more than one is wired (on the strip) in series. Your strip only has one LED, so the pads are between each LED.
 

Audioguru again

Joined Oct 21, 2019
5,420
You wrote that your Li-Po battery is only 3.87V which is near half charged. Have you tried it powering the LEDs when it is fully charged at 4.2V? Or is the charger defective and charges to only 3.87V?
 

Thread Starter

dameo182

Joined Nov 12, 2022
35
18W per 90 LED - 6W per 30LED = 1.2A per 30LED = 40mA per LED @ 5v, assuming LED Vf = 3v then R =50ohm. @3.7v, R drops 0.7v = 0.7/50 = 14mA per LED in theory assuming guess for Vf is right. However, your measurements show 9.4mA per LED, which suggests Vf is nearer 3.25V or R = 75ohm. You could measure the resistors on the strip and Vf when running on 5v, but TBH its all moot. If they're bright enough on 3.7v then go with it.
So you divided the 6watts by 5volts to get 1.2amps, then divided 1.2 by the number of LEDs to get the amperage draw of 1 LED? And that method works for any voltage of LED strip or light? Thanks for showing that working out, and yeah I'm goin to just use it as it is apart from just adding the boost converter, I haven't got that far in the testing yet but I imagine I'll need as much light from that strip as possible due to it's placement, but I'll know for sure as I get further into the build. Thank you.
 
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