LED Questions for 3d printed letters

Thread Starter

rdnktrkr

Joined Nov 6, 2022
4
I've 3D printed some letters for the grandkids and they call for 5mm LEDs for a diffused backlight, one has 21LEDs the other 26, I purchased a kit containing 5 colors RGBYW, according to the data sheet they all are 20ma 2.0-2.2v, whereas research shows different color LEDs have different voltage drops from 2.0-3.6
According to a LED calculator resistance would vary from 30ohm (2x2.0v) to 150ohm (1x3.6v) Questions are what resisters should I use? Should I trust the label on the LED box or the voltages from my research? I was going to run a 5v power supply but at this stage I can change to another voltage, Would that be advantageous?
 

Ya’akov

Joined Jan 27, 2019
6,856
Should I trust the label on the LED box or the voltages from my research? I was going to run a 5v power supply but at this stage I can change to another voltage, Would that be advantageous?
Using a DMM with a diode test function you should be able to find the forward voltage for each LED type empirically. You could also use a component tester like this one. That particular one is more expensive than many but I can recommend it as well made and very functional. They are handy to have around anyway.
 

MisterBill2

Joined Jan 23, 2018
13,798
the 20 ma is the part that you can trust. Are the resistance values given the series dropping resistor or the effective LED resistance? That is not made clear.
Are you seeking resistor values to have 20 ma current in each? or what??
 

WBahn

Joined Mar 31, 2012
27,893
I've 3D printed some letters for the grandkids and they call for 5mm LEDs for a diffused backlight, one has 21LEDs the other 26, I purchased a kit containing 5 colors RGBYW, according to the data sheet they all are 20ma 2.0-2.2v, whereas research shows different color LEDs have different voltage drops from 2.0-3.6
According to a LED calculator resistance would vary from 30ohm (2x2.0v) to 150ohm (1x3.6v) Questions are what resisters should I use? Should I trust the label on the LED box or the voltages from my research? I was going to run a 5v power supply but at this stage I can change to another voltage, Would that be advantageous?
Can you provide a link to the kit? We might be able to track down additional information about it.
 

ThePanMan

Joined Mar 13, 2020
520
First - a RED LED made by manufacturer X and a RED LED made by manufacturer Y can have fairly differing forward voltages (Vf). If you have a data sheet then chances are whomever sold you those may just know a little more than what you find on the internet. Nevertheless, finding their Vf is important.

Another important detail, maybe I'm missing something, but what voltage are you powering them from?
I was going to run a 5v power supply but at this stage I can change to another voltage
OK, I found the missing detail.

First, are you going to put them ALL in parallel? If so then it's recommended you have a resistor for each LED. Or are you going to put (for instance) three LED's in series with a single resistor, build up several sets of three and then put them in parallel? If so - you will need a higher voltage power supply. I chose 3 series LED's as an example, but you can put more. Some strings could be 5 LED's as long as the Vf does not exceed your power supply voltage. Or you could use three strings of 4 LED's, three strings of 3 LED's for a total of 21 LED's. Each string would need its own individual resistor. For the letter with 26 LED's, a combination of 3's and 4's works like this: five strings of 4 LED's and two strings of 3 LED's will give you 26 LED's in that letter.

But all that depends on what voltage you're starting with. Even 12 volts might not be high enough. IF you have a "4 string" of LED's all with 3.6Vf then 12 volts won't be enough. You'd have a total Vf of 14.4V. I'd start with 18 volts just to have sufficient headroom for the project. 18 volt power supplies are easier to find than 15 volt or even 16 volt PS's. I happen to have an 18 volt PS laying around somewhere in my junk pile. They CAN be found. An old printer might have a PS that has several different voltages to choose from. You may just find what you need. Just make sure the PS is not only high enough in voltage but also have enough power (in watts (amps times voltage) ) for the total amount of current you're going to draw. 20mA with five strings of 4 LED's comes to 100mA. Without knowing all the details, your project is running close to half an amp (500mA). You shouldn't have a problem finding a PS for that amperage.

Good luck. Take pictures. Post them when your project is done. We LOVE to see other people's projects when finished.
 

Tonyr1084

Joined Sep 24, 2015
7,199
1667784992236.png
Now all that matters is how you intend to set up your LED's. What colors and what forward voltages each has.
In the top row (4 series LED's) each has a Vf of 3.6.

In the case of the 4 LED's in series
3.6Vf x 4 = 14.4Vf.
18V - 14.4Vf = 3.6 working volts.
3.6V ÷ 20mA (0.02A) = 180Ω

In the case of the 3 LED's in series
3.6Vf x 3 = 10.8Vf
18V - 10.8Vf = 7.2 working volts.
7.2V ÷ 20mA = 360Ω

Each string of LED's is 20mA.
The TOTAL amperage of the circuit is 120mA.

This is known as a series/parallel circuit. Some may call it a parallel/series circuit; same thing. It consists of LED's in series with a resistor. Each series set (LED's and resistor) is connected parallel to the next. Whether you use 3 series LED's or 4, the math works out as shown. For a beginner, this is the most basic example of Ohm's Law.

Now you know how to fish. Figure out how to hook up 26 LED's.
 

AlbertHall

Joined Jun 4, 2014
12,188
Note that the different colour LEDs, running at the same current, will not have the same brightness, If the intention is to match the brightness then some experimentation will be required with the resistor values. Do not exceed the 20mA for any LED.
 

MisterBill2

Joined Jan 23, 2018
13,798
Note that the different colour LEDs, running at the same current, will not have the same brightness, If the intention is to match the brightness then some experimentation will be required with the resistor values. Do not exceed the 20mA for any LED.
that 20 mA current rating for LEDs is not a "Max" limit, but rather a nominal value, which quite a few devices are intended to operate at higher current. 30 years ago it was different. 20 is always "safe" but quite a bit less critical for many applications.
 

Thread Starter

rdnktrkr

Joined Nov 6, 2022
4
Sorry for the delay, I had a tree take out my power/internet lines. Thankyou for the suggestions, I'm thinking that 12v would be a much better choice on voltage than 5v.
This is the link for my project https://www.printables.com/model/89915-glowing-led-all-alphabet-letters-and-all-numbers-6, I have a couple spools of 30 and 28ga solid wire to connect everything. My mother and wife are impressed with the 3d printed letters so far
This is the link for the LEDs https://a.co/d/81S3TeN
I have a box of 12v power supplies from an auction that have never been opened 300ma, 1a, 2a, and 5a, the majority is 2a if those could be used.
I could also purchase some LED strip lights and use those but I think that would be almost like cheating.
 

Tonyr1084

Joined Sep 24, 2015
7,199
I'm thinking that 12v would be a much better choice on voltage than 5v.
If you're limited to 12V then you also limit the number of LED's with a Vf of 3.6V to a string no longer than three LED's. IF you use the LED's with a Vf of just 2.0V then you can get away with no longer than five.
I could also purchase some LED strip lights and use those but I think that would be almost like cheating.
That certainly would be easier.

I have colored LED's in my down stairs bathroom, the bathroom with the jetted tub. When in the tub my wife (and I) like the changing colors. You can choose a color or set it to random changing colors. You can vary the speed at which they change. I prefer the long slow fade from one color to the next.

One thing I didn't show in my drawing was the use of multiple color LED's in a single string. You can mix and match however you like. For instance, you can have one color at 2.0Vf each mixed in with another color at 3.6Vf. I'm only using those two forward voltages purely as a reference. The final calculations will depend on the actual Vf of each LED you're using. Nevertheless, if you have two 2.0Vf LED's in a string with two 3.6Vf LED's you have a max of 11.2Vf. With a stout 12V PS you can get away with that kind of mix/match LED arrangement. Of course however you choose out your LED's you will need to add up the total Vf of any given string, then calculate the closest resistance for the brightness you prefer. 20mA is very commonly a "Safe" current. Just about every LED I've ever used has had a stated MAX current of 30mA. Two things: At 30mA, they can be quite bright. Even potentially hazardous to the human eye. They also run hotter, and therefore have a shorter lifespan. I've seen LED's run at very low currents. 5mA on a super bright LED can be plenty bright as an indicator lamp. I think the lowest I've ever personally run an LED was somewhere down around 3mA and found it to be bright enough for the purpose I intended.

So by now you should know how to string series LED's with a single resistor and calculate that proper resistance. You should also know by now that putting those constructed strings in parallel you add up the amount of current to get a total amperage. IF your added total is greater than the PS you have in hand then you know you need a higher powered PS.

Let us know how you intend to add up your LED's and we can check your math before you apply power. We'll let you know if you're on the right track or if you've made a fundamental error.
 

Thread Starter

rdnktrkr

Joined Nov 6, 2022
4
Assuming R=2.0v, G=2.1v, Y=2.1v B=3.6v and W=3.6v
1st series = R-B-Y-W and 43ohm resister
2nd series= G-R-Y-W-R and 10ohm resister
3rd series= B-W-G-R and 36ohm resister
4th series= W-B-Y-R and 15ohm resister
5th series=G-Y-W-B and 10ohm resister
I found a LED calculator online and have been typing the totals in and those are the resisters it shows,
How precise does the resistance need to be? Can I purchase 1 pack of 39ohm instead of a pack of 36 and 43? What about 10 and 15ohm, can I use 12 or 13?

A little background about me, I haven't done much since I was in school and was going to trade school and worked in a TV shop (remember the works in a drawer TVs?), mostly what we did was replace boards and send them out to be rebuilt, I still have a Kennedy toolbox with vacuum tubes, RCA VTVM, Heath Kit O scope and all my tools from school. Back then we were still amazed at "Pong". Recently I've been using white LED strips to light under and inside our cabinets and my safes
 

Tonyr1084

Joined Sep 24, 2015
7,199
An insignificant change in the resistance will have an insignificant change in the amperage.

The closest resistor to 42Ω in MY stock is 47Ω. Here's how the numbers work out both ways:
(12V - 2Vf -3.6Vf - 2.1Vf - 3.6Vf) ÷ 42Ω = 17mA
(12V - 2Vf -3.6Vf - 2.1Vf - 3.6Vf) ÷ 47Ω = 15mA
You won't notice the difference in brightness between 17mA and 15mA.
If you were going for 20mA then the correct resistance would be:
(12V - 2Vf -3.6Vf - 2.1Vf - 3.6Vf) ÷ 20mA = 35Ω

Let me break this down for you:
(12V - 2Vf -3.6Vf - 2.1Vf - 3.6Vf) = 0.7V That's not a lot of head room. Personally I'd like to see a few spare volts, but that's just because I'm that way. Nevertheless, if that is how you want to arrange them, make sure the math works out.

Notice the starting supply voltage is 12 volts. Each forward voltage of each LED in that string is subtracted from the supply voltage. In theory if you go all the way down to the exact starting voltage, without a resistor you're flirting with disaster. That's why I like to see a few extra volts in reserve. That way a single resistor can be counted on to be fairly safe controlling the amount of current going to the LED string.

It's my dinner time, so I'm not going to take your numbers and see how they add up in different combinations. I'll leave the work up to you.

Ciao.
 

MisterBill2

Joined Jan 23, 2018
13,798
Consider that most resistors are either 5% tolerance or 10% tolerance, and that the steps in common values are created to allow "coming close" to a calculated value. The small variation will usually have no effect that matters.
Designing for the real world almost always allows for at least 5% variability. That is one requirement for adequate production yield.
 

Tonyr1084

Joined Sep 24, 2015
7,199
OK, this morning I'm running your numbers. I've changed the order around a little just to make it easier to calculate the data.

In set #2 you have five LED's. The total Vf for that column is 11.8V. You're starting with 12 volts, which leaves only 200mV headroom. Personally I think that's too close. Others may offer their opinions which may vary from mine. That's fine. I'm not the final expert on this.

In the chart below you see the ohmage values I've calculated. I've also calculated wattage for each string at 240mW. You can get away with 1/4 watt but they're going to be running near full rating. The next size up in wattage would be 1/2 watt, and for sure, that is what you should be using.
1667927652376.png
And given the comment by @MisterBill2, you can expect some variation in final amperage for your strings. With such low resistance values the percentage of deviation will likely go unnoticed. But just so you understand, a RED LED might not appear as bright as a WHITE LED running at the same current. Perceived brightness doesn't notice small variances, but the difference between white and red; one can be blinding while the other is a little uncomfortably bright, while another color can be a pleasant brightness.

Still, as far as the actual Vf of each color, you really need to verify for yourself exactly what each is. If you've bought these from a highly reputable source then chances are good that the manufacturer's data sheet is accurate. But far too often we've seen data sheets that blow smoke up your back side. Then the parts start blowing smoke. So take this as a learning experience before you waste your materials and time, and get to know your exact Vf numbers BEFORE you proceed.
 

Thread Starter

rdnktrkr

Joined Nov 6, 2022
4
Sorry for the delays, its been a little busy around here and the Grandkids are staying with us for the holidays.
I checked my power supplies and found 4 that measured 11.7v so after the holidays I will start assembly and see how close I can get to something I think they will enjoy. Thanks for everyone's help, I will update when I can Greg
 

MisterBill2

Joined Jan 23, 2018
13,798
Sorry for the delays, its been a little busy around here and the Grandkids are staying with us for the holidays.
I checked my power supplies and found 4 that measured 11.7v so after the holidays I will start assembly and see how close I can get to something I think they will enjoy. Thanks for everyone's help, I will update when I can Greg
If the supplies mentioned are the non-regulated "wall wart" type, the unloaded voltage is always above the voltage resulting with some load drawing power. Presently there are both varieties common, although the light weight switching supplies are the most common. Mostly, the heavier devices that include an internal transformer are not well regulated.
 

Audioguru again

Joined Oct 21, 2019
5,418
Those will be very bright letters. The heating produced in the fairly small size will also be very high. How will you cool them?
Make only one letter then try it. You will probably reduce the number of LEDs and reduce their current.
 

MisterBill2

Joined Jan 23, 2018
13,798
You may need to increase the value of some of the resistors a bit. Non-regulated voltage sources tend to often provide voltages above what the markings claim. That is why they specify the load current for the output claimed.
 
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