# Resistor for 20 LEDs in series

I am reverting here for some advice on a LED panel circuit that I am trying to build as I cannot find any concrete answers anywhere else online. My apologies as my understanding of electronics is very limited and I lack some of the basic understanding around voltages and current and how this is consumed by components. This is why I am reaching out to the experts.

The complete circuit that I want to build will have 100 LEDs. The specs of the LEDs are as follows from the datasheet:

Forward Voltage (VF) - Typ: 3.30V - MAX: 3.80V

Forward Current (IF) - Typ: 20mA - MAX: 25mA

The datasheet can be found here if it helps at all: http://www.luckylight.cn/UploadFiles/484WC1C-W5-3P.pdf

My source voltage is a 5V 7A power supply.

Im not sure if I have the right forward voltage value from the datasheet though. I built a test circuit with 10 of these led's in parallel and they all lit up quite brightly. The test circuit got its power from the 5V pin on a raspberry pi which outputs 5V at about 200mA. When I measured the output on these pins using a multimeter before the LEDs were connected I got a reading of 5.22 V and after I connected the 10 LEDs in parallel I received a voltage reading of 5.17V indicating a voltage drop of around 0.05V for 10 LEDs. This is where I am a bit uneducated as the way I understand it is that if I have 5 volts, I will only be able to power to LEDs with a forward voltage of 2.5V each and it baffles me as to why a circuit with 10 LEDs with a 3.30V forward voltage will light up so brightly on a 5V power supply.

What I would like to accomplish is to split the 100 LEDs into 5 rows of 20 where each row runs in series on its own resistor (just one resistor) due to the fact that the space on the circuit that I have is very limited. I then take that one row of 20 LEDs in series and duplicate it 5 times which gives me the 100 LEDs. I guess you can then say I have 5 isolated LED circuits all connected to the same power supply.

I have tried calculating the resistor value that I need to run all 20 LEDs on online calculators but they all provide me with a parallel diagram where each individual LED has it's own resistor which is not what I want. I want to run all 20 LEDs on one resistor so I need to calculate which resistor I need to do that.

My questions basically sums up to this:

1. How can I calculate the resistor needed to drive the 20 LEDs in series with all 20 LEDs using only one resistor?
2. I have 5V 7Amp on the power supply available in my project and at 25mA the entire circuit should utilize 2500mA or 2.5 Amps which should be fine. Will the 5V be enough to drive the 100 LEDs at 3.5 to 3.8 forward voltage per LED though? This is where I lack a basic understanding of how it works. I do have access to a 12V 3A power supply in the same project if this is needed.
3. Will it be a problem if I build the 20 LED circuit, duplicate it 5 times and connect them all to the same power supply?
4. Is it necessary to use a resistor at all with all these LEDs running on the same supply?

Thanks for the assistance.

• 20 LEDs in series, each with a forward voltage of 3.3V would result in a total voltage drop of 66V. So, no, you can't do that with a 5V supply (or a 12V supply). Commented Apr 15, 2016 at 6:41
• I hear you. Have I got the right value from the datasheet though? The reason I am asking is because I built a test circuit with 10 of these led's in parallel and they all lit up quite brightly. The test circuit got its power from the 5V pin on a raspberry pi which outputs 5V at about 200mA. When I measured the output on these pins using a multimeter before the LED's er connected I got a reading of 5.22 V and after I connected the 10 LEDs in parallel I received a voltage reading of 5.17V indicating a voltage drop of around 0.05V for 10 LED's? Commented Apr 15, 2016 at 6:51
• Maybe you should rephrase your question to: "I have 5V 7A psu (or 12V 3A psu), how to light up n LEDs?" of something like that.. It's a simple issue that you managed to transform into a wall of text. Commented Apr 15, 2016 at 6:53
• @JoachimPrinsloo that voltage droop is from a 0.2A load on the wires/power supply. That's not the LED's forward voltage drop. Also, you don't have to power them at 20 mA. Leds will be quite bright at lower currents, even down to 0.1mA. Commented Apr 15, 2016 at 6:54
• Do they all need to be on at the same time? If not, you could multiplex them. What's the final application? Commented Apr 15, 2016 at 6:56

When you connect the LEDs in series, the forward voltages of the LEDs add up. If you have to connect 20 LEDs in series, the forward voltage of the string of LEDs is 20 * 3.8V = 76V. But then you only need 20mA per series of LEDs! (You got that backwards)

If you instead contact 20 LEDs in parallel, the forward voltage is 3.8V, but now the current is 20 * 20mA. You now can use a 5V supply (since 5V > 3.8V). In that case, you need a resistor in front of every LED to "take care" of the difference of your 5V supply and the 3.8V LED's forward voltage (5V - 3.8V = 1.2V), that's what all the LED-Resistor-Calculators show you.

So, to make LEDs light up in series, you need to supply 20mA, independent of the number of LEDs. An LED driver does that. It supplies 20mA and "adjusts" the voltage to make that happen. You normally would not put 20 LEDs in series, since your LED driver had to supply ~76V (which the power supply has to supply in the first place).

A 5V supply can do 1 LED in series (5V > 3.8V). A 12V supply can do 3 LEDs in series (12V > 3*3.8V). For every string of LEDs you than provide 20mA each.

• Not the answer that I would have liked to here but the correct one nevertheless! This clearly explains why my test circuit worked as well. I basically have one of two options here. one is to wire all the LEDs in parallel and the other is to go back to the drawing board and find another solution. Thank you so much. even the voltage makes a lot more sense to me now! Commented Apr 15, 2016 at 7:41

You CANNOT connect 20 3.3V Forward Voltage leds in series with a 5V power supply.

VSource - VForward
5 - (3.3 * 20) = 5 - 66 = -61

That's a 61 Volt deficit. Not enough volts to trigger the forward characteristics of the LEDs. No current will flow.

The only way to do this, is by using a voltage booster, like a Step-Up Switching Regulator. It will need (3.3V * 20 Leds) + 2V (For the Resistor to work with) = 68 Volts * 0.02 Amps = 1.36 Watts input, plus any efficiency (Typically 85%) so ~1.56 Watts input. At 5V, that's 1.56W / 5V = 0.312 Amps input. Now 4 of these strings, that's 1.25 Amps @ 5V.

Update: The 0.05V drop you see on the 5V supply is not the LED forward current drop. That drop is from the power draw over the USB wires, which are non-perfect conductors, aka resistors. That's normal.

• I hear you. Have I got the right value from the datasheet though? The reason I am asking is because I built a test circuit with 10 of these led's in parallel and they all lit up quite brightly. The test circuit got its power from the 5V pin on a raspberry pi which outputs 5V at about 200mA. When I measured the output on these pins using a multimeter before the LED's er connected I got a reading of 5.22 V and after I connected the 10 LEDs in parallel I received a voltage reading of 5.17V indicating a voltage drop of around 0.05V for 10 LED's? I added this to the question. Commented Apr 15, 2016 at 6:54
• @JoachimPrinsloo that voltage droop is from a 0.2A load on the wires/power supply. That's not the LED's forward voltage drop. Also, you don't have to power them at 20 mA. Leds will be quite bright at lower currents, even down to 0.1mA. AND you probably don't want to use the RPI's 5V pin for this, as you could possibly trip the power fuse of the RPI, at these high currents. Commented Apr 15, 2016 at 6:56
• Thanks Passerby. This seems to be going in the right direction. How will I then determine the forward voltage of the LED? can I assume that the measurement that I took on the multimeter is the correct forward voltage and use that to calculate my resistor? IE: (5.22V - 5.17V) / 10? PS: thanks for the tip regarding the RPI. did not think that would matter :-) Plus I did it on my new RPI3. would be a shame if the magic smoke that makes it work escaped from that one. Commented Apr 15, 2016 at 7:03

I would use a 24V supply and wire 7 or 8 LEDs (8*3V or 7*3.3V = 24V) in series. Do three series wwith 24 LEDs. Add a 10 to 30 ohms resistor to each serie. It helps regulate current accross all series (theoricaly).

I did it with 12V supplies for Christmass.