See my edit above. I left out the resistor value! Sorry about that.

I forgot to specify the resistance value. That's what happens when I write something during a massive blackout at 4am with 60 mph winds in a house surrounded by 100' fir trees!


For LEDs with Vf between 3.0 – 3.6 volts, the current will be between about 10 – 13 mA with 150 ohm resistors. This is perfect with modern LEDs.

 

View attachment 343195

Being a newb, I made some ignorant errors and I'm sorry for that. I had initially used Yellow LED's and assumed, all LED's had the same forward Voltage. But I rechecked and the forward voltage for the Warm white LED is indicated as 3.0 to 3.6 VF. So, from my very very limited knowledge and some research online, I would assume, if I'm using 5V power supply then, I would probably need a 150ohm resistor per LED.

The main reason I want to use a USB-C over the DC jack is because the DC jack is too big for the lamp and i need something more compact. So. is there anyway I can use the USB-C with the LED's? Thanks

If your LEDs have a 3 V drop, then two of them in series is already greater than the 5 V available to you. You can only run a single LED (with its resistor) in each branch.

As several of us have been asking, we need to know what current the LEDs are rated for. Some LEDs are designed for hundreds of milliamps and others need less than one milliamp. The value of the resistor depends on the current that you want that LED to have.

 

Hi,
The TS posted this info:

they have no data sheet. The only information given in english is :: "Input- 3-3.6 Volts DC. Working voltage 50MA Max current. "

E

 

Since I was playing with these diodes to see how to get them to work, I got the chinese ones and they have no data sheet. The only information given in english is :: "Input- 3-3.6 Volts DC. Working voltage 50MA Max current. "

I understand its not possible to connect two LED's to a 5V supply, so I could connect each LED to a 150Ω resistor. Unless theres a better way to do it.

The LEDs are rated for 50 mA maximum current. If you want longevity, you do not want to run them near their max rated current. See if they are bright enough at 20 mA and, if not, go up as high as 30 mA.

If you have a 5 V supply (that is actually 5 V), then the nominal resistor that you need can be determined using an assumed typical forward voltage of 3.3 V (midway in the stated range).

From Ohm's Law:

V = I·R

Where V is the voltage across the resistor and I is the current through the resistor.

So

R = V/I

The voltage across the resistor is Vcc - Vf, which for you is nominally 5 V - 3.3 V = 1.7 V.

The current is the desired current, which for you, as a start, is 20 mA.

Hence,

R = 1.7 V / 20 mA = 85 Ω

For 30 mA, it would be 57 Ω

Choose a standard resistor value near the nominal value.

If you use a 56 Ω resistor with a 5% tolerance, then your resistance can be anywhere between 53.2 Ω and 58.8 Ω.

Worst case low current is with a low supply voltage (4.75 V), a high Vf (3.6 V) and a large resistor (58.8 Ω), which works out to 19.6 mA.

Worst case high current is with a high supply voltage (5.25 V), a low Vf (3.0 V) and a small resistor (53.2 Ω), which works out to 42.3 mA.

That's more than a 2:1 range, but may be good enough for your lamp.

You can also see how designing for a current of 30 mA, only 60% of the max rated 50 mA, can still result in your actual current being nearly 85% of the rated max if all of the tolerances happen to work against you.