Custom LED Lamp

Okay, everyone can see: it's a UV lamp. You will need a UV lamp in the following cases:

• checking banknotes
• doing your nails with fancy stuff
• curing resin 3D prints

You guessed right: I got an SLA printer!

But this turorial will work with any LED. You need a video lamp? A work lamp? Grow lamp? The principle is the same. Do it right!

LEDs

Proto PCB

A DC power supply

LEDs are diodes. (it's in the name, Light Emitting Diode, duh)

And diodes are fun to work with. Unlike classic lamps, you can't dim LEDs by just turning the voltage a little lower and wait until

less U ➔ U=R*I ➔ less I ➔ P=U*I ➔ P gets MUCH lower

kicks in. Diodes act like dams:

• water level (U) below the dam's top (diode forward voltage) ➔ no flow
• reach the top ➔ very little flow
• just a bit over the top ➔ max current

You can check a LED's forward voltage by looking at the datasheet or making it glow and measure the voltage across the anode and cathode. If you find some random stuff in the drawer: good luck with the datasheet! Just be sure: you won't let too much current through, use a relative big resistor! I had a 4.5V supply paired with a 330Ω resistor. Short-circuit current about 14mA, an average LED operates with 10-20mA. It can't kill any LED.

My UV led's forward voltage is ~3.23V. Some need a bit more, some less, so we need to check later.

The wall wart from the first chapter is not really beefy, but can produce "pretty high" voltages, compared to a "simple" 5V USB power supply.

Current generates the heat, keep it as low as possible, therefore you can get thinner wires. No wonder the power grid has segments with hundreds of kilovolts! For the same power use higher voltages and lower (milli)amps. Don't be afraid to connect LEDs in series, it will raise the forward voltage, but the current will be the same.

I have a salvaged power supply, and it can produce up to 32V and can handle a few dozen of milliamps I will need. My lamp will have ~30 LEDs, so arrange them! No, not 30 LED in parallell and 3.23V - 300mA!

If you have lower voltages, you can try branching. A common point in the middle and some LEDs to the left, some to the right, making it a simpler circuit while having as much LEDs in series as possible. However if you have much more voltage, you can connect even 9 parts in series! (3.23V * 10 is over my power supply's theoretical 32V limit)

Solder the LEDs and just the LEDs!

9 * 3.23V = 29.07V in theory. We may have some room for a 10th LED. And yes, we have!

10 LEDs in series came out around 31.5V and the power supply also doesn't stop at 32.00000V. The voltage dropping on the limiting resistor is around 1.9V, so it gives around 33.4V! You know, in theory, it couldn't work, but in practice.

We have no voltage left for an 11th LED, so start calculating! My goal is at least 10mA for each line. U=R*I, 1.9V/10mA gives us 190Ω. But we have no 190Ω!

200Ω? Don't mock me! AT LEAST 10 mA! 160Ω? Well, it's in tolerance: 1.9V/160Ω ~= 12mA. That's no overdrive!

Grab some matching resistors.

Now you can solder the resistors and the wires. If everything went well, you have a custom lamp!