0
\$\begingroup\$

A few days ago the electric circuit of a T8 led tube broke, but when I measured the led all were ok. Then I came up with the idea of using them in an experiment and turning them into a usb or battery lamp.

The tube consists of 30 smd led 2835 connected, two lines of 30 leds connected in series, and they connected in parallel with each other. So I have had to change this and connect all the leds in parallel, one by one. And I have managed to light the lamp well, so far, I still have 30 LEDs to connect.

enter image description here

My question is if I need any resistance or capacitor, because when I connect them with a usb these will warm up a bit, and I do not want them to get lost.

Here I leave a picture of how they are staying, lit with an old tablet battery.

enter image description here

\$\endgroup\$
1
\$\begingroup\$

LEDs should be run from a current limited or constant current supply. If you examine your original tube you should find some additional electronics to provide this current limiting.

In your setup you are running at constant voltage. This generally doesn't end well as small changes in voltage result in large changes in current.

enter image description here

Figure 1. IV curves for various coloured LEDs.

The voltage drop across an LED depends on the current but also on the chemical doping which provides the LED colour. (It's not just the colour of the lens that determines its appearance.)

If you follow the 'W' (white) curve for the sample LEDs on Figure 1 you can see that at 3.5 V it will pass 40 mA but at 4.2 V (20 % higher voltage) the current doubles to 80 mA. This small change in voltage may be enough to destroy your LEDs. Your LEDs are more powerful so the currents will be much higher.

Also bear in mind that the LEDs will vary a bit - even within a batch - and at a given voltage some will pass more current than others.

My question is if I need any resistance or capacitor, because when I connect them with a usb these will warm up a bit, and I do not want them to get lost.

Normally each LED or series string will have an in-line resistor to limit the current in that string.

USB provides 5 V at up to 0.5 A (500 mA) on a standard port. By feeding at 5 V you may get close to destroying the LEDs but since you have so many in parallel it is more likely that the USB port will shut down on current overload.

If you read the Edison-Opto datasheet for those devices you will see that they run at about 150 mA each. That means that your USB port could power three of them at full brightness.

You will also see that at this current they will drop a maximum of 3.0 to 3.6 V (depending on the "bin" grading that you purchase). That means that if you connect them to a 5 V PSU they will try to draw much more than 150 mA each. If you have a good enough PSU it will be able to burn out all of your LEDs.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 2. Adding a 15 Ω resistor in series with each LED will limit the current to < 150 mA.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.