0
\$\begingroup\$

I'm having a hard time understanding these things.

Here's my situation:

I have 10 white LEDs. I bought them from China, and they have no rating and no one knows for sure their maximum voltage or amperage.

I'm using a 5V, 500mA power supply.

I first wired them in series of 2 and then in parallel (I hope I'm using at least partially the right terms), like this:

schematic

I didn't use any resistors because this should only apply 2.5 volts to each LED, and a white led is usually 3 to 4.5 volts? Right?

But the light was very dim, so I rewired everything in parallel and applied the full 5 volts to them.

Now I'm happy with the brightness, but the hot glue I used to attach them to the board melted.

So the question is, is the glue gun the wrong glue to use, or are they getting way too hot?

The amperage should be ok, 10 LEDs x 20mA should only draw 200mA, way below the 500mA the power supply can supply, or did I get this wrong too?

\$\endgroup\$
5
  • \$\begingroup\$ What was your reason for buying them? \$\endgroup\$ – Andy aka Nov 16 '20 at 11:02
  • \$\begingroup\$ They were cheap and I thought a led is a led, I didn't think the numbers could vary much between types/shapes/manufacturers \$\endgroup\$ – Syco Nov 16 '20 at 11:04
  • 1
    \$\begingroup\$ Buy some LEDs with proper data sheets is my advice sold through a reputable source and from an original supply that has a recognized quality system and some reputation in the field of making LEDs. Use series current limiting resistors too. \$\endgroup\$ – Andy aka Nov 16 '20 at 11:06
  • 2
    \$\begingroup\$ If you apply 2.5V to an LED that usually has 3-4.5V then of course it's dim. If you apply 5V to an LED that usually has 3-4.5V then of course it's bright. Bright also means hot and the smoke might come out sooner. \$\endgroup\$ – user253751 Nov 16 '20 at 13:19
  • 2
    \$\begingroup\$ 10 leds x 20mA should only draw 200mA, No, LED's will "take all the current they can get", LEDs will simply destroy themselves and not say: oh, 20 mA is enough for me! Instead you have to feed them only 20 mA and not more. A resistor in series with the LED is enough to do that. \$\endgroup\$ – Bimpelrekkie Nov 16 '20 at 13:43
3
\$\begingroup\$

You're treating the LEDs as if they're (2.5 V - 3 V) lightbulbs. Lightbulbs do not mind if the voltage is a bit lower or higher but LEDs do.

LEDs respond very strongly to the applied voltage. Apply a voltage that is too low (5 V for 2 LEDs in series) then almost no current flows and the LEDs burn very dimly. Apply too much voltage ( 5 V for 1 LED or all LEDs in parallel) then way too much current will flow and the LEDs will overheat and burn out.

The fact that your 5 V supply is rated for 500 mA doesn't matter as the LEDs will want to draw much more than 500 mA and the supply will be overloaded. If you're lucky the supply will switch off. If you're unlucky the supply could catch fire.

You should NEVER apply a voltage directly to a LED. Instead, they need a current. what is also OK is to connect a series resistor to limit the current and then apply a voltage to LED + series resistor. The resistor will limit the current and prevent the LED from destroying itself. How to calculate what resistor is needed (value and power dissipation) can be found on this site as it has been asked many times before.

Example of a quick calculation: if one LEDs needs 3.5 V, that means there needs to be 5 V - 3.5 V = 1.5 V across the series resistor. You want 20 mA current per LED that means a series resistor of 1.5 V / 0.02A = 75 ohms in series with each LED. That 75 ohm resistor will dissipate 1.5 V * 0.02A = 0.03 W which is quite low so any resistor can do the job (without getting too hot) even a small 1/8 Watt resistor.

\$\endgroup\$
4
  • \$\begingroup\$ I had to read that a few more times to get it.. other wizards online suggest 82ohms, like this i.imgur.com/0gRV2sr.png, but I get the point now. One last question, can I replace the 10 resistors with one bigger one? Thanks \$\endgroup\$ – Syco Nov 16 '20 at 17:29
  • 1
    \$\begingroup\$ 82 ohms will be OK as well, you won't be able to tell the difference. can I replace the 10 resistors with one bigger one? Yes but only if the LEDs are identical. Anyway, I would not recommend using one resistor. \$\endgroup\$ – Bimpelrekkie Nov 16 '20 at 18:24
  • 2
    \$\begingroup\$ Also, you don't need a "bigger" resistor. Resistor size relates to how much power they can dissipate, not what value they have. And you would need a smaller value resistor, for 10 LEDs you need 75 ohms / 10 = 7.5 Ohms Power dissipation will be 10x times more (as it replaces 10 resistors) so 0.3 W so you will need a 0.5 W resistor. But I don't recommend this, just use ten 75 (or 82) Ohms resistors. They only cost a few cents each. \$\endgroup\$ – Bimpelrekkie Nov 16 '20 at 18:25
  • \$\begingroup\$ Ok, but just to get the final picture, it's either (5V - 3.5V) / 0.02A = 75ohms or (5V - 3.5V) / (10 x 0.02A) = 7.5ohms. Because I only have 10ohms and 100ohms resistors laying around, I could use 1x10ohms or 10x100ohms and they would be able to dissipate more than the 7.5ohms or 75ohms that I need, and that's a good thing.. \$\endgroup\$ – Syco Nov 16 '20 at 19:10
2
\$\begingroup\$

White LEDs usually have the blue LED behind luminophor so the usual drop voltage 3.2-3.5 volts at nominal current. The current depends of device size. 5 and 3 milimeters maximum constant current 20 mA, but 10-12 mA is good enough. Bigger LEDs, like I used 8 milimeters, description said 0.5 W, but it is on peak. Permanently 50-60 mA/180-200 mW should be. They are not reliable, make it possible easy to replace. Resistor in series must be, connection LED in parallel is bad idea. And if glue from glue gun melt, the diodes is too hot.

\$\endgroup\$
0

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.