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LED's in a series with one resistor question — Parallax Forums

LED's in a series with one resistor question

markmcleod50markmcleod50 Posts: 27
edited 2004-11-19 06:19 in BASIC Stamp
I found this article on the web and wanted to make sure I was reading it right, I will be buying 225 LED's that run 2.3v@20mA
I want to connect as many as I can together and use one resistor per grouping. I will be using a 12volt switchable wall transformer rated at 500mA.
From my calculations I can connect 5 LEDs with one 100ohm resistor? But it seems the power supply should be able to handle a lot more than just 5 leds, what am I missing? Heres the example I found. I will need to connect 200 LED's and try to run them off as few transformers as possible, is there something else i could use to get my current up without having to buy a 4amp (200 x 20mA) transformer?

A red, a yellow and a green LED in series need a supply voltage of at least 3

Comments

  • Eric REric R Posts: 225
    edited 2004-11-19 04:43
    According to my calculations for 225 LED's in series with 1 resistor @ 150 ohms you would need a source voltage of ·520v. If someone finds this wrong, let me know...
  • ubikuberallesubikuberalles Posts: 2
    edited 2004-11-19 06:19
    I think you will need to experiment a little before coming up with your final design. If you connect your LEDs all in series keep in mind that each LED will sink about 1.7 volts (depending on the LED. The super bright LEDs sink up to 3 volts each!). So, if you put 5 LEDs in a row, you will need at least 8 V to cover the voltage drop across the LEDs. 225 LEDs in a row? You'll need 380 Volts just to cover the voltage drop across all the LEDs. I wouldn't recommend that much voltage and so you should consider hooking them up in parallel which means a lot of wires. You'll also need a reistor for each LED in the parallel circuit or you may burn out the resistor.

    A combination of LEDs in parallel and serial would most likely be the best bet. One circuit of 5 LEDs in series with a resistor could be hooked up in parallel with another 10 LEDs and a resistor in series.

    I just did an experiment and I could only get 5 LEDs lit up with a 9 V battery and a 220 Ohm resistor (a 470 Ohm resistor worked with 5 LEDs but they were just a little dimmer). If I added a 6th LED to the circuit, none of them would light up. Hooking up an additional 5 LEDs and a 220 Ohm in parallel to the other circuit also worked.

    Post Edited (ubikuberalles) : 11/19/2004 6:31:07 AM GMT
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