Basic LED circuit

Discussion in 'General Electronics Chat' started by sanmakk1001, Oct 3, 2014.

  1. sanmakk1001

    Thread Starter New Member

    Jul 13, 2014
    Hey guys I hope you are fine.!! Anyhow let's get to the point. I am working on a very basic project. In which I will connected 30 LEDs of different colours together. I decide to go for parallel circuit. The LEDs I am gonna used are red green and blue. I have and AC to DC adapter which gives me output of 4.5v and current 750 mA.

    I didn't started work on the project because I have couple of questions goin on in my head. I am all confused I search the internet but couldn't fine approximate answers. I have created a list of question. I hope you will help me.

    1: What is ampere
    2: what is difference between voltage and current.
    3: how can I measure ampere of LEDs. My led dont come with spec
    4: How can I measure LED voltage.
    5: what is meant by forward current
    6: on which terminal resistance should be contacted.
    7: I have a multi meter do please guide me how to perform correct measurements. So that I could build up the circuit with appropriate resistance.
  2. #12


    Nov 30, 2010
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    need 30 x 240 ohm 1/8W resistors.. 1 per LED (assuming regular LED with 2.1Vf @10mA)
    or 30 x 120ohm 1/8W resistors for "full" brightness.. (assuming regular LED with 2.1Vf@20mA)
  4. shteii01

    AAC Fanatic!

    Feb 19, 2010
    1) Current unit of measurement.
    2) One is a field, the other is not.
    3) You don't. Manufacturer provide it. Generic led use between 10-25 mA.
    4) You don't. Manufacturer provide it. Generic led use between 2-4 V.
    5) Nothing.
    6) Does not matter.
    7) Go watch youtube.
  5. #12


    Nov 30, 2010
    Red, green, and blue have different forward voltages. You're going to set up a circuit with your alleged 4.5 volts, a 240 ohm resistor, and one of each color to find the voltage for each color. You will also measure the 4.5 volts to see if it is really 4.5 volts when you have only one LED connected. Wall warts are notorious for delivering higher than the label voltage when there is very little load.

    After we get those numbers and you find out if the LEDs are responding well to .01 amps, we can finish the math.

    ps, I can get rather cryptic, but this time, I think shteii01 out did me. :rolleyes:
    Don't worry. We'll explain all that in due time.
  6. wayneh


    Sep 9, 2010
    1: 1 amp = 1 Coulomb per second = charge flow per unit time
    2: In analogy to a waterfall, voltage is the elevation drop, current is volumetric flow, charge is an amount of water.
    3: A cheap multimeter in series with the LED will tell you the current, but not the specification. A typical, cheap LED is lit by 5-20mA and might be destroyed by >25mA. You must limit current in order to prevent this.
    4: While lit by the proper current, measure the voltage drop across the LED.
    5: The current passing through the LED.
    6: Resistance to limit current may be placed anywhere in a series circuit. It restricts flow like a water valve.
    7: That takes more than a simple answer. But the most common newb mistake is to attempt to measure current by placing the leads in PARALLEL to the circuit, shorting it. To measure current, the leads must be in SERIES with, and part of your circuit. Voltage and resistance measurements are performed in parallel, but not current.
  7. #12


    Nov 30, 2010
    More than a simple answer: If you can figure out the voltage scales and use a known resistor value, the current in every position can be calculated. This is the important one.

    If you can figure out the current scales, that will be good, but not a critical function today and definitely not enough to calculate everything we need.