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Learning Electronics, need help with my noob circuit

Discussion in 'General Electronics Discussion' started by otacon, Nov 7, 2014.

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  1. otacon

    otacon

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    Nov 7, 2014
    Probably a silly question but you got to start somewhere. I'm stuck adding more power to my circuit.

    At the moment I've managed to power 3 LED lights in series with a 9v battery. If I try to power a 4th LED, I don't have enough current to power all 4 of them. This is where I'm stuck. What's the best way to provide my circuit with more power? I know how to calculate all the necessary information using ohms law, but don't know how to physically set up a second battery, or if there's a better way of adding power. I'd love to power more than 3 LEDs :)

    Also, what's safest voltage to work with while learning?

    Thanks all!
     
  2. Bluejets

    Bluejets

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    Oct 5, 2014
    Depends on the LED's.
    What forward voltage are they.?
    What current rating.?
    What battery are you using.?
    It is more likely you do not have enough voltage, not necessarily current in the first instance although this may be an issue further down the track if your 9v supply is a 216 battery.
    Three is series(group 1) then another 3 in series (group 2) connected in parallel.
    Also you will have to take into account current limiting resistors.

    Safe voltage is low voltage such as you have now although some battery packs can deliver rather high discharge currents if you start shorting things out by mistake.
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Hi there and welcome to Electronics Point :)

    The problem is you don't have enough voltage, not current. Each LED requires a certain amount of voltage across it. This voltage is called the "forward voltage", VF and you can measure this with a multimeter on volts range when the LED is glowing.

    Different types of LED have different forward voltages; generally, infra-red LEDs have the lowest VF at about 1.2V, then red at about 2V, then yellow and green, then blue and white, at around 3.5V typically. But there are exceptions.

    If your LEDs need, say, 2.8V each before they will even light, then three of them in series need 8.4V (voltages in series add together), which a 9V battery can provide, but four of them in series will need 11.2V; the 9V battery doesn't have enough voltage to make current flow through them.
    Ohm's Law doesn't directly apply to LEDs because they are not ohmic, i.e they do not behave like resistors. But it is relevant when you're calculating the current limiting resistor that you should connect in series with the LEDs. If you don't have a current limiting resistor now, you are relying on the LEDs and the battery to limit the current, which is not wise and can damage the LEDs.
    Anything less than about 40V DC. 12V is a good voltage to choose as most electronic components (ICs etc) can withstand 12V easily, but it's still enough that you can do useful things with it.

    For the answers to all your LED questions, see Steve's tutorial at
    https://www.electronicspoint.com/resources/got-a-question-about-driving-leds.5/
     
    otacon and davenn like this.
  4. otacon

    otacon

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    Nov 7, 2014
    Thank you both for your answers. So if it takes 2.8v (example) to light up one, and I got three LEDs (2.8 * 3), and that result is the voltage required for the circuit to run?

    I still don't know how to add more power to my circuit though, unless I'm missing something :(

    I'm using a 9v energizer alkaline battery, 100 ohms resistor, three LEDs in series.
     
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    I pointed you to that tutorial because it will answer all your questions, including questions you didn't even know you needed to ask, which is even more important. Read it until you understand it, and if you have specific questions that it doesn't answer, then ask them, so Steve can update the resource.
     
  6. otacon

    otacon

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    Nov 7, 2014
    Thanks Kris, I'll read the tutorial right now.
     
  7. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    10LEDsOn9v.gif
     
    otacon likes this.
  8. BobK

    BobK

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    Jan 5, 2010
    A Joule thief on steroids!

    Bob
     
  9. otacon

    otacon

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    Nov 7, 2014
    Thank you for sharing. I'll take a look at it after I understand the tutorial Kris suggested.
     
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