Need assistance with a simple parallel circuit question.

Discussion in 'General Electronics Chat' started by agroom, Jul 27, 2011.

  1. agroom

    Thread Starter Active Member

    Oct 15, 2010
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    1
    I want to make a string of 15 3.6v LEDs. I need to drive it with 3 1.2v batteries, but I want to space them out evenly along the circuit. I'm stuck on how to make the circuit because the LED's all have to be run in parallel, but the batteries in series. Is this possible?

    The layout would be: battery, 5 leds, battery, 5 leds, battery, 5 leds

    Thanks for any help!

    [EDIT] Sorry, in hindsight I should have been more clear about what I'm asking, what the project is and why I would consider the above scenario.

    I want to power a string of 3.6v, 20mA LEDs using 1.2v batteries that must be placed throughout the circuit. Ideally, using 4 batteries and individual resistors for each LED, the 4 batteries create a combined voltage of 4.8v, so the LEDs must be connected in parallel and the batteries in series. This would allow me to create enough voltage where I can attach a 68 (I think that's the closes to 60) ohm resistor to each LED to reduce the voltage and control current in the circuit.

    I know how to make it if all the batteries were in one location, but the 4 batteries need to be evenly distributed throughout the string of LEDs, so that I have: Battery - 4 LEDs - Battery - 4 LEDs - Battery - 4 LEDs - Battery - 4 LEDs (Not shown are the resistors attached to each LED). The project is for a very small diameter, light weight LED hula hoop, so weight distribution is very crucial. I also need to use the lower voltage batteries so that they fit inside the tubing.

    My initial strategy was to use only 3 batteries which will 'just' power the LEDs in an attempt to keep the weight as low as possible. I'm using a very small gauge wire and 3mm LEDs to also help compensate for both weight reduction and ability to fit inside the tube. I have made several LED hoops already, all using 2 3v batteries in series, located in 1 place then a string of parallel LEDs following it, each with it's own resistor. Each LED/Resistor combo was soldered to the wire with heat shrink applied around each connection to insulate the exposed wire and add stability.

    I'm aware of the implications the 3 battery setup can cause, but I have read up on the "real world" application of using a batteries internal resistance to control the current and in such a minor application such as this I was told that it's most likely it will work just fine.

    However, my real issue simply comes down to not knowing how to create a circuit that the batteries are evenly placed along the circuit rather than all in one location, while being able to combine the batteries voltages yet keeping the LEDs in parallel. Is this circuit possible and if so, would anyone be willing to provide me with a wiring diagram?
     
    Last edited: Jul 27, 2011
  2. #12

    Expert

    Nov 30, 2010
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    No. It won't work. You barely have enough voltage to run an LED and nothing extra for the resistor that keeps the LED from burning up.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Well, that wouldn't really work, as your batteries when fully charged after being allowed to sit for a period of time would measure around 1.2v each - so that's 3.6v, about the same as the Vf of your LEDs.

    However, the led Vf will vary, even in the same batch. If you have LEDs in parallel, and one LED with a lower Vf than the remaining LEDs, the one LED will hog the current - and it'll burn up quickly.

    Also, if your batteries just came out of the charger, they might be more than 1.2v each. The current through your LEDs would be very high then.

    Also, if alkaline batteries were used instead, you would have 3x1.5v = 4.5v across LEDs rated for 3.6v. They would not last long.
     
  4. agroom

    Thread Starter Active Member

    Oct 15, 2010
    60
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    I understand the reasoning you say I can't do this. It would create a circuit with no resistance and short all the LEDs. In practice though the internal resistance of the batteries and the wire creates enough resistance this does not happen. I have done this multiple times as I'm sure many of you have. Anyone ever bought one of those 9LED flashlights? Of the 10+ I have bought and modded, they use < 4.5v LEDs but drive it with 3 AAA batteries and no resistors. I have also replaced them all with 3.2-3.6v LEDs and have worked for well over a year now. I'm not saying it's an acceptable practice, but in reality it is a practice.

    For argument sake then, say I want the same setup but with 4 batteries and each LED has the appropriate resistor, batteries create a constant voltage even at full charge, etc. I would simply like to know how to drive a circuit of parallel LEDs with multiple batteries placed throughout the circuit.
     
    Last edited: Jul 27, 2011
  5. SgtWookie

    Expert

    Jul 17, 2007
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    If you have three 1.2v batteries, and you want just a simple DC circuit, about the only thing you can do is put the three batteries in a series string, and all of the LEDs in parallel with the battery string.

    The LEDs's current will not be controlled except by the battery internal resistance, as you mentioned already.
     
  6. agroom

    Thread Starter Active Member

    Oct 15, 2010
    60
    1
    There is no way to do this without putting all the batteries in 1 location then? They cannot be individually placed throughout the circuit?
     
  7. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    The batteries can be anywhere you want, but the LED's must not connect to any of the wires BETWEEN the series of batteries. You will simply have more wire in your hook up arrangement to accomplish the series battery connections.
     
  8. agroom

    Thread Starter Active Member

    Oct 15, 2010
    60
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    I was afraid of that, though being a novice in circuitry thought there may be a way I wasn't seeing. Thanks!
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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  10. PeeSeeBee

    Member

    Jun 17, 2011
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    You could also try using the guts of a solar powered garden night light.

    I have one which powers a set of 50 white LEDs. It uses an ANA608 chip. It is similar to the circuit shown on page 4 here:-

    http://www.nonstopsystems.com/radio/article-magloop-twisted-gamma.pdf

    There are a few differences to mine. Mine uses a 1N5819 Schottky, a 47μH inductor, runs from 3v, & has the string of LEDs connected across the inductor.

    If you have the equivalent in the USA to our £1 stores ($1 stores?), it might be worth buying one or 2 to experiment with.
     
  11. KJ6EAD

    Senior Member

    Apr 30, 2011
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