Questions regarding a very simple circuit

Discussion in 'The Projects Forum' started by lotsoflemon, Apr 22, 2011.

  1. lotsoflemon

    Thread Starter New Member

    Apr 22, 2011
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    Hello! I have just joined this forum, in order to post this one question :)

    We are a couple of guys, trying to make a very simple light using a switch, three AAA batteries, three blue LED's and three resistors. The entire thing is to take up as little space as possible. We visited our local electronics store, and walked away with the different parts. We wired it all together, and it worked for 20 min. But then it wouldn't light at all, and two of the batteries became very hot, even after we switched it off.

    I have included a wiring diagram of the parts. If for some reason you cannot see this, it is 3 AAA batteries soldered directly to each other. The switch is connected through a wire, and the three LED's are soldered to a resistor each. I believe the resistors are a 100 Ω, they have the following ring colors: Brown, black, brown, gold and white. The LED's and resistors are thus connected to the power supply (and switch) in parallel. I don't know anything about the LED's, except that they are blue.

    This truly is a neewbish question. If you know what went wrong and what to do, your answer is much appreciated. If you feel that i just wasted five minutes of your life, i am sorry :)

    I hope someone out there will respond! Thank you very much.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You have the voltage backwards. The arrowhead on the diode points to the cathode (negative voltage terminal). Reverse the batteries and you should get light.
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Well, you certainly did not wire it the way you have shown it. The diagram shows the diodes connected the wrong way round. No question of them lighting like that.

    Probably you had the LEDs the right way, but the real problem is likely to be soldering the batteries. It is never a good idea to solder them directly, as they can indeed fail, and even sometimes burst. Please do not do that again - try to get a battery holder.
     
  4. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    The circuit looks okay... and it worked.

    It is not a good idea to solder directly onto the battery end. This can cause damage to the battery. My guess is that your solder bloob has shorted the positive terminal to the shell of the battery. If it is the negative terminal then a short may occur... twice.

    Just a guess,
    Ifixit
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The OPs actual circuit probably was OK, but his diagram clearly shows reversed diodes.
     
  6. lotsoflemon

    Thread Starter New Member

    Apr 22, 2011
    4
    0
    Thank you for all your answers! That was very quick.

    You are right, the diagram was wrong. I did manage to get 20 min. of light.

    So the direct soldering on the batteries is the problem? It is true that my solder bloobs are quite large and do indeed touch the shielding of the batteries. <snip>. Have i broken just the batteries, or the diodes too? And does the resistors match somewhat with the diodes?

    Thank you!
     
    Last edited by a moderator: Apr 22, 2011
  7. mbxs3

    Active Member

    Oct 14, 2009
    141
    3
    Usually the LED packaging will provide the specs such as forward operating voltage as well as current rating in mA. With these numbers you can determine the correct resistor needed for each LED. Do you have any of that information on the LEDs? Also, as mentioned before, you should get a battery pack.
     
  8. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Instructions: (Poor Man's Battery Holder)

    (1) Cut an appropriate 3/4" thick board to desired dimensions.
    (2) Separate paper clip loops until you have a 90° angle.
    (3) Screw the large loop down on the board with washers. This will hold the clip and conductors.
    (4) Do the same at each end of the cells.
    (5) The same approach, used on the sides of the cells, can be used two keep them cradled.

    As a kid, I fished with bent sewing needles. Most of them got away!:confused:
     
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    If the battery cells are alkaline and are large enough to provide the total current then their total voltage is 4.5V.
    A blue LED uses about 3.4V and the 100 ohm resistor limits the current in each LED to (4.5V - 3.4V)/100= 11mA which is fine.

    NEVER solder to a battery because it might melt the plastic insulation inside.
    Learn how to solder without creating huge blobs of solder that short things.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If the last color band on the resistors is silver instead of white, then the resistors are actually 10.1 Ohms, 10% tolerance.
    Here is a graphical 5-band resistor color code decoder:
    http://samengstrom.com/15924820/en/read/5_Band_Resistor_Color_Codes?history=24614782,15924820
    If they were 4-band resistors, then they would be 100 Ohms.

    Blue LEDs might have a Vf of 3.2v to 4v, and the rated current might be 20mA.
    Let's just say that they have a Vf of 3.4 for the moment.
    4.5v - 3.4v = 1.1v to drop across the resistor.
    1.1v/100 Ohms = 11mA, which would give the LEDs about 1/2 their normal brightness, but they would last a long time.
    1.1v/10 Ohms = 110mA, which would be about 5.5 times their rated current, and they would burn out pretty rapidly. LEDs frequently short out when they fail. So, you would get no light output, and your batteries would get quite warm.
     
  11. lotsoflemon

    Thread Starter New Member

    Apr 22, 2011
    4
    0
    Thank you for all your replies :) For some reason i didn't get a notification, so sorry for my late answer...
    My problem with the batteries is that i have very limited space. The three AAA batteries will need to sit in a triangle, with the switch in the middle and the LEDs and resistors around it. My options are very limited unfortunately.
    I actually purchased a battery holder, but it doesn't fit. So i somehow need to figure out how to connect the three batteries in a triangle, without any soldering. What would be the optimal design?
     
  12. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    You can buy battery holders that hold only one cell. They will add about 3/8" to 1/2" to the length of each cell and probably 1/8" to 3/16 to the width. You would then use short soldered jumpers between them.
     
  13. rfordh

    New Member

    Oct 26, 2010
    6
    3
  14. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
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  15. lotsoflemon

    Thread Starter New Member

    Apr 22, 2011
    4
    0
    Thank you :) Unfortunately i live in Denmark, and home electronics like this is quite expensive. I ended up with simply bending a few paper clips in exact shapes, that matches a triangle. I then taped them directly to the batteries. It's working great, thank you for all your help!
     
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