9 volt battery getting way too hot in LED circuit...

Discussion in 'General Electronics Chat' started by critiera119, Mar 14, 2011.

  1. critiera119

    Thread Starter Active Member

    Nov 21, 2008
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    The problem: 9 Volt battery powering circuit is getting extremely hot, can't even touch. Also, resistor array built for circuit is warm to touch.

    I am trying to figure out if I have a short or if I should have used 1/2 watt instead of 1/4 watt resistors.

    Circuit:

    5mm white x 2
    3mm yellow x 2
    3mm red x 7
    3mm green x 1
    3mm orange x 1

    Each of the 13 LEDs has it's own 1/4 watt size 150ohm resistor. Was 150ohm too small?

    I believe that battery is getting hot when the switch is in the off position as well. Perhaps my switch is not connected correctly...
     
    Last edited: Mar 14, 2011
  2. Audioguru

    New Member

    Dec 20, 2007
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    Of course the value of the 150 ohm resistors is too low.

    The white LEDs have a voltage of about 3.5V so their current is (9V - 3.5V)/150 ohms= 37mA each and they will burn out soon. The 150 ohm resistors dissipate 37mA squared x 150 ohms= 0.2W each so a 1/4W resistor will be very warm.

    The other LEDs have an average voltage of about 2.1V so their current is (9V - 2.1V)/150 ohms= 46mA so they will burn out very soon. The 150 ohm resistors dissipate 46mA squared x 150 ohms= 0.32W so a 1/4W resistor is overloaded and will faill soon.

    The total current is about 562mA which is way too much current for a little 9V battery so it gets hot. Its voltage drops quickly with such a high current as shown on its datasheet:
     
  3. critiera119

    Thread Starter Active Member

    Nov 21, 2008
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    Think 330ohm 1/4 watt will do?
     
  4. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    Driving those LEDs at full brightness will draw 20mA x 13 = 260mA. That 9V won't last very long. How long do you want the LEDs lit for?
     
  5. critiera119

    Thread Starter Active Member

    Nov 21, 2008
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    More than 15min would be nice.
     
  6. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    The white LEDs have a FV of 3.5V so...
    9V - FV = 5.5V
    5.5V / 0.02A = 275 ohms

    so 275 ohms would be ideal for a 3.5V white LED. Different colored LEDs have different FV so you'll have to calculate them.

    As for wattage

    5.5V (the voltage dropped through the resistor) * 0.02A (current through the resistor) = 0.11W so a 0.250W (1/4W) resistor is fine.

    330 is close enough for the white LED
     
  7. critiera119

    Thread Starter Active Member

    Nov 21, 2008
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    Ok, will make the resistor change.

    Now can we figure out why my 9volt battery is making bubbling sounds while the SPST one position slide switch is in the off position?

    Here is how I have it connected. And why is there a third prong on this slide switch?

    See image:
    [​IMG]
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    As I showed on the graph of the battery, little 9V alkaline battery can drive 400mA for 15 minutes but the voltage drops the entire time so your wiill see the LEDs get dimmer and dimmer.
    Since the battery will be destroyed in only 15 minutes then it will get hot but not too hot if it has some airflow.

    Did you do the simple calculation to see the currents and dissipation if 330 ohm resistors are used? A 1.8V red LED will have a current of 22mA and its resistor will dissipate 0.16W. A white LED will have a current of 17mA and its resistor will dissipate 0.1W.

    The total current for the 13 LEDs from a new 9V battery will be about 255mA. It will last about half an hour when the LEDs will be dim and the battery will get fairly warm.

    EDIT: Your slide switch is SHORTING (!) the battery.
     
  9. critiera119

    Thread Starter Active Member

    Nov 21, 2008
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    Saw your graph, saw the formula.

    I figured an average of 2.8 for all 13 LEDs and 20ma draw from each and did something even more simple and plugged that data into an online calculator which gave me the 330ohm 1/4watt solution for each LED.

    Now to figure out how to connect slide switch correctly...
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    An on-off switch is connected in series with the load. Then when the switch is turned on the battery is connected the load like a piece of wire. When the switch is turned off then the battery is disconnected from the load.

    Your slide switch is a "double-throw" switch. It can turn on one thing when switched to one side then it can turn on a second thing when it is switched to the second side.
     
  11. critiera119

    Thread Starter Active Member

    Nov 21, 2008
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    There is no middle position though. Just one slide on and one slide off.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    If there are three terminals on the switch in a line, then the middle one is the "common" terminal.

    Connect the anode side resistors of the LEDs to the common terminal.
    Connect the battery + terminal to one of the other switch terminals.
    Connect the battery - terminal to the cathode side of the LEDs.
     
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  13. critiera119

    Thread Starter Active Member

    Nov 21, 2008
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    Exactly the info I wanted. I was also going to try that next actually. Thank you all.
     
  14. Wendy

    Moderator

    Mar 24, 2008
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