Do I need a transistor with this LM3914?

Discussion in 'The Projects Forum' started by speemike, Jan 1, 2010.

  1. speemike

    Thread Starter New Member

    Jan 1, 2010
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    Let me start by sayting that I am a mechanical engineer that loves to tinker with circuits. With that being said I am not very good.

    I am trying to drive a 10 LED automotive light bar using a LM3914. The light bar says it draws a max of 250mA when all lights are on. I plan on only using 5 of the 10 outputs on the LM3914, so that means each of the 5 outputs will drive 2 LED's. I am concerned that the current draw may be to much for the LM3914, but I am not sure. I am also concerned that having 1 output drive two LED's may get the LM3914 to hot. I have looked at the LM3914 spec sheet and I am totally lost.

    Should I drive the 10 LED's off of 5 transisters?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Why don't you drive the two LEDs in series instead of parallel?
     
  3. speemike

    Thread Starter New Member

    Jan 1, 2010
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    I guess I could do that, I will have to see how bright they are.
     
  4. Audioguru

    Expert

    Dec 20, 2007
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    I use an LM3915 in a VU meter type of circuit. Each output has two 2V red LEDs in series at about 25mA. I use a 9V supply and when all outputs are turned on the total current is 250mA and the LM3915 would dissipate 1.25W which would make it too hot. So I use a 10 ohm 1W resistor in series with the supply to the LEDs to share the heat.
     
  5. speemike

    Thread Starter New Member

    Jan 1, 2010
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    Great thanks guys!
     
  6. speemike

    Thread Starter New Member

    Jan 1, 2010
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    I am using 10 red clear LED's which each has a 2v drop. On each of the 5 outputs of the LM3915 I have 2 LED's in series and a 100ohm resistor also in series. I am using a 12 volt input. I have hooked it up and put all 10 LED's on and it got warm but not hot. I think I should be ok what do you guys think?
     
  7. Audioguru

    Expert

    Dec 20, 2007
    10,605
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    The outputs of the LM3915 have current regulation so each one does not need a resistor. Since the supply is so high at 12V then a single power resistor can feed reduced voltage to all the LEDs. The power resistor shares the heat.
     
  8. speemike

    Thread Starter New Member

    Jan 1, 2010
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    Got it thanks! I already soldered in 5 100 ohm resistors into the circuit so I will just leave them. I may also add one more resistor to the power supply like you suggested, this will further help lower the supply.
    Thanks again for all your help.
     
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