Indicator circuit help

Discussion in 'Homework Help' started by del1803, Dec 3, 2007.

  1. del1803

    Thread Starter New Member

    Dec 3, 2007
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    I am designing 2 circuits, one a flashing light circuit using transistors and one using a 555 timer, on the 555 timer circuit i blow the relay switch because 7 amps is running through it when it takes a maximum of 5amps. On the transistor circuit i can't get the lights to turn on, can anyone help with the attachments
     
  2. eeboy

    Active Member

    Sep 27, 2007
    90
    1
    I don't see any current limiting resistors on the LED's...

    Throw a 1k ohm resistor in series with each LED.
     
  3. eeboy

    Active Member

    Sep 27, 2007
    90
    1
    Also, on the transistor circuit, is that truly an NPN with the emitter tied to the two switches on the right?

    You should have the collector tied to the cathode of all the LEDs and the emitter tied to ground. The annode of the LEDs should be tied to +12 (behind the switches that is). Again, you'll need current limiting resistors on each LED.
     
  4. del1803

    Thread Starter New Member

    Dec 3, 2007
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    Put the 1k resistors on the lights but then lights won't power up at all now
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    The symbols are for incandescent lamps. Are you sure they are LED's? They are labeled in watts, which is not normal for LED's, but is for incandescents.

    The high currents are also typical of incandescents. At turn-on, before the filaments heat, there is a large current surge. That may be what is harming your contacts and transistors.
     
  6. del1803

    Thread Starter New Member

    Dec 3, 2007
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    Change my transistor circuit and that doesn;t work either, new circuits attached
     
  7. eeboy

    Active Member

    Sep 27, 2007
    90
    1
    Sorry... I thought I read LEDs in your original post. These are small incandescent bulbs right? I should have questioned the 21W and 5W.

    Depending on the switch configuration your circuit could draw approximately 8 amps (94W). Again, I am making some assumptions that the 21W and 5W identifies the wattage of the individual bulbs. This puts you over the rated current of the relay. You'll need to make some changes to your load (maybe remove a few of the 21W bulbs).
     
  8. del1803

    Thread Starter New Member

    Dec 3, 2007
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    there not led's they are lamps is that a problem?
     
  9. del1803

    Thread Starter New Member

    Dec 3, 2007
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    I need all the lamps in because they are for a car indicator circuit
     
  10. eeboy

    Active Member

    Sep 27, 2007
    90
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    What transistors are you using? That's an awful lot of current! I have a feeling you are using something far too inadequate.

    Your transistor circuit is still incorrect. The collector needs to tie to the "bottom" of the lamps. Your emitter is correctly tied to ground. Tie the "top" of the switches to 12V.

    Also, remove the 1k resistors. I don't know where I got LED's from.
     
  11. eeboy

    Active Member

    Sep 27, 2007
    90
    1
    If you need all the lamps then you'll need to size your relay appropriately. A 15 amp relay for automotive purposes should be easy to come by.

    You can remove the lamps for testing purposes at this point and place them back in the circuit when you have proven it and you have your new relays.
     
  12. del1803

    Thread Starter New Member

    Dec 3, 2007
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    Ok will do that for my 555 timer circuit, i have attached my transistor circuit again this time my capacitor blows and 4.0153v max allowed 3v, and the transistor blows at 2.5488A max allowed 2A
     
  13. eeboy

    Active Member

    Sep 27, 2007
    90
    1
    Yeah... you have the same problem.

    Your load is the cluster of lamps. In the case of the 555 circuit you are supplying the load via a relay. In the case of the transistor circuit you are supplying the load with the transistor. They both must be rated for the load. You can calculate the current as you know the bulb wattage and the voltage applied. In your case you have 4-21W bulbs and 2-5W bulbs (worst case). The sum is 94W. Knowing P=VI, you can calculate I. In this case I is 7.8A. Your component rating must exceed 7.8A.
     
  14. del1803

    Thread Starter New Member

    Dec 3, 2007
    7
    0
    Ok cheers for your help its much appreciated
     
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