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Transistor selection help

Discussion in 'General Electronics Discussion' started by Tbrewer409, Nov 29, 2014.

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  1. Tbrewer409

    Tbrewer409

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    Nov 23, 2014
    Im in need of some help. Desinged a circuit that uses a 4017 counter for some 750ma leds. 3 colors, each time you hit the momentary switch it changes to the next color ( q0=nothing hooked up q1= blue 750ma led q2= red 750ma led q3=white 750 ma led). Simulated in yenka and multisim and it works correctly. Problem I have is, I had to use custom tranistor parameters, im local to parts express and they do not show the values of their transistors, and I really dont know which I would need as im still very new to electronics. Circuit is going to run about 13.5 vdc (car) collector hooked to negative of led, base to output pin on 4017 with 470ohm and diode on simulator, emitter to ground. Thanks for the help and sorry if I have given useless info.
     
  2. Tbrewer409

    Tbrewer409

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    Nov 23, 2014
    This is the circuit. Thank you for any help.
     

    Attached Files:

  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Diodes D1 to D5 are not required.

    Completely remove R5 and leave Q4 connected directly to MR with no other connections to either.

    CP0 needs a pullup resistor and the switch S1 will bounce causing multiple transitions. Each time you press it the result will essentially be random. However the clock input has a schmitt trigger input, so if S1 is a SPDT switch you can make a simple debounce circuit with a resistor and a capacitor,

    Q1, Q2, and Q3, along with R1, R2, and R3 are not really great at switching and controlling the current for the LEDs. You'll find that the current will differ between the red and the blue LEDs.

    You'll waste a lot of power in those resistors (which will need to be rated at 10W (minimum)). I would replace the transistors Q1 to Q3 with logic level mosfets. It would be better if you drove the LEDs using a circuit like figure 3.5 here.
     
  4. Tbrewer409

    Tbrewer409

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    Nov 23, 2014
    Holy crap, I'm way off. Could you possibly pencil the spdt that you are referring to. I am definately going to read that post, those resistors are huge and take up way to space also.
     
  5. Tbrewer409

    Tbrewer409

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    Nov 23, 2014
    BTW, Thank you so much for your help.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Even without those resistors, something will be dissipating 10W or so when you turn on a LED. If you use a mosfet as I've suggested in the other thread it will be the mosfet dissipating this power (so it will need a heatsink).

    To drive the LEDs more efficiently you could drive them from a lower voltage source (perhaps 6V).

    4017clock.png

    Please excuse the rough drawing. 100k and 0.47uF should be fine.
     
  7. Tbrewer409

    Tbrewer409

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    Nov 23, 2014
    I am wanting to use a momentary contact push button. Would my normally closed contact be to the positive side just like your drawing?
    Also you mentioned heatsink, am i going to need one whether i use the mofset with voltage regulator, or the resistor-transistor. Reason I ask, is this is going into a 4 square pvc electrical box (it's a dome light for my police car). Again I really appreciate your help.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I drew the switch at random to illustrate the technique. I think you showed the switch closed in your original drawing, but I did not consciously follow it.

    Edit: I just checked the datasheet to be completely certain. The 4017 is clocked on the positive edge of the signal. This means you should have the switch exactly the opposite way around.

    Also since you're running this from a car electrical system, you will need to protect the electronics from electrical noise that could either make it operate unusually or destroy components.
    I would recommend you find a 12v to 5v dc to dc converter. These often have a usb socket on them and fit almost entirely within the cigarette lighter socket. I'm sure you know what I mean. One rated for 2A output would be great for this. You could either take it apart and mount the electronics inside the sealed box or run a cord to the cigarette lighter socket.

    If you run your device from 5v then the losses will be much lower and you may not require heatsinks on anything other than the LEDs.

    It would also be advisable to decouple the 4017 power supply from the LEDs. This might be as simple as producing the logic power supply using a 100 ohm series resistor and a 100uF capacitor so that switching the LEDs doesn't cause glitches.
     
    Last edited: Nov 30, 2014
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