Problems driving a ULN2803

Discussion in 'General Electronics Chat' started by mangopuppy, Dec 15, 2013.

  1. mangopuppy

    Thread Starter New Member

    Dec 15, 2013
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    I have a 555/4017 board that I'm trying to drive a ULN2803 to power an LED array. Is the 4017 so "weak" that it cannot signal the ULN? I get no response from my circuit, past the indicator LEDS on my chaser board.
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    A schematic is in order. Be aware the 2803 will not drive high. It needs a pull up.
     
  3. t06afre

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    Can you post a schematic
     
  4. t06afre

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    That will depend on of how you use(connect) it
     
  5. Alec_t

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    Sep 17, 2013
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    The '2803 input draws about 1mA, which the '4017 should be able to provide.
     
  6. mangopuppy

    Thread Starter New Member

    Dec 15, 2013
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    This is my board, which is already soldered onto perf. I'm using it's LED path to connect to the 2803's inputs (on bread). Nothing happens. I'm wondering where the 9V power on the perf went! Hardly anything is left to hit the 2803, I guess.
     
  7. GopherT

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    Nov 23, 2012
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    if you are measuring on the ground side of the LED, you will see nothing.

    If you are measuring the chip side of the LED, you will see only 2 volts.

    if you add a 330 ohm current-limiting resistor in series with each LED and then measuring the output at the CD4017, you will see a reasonable voltage.

    NOTE: you should not connect LEDs without a resistor in series to limit current.
     
  8. tubeguy

    Well-Known Member

    Nov 3, 2012
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    If you are connecting the LED's to the 2803 outputs the same way as in the CD4017 connection in your schematic, they won't light up. This is because the ULN2803 only sinks current.

    Do you understand the difference between sinking and sourcing ?
     
  9. mangopuppy

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    Dec 15, 2013
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    Yes. I understand the 2803 is not a power source, only a relay to a ground for each LED. I have a bank on LEDs in parallel with a 300 ohm resistor at each output of the 2803 with a separate low volt, high current power source for the array. (9v at 2.2A)

    What I do not understand is while measuring the voltage at the outputs of the 4017, I read less than 0.5v. I think according to the specs on the 2803, 2v is the min input to drive it?
     
  10. tubeguy

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    Nov 3, 2012
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    Can you please post your complete schematic as built ?

    If the CD4017 is connected directly to the ULN2803 inputs and is not broken, the CD4017 output voltage should be near it's V+ supply when the CD4017 output is high and in a static condition.
    But if chasing, you will get a lower reading.
     
    Last edited: Dec 15, 2013
  11. mangopuppy

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    Dec 15, 2013
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    In a nutshell....
     
  12. tubeguy

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    Nov 3, 2012
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    As mentioned earlier, you have LED's connected to the CD4017 outputs without current limiting resistors. You must use current limiting resistor for those LED's also.
    Those LED's are lighting properly, correct?

    Were is the ground connection on the 2803 ?
    The C (common) connection goes to internal diodes. It should be on V+. -- Look at the datasheet.
    But, if connected as shown the LED's in the array should be lit constantly through those diodes.
    Is the LED array polarity correct?
     
    Last edited: Dec 15, 2013
  13. mangopuppy

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    Dec 15, 2013
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    As referenced, no, no resistors for the LEDs on the perfboard. I will add the appropriate rating (~330) tonight.

    Yes, they are lit up well, perhaps I've been lucky not to burn them out, but I ran the board until the battery died with no issues.

    The "9" pin on the 2803 is grounded to both the high current source, as well as the perfboard. My understanding of this chip is the "10" pin is not needed (high current source) with driving LEDs. It was designed for induction protection when controlling motors, so it is not connected.

    The pos side of the PS is routed to a common anode rail of LEDs, then 300 ohm resistors on each cathode, and then to 2803 outputs. Is this incorrect?

    I will construct the layout in detail for the Array and post it here.
     
  14. tubeguy

    Well-Known Member

    Nov 3, 2012
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    I see now that the C could be a G, so that is correct. Your understanding of the ULN2803 is accurate.

    Common anode wiring of the array is correct.

    It's possible the LED's on the CD4017 without limiting resistors are preventing the Darlington's in the ULN2803 from turning on completely.
     
  15. mangopuppy

    Thread Starter New Member

    Dec 15, 2013
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    Thanks for your help. I've annotated my layout to make it more descriptive. When I finish adding the resistors to my perfboard, I'll post the result.

    Am I correct by pulling the 2803 inputs in a parallel manner from the 4017? I guessed that the drop would be too much if I tied the inputs to the perf LEDs in series.
     
  16. elec_mech

    Senior Member

    Nov 12, 2008
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    The ULN2803 is designed to work with TTL voltages, roughly 5VDC. You are using a 9VDC power supply which is outside its designed range (to the best of my knowledge). I suggest using a ULN2804 which is designed to work with 6-15VDC.

    Also note CMOS ICs and 555's are inherently noisy and may affect operation of the other. Add a 0.1uF ceramic capacitor across the power leads of the 4017. Add a 0.1uF ceramic AND 1uF electrolytic across the power leads of the 555.

    Are you using two sets of LEDs - one on the output of the 4017 and one on the output of the ULN2803 as shown in your schematic? If so, why?

    What is the forward voltage and current rating of your LEDs? If you're using two different types of LEDs, please post the specs for both.

    Post a picture of your set-up - we might be able to spot something there as well.
     
  17. t06afre

    AAC Fanatic!

    May 11, 2009
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    The 2803 and 3804 differ only by the series resistance in the input(Base on the darlington transistor) The 2803 use a 2.7K resistor, and the 2804 use 10.5K resistor. By adding a series resistor to input of a 2803 you can use the 2803 safe at higer voltages
     
    elec_mech likes this.
  18. mangopuppy

    Thread Starter New Member

    Dec 15, 2013
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    I have a bank of LEDs on my perfboard as a monitoring tool. Orange? LEDs I'm assuming 3.4v FV. I had them laying around. The output array will contain 48 white LEDs (3.4v).

    The one issue I have is why the outputs of the 4017 will not trigger the 2803. Another member suggested adding resistors after the perfboard LEDs, so I'm in the process of trying that idea. Currently, I have no limiting resistors for those orange LEDs. The ULN2803 has it's standalone PS of 9v @ 2.2A. I believe the specs on the chip have a base voltage of ~2v for inputs, so if I cannot generate that, there lies the problem. This is still theory. It cannot be a bum IC, because I've tried a few others already with the same result.

    It was posted earlier that whatever voltage hits the 4017, should be roughly equal to it's outputs. The NE555 I think gets the 9v PS dropped to roughly 5v, so the 4017 output may be close to that BEFORE the LEDs. Doesn't make any sense, but If that's true, I suppose I cannot use the 2803, and must use a few transistors instead. Guidance would be great.

    I attached my setup in a post a little earlier.
     
    Last edited: Dec 16, 2013
  19. ScottWang

    Moderator

    Aug 23, 2012
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    Did you still connecting the 10 LEDs from the output of CD4017 to GND?
    If you want to try the ULN2803 then you better take the LEDs away.
     
  20. mangopuppy

    Thread Starter New Member

    Dec 15, 2013
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    Kind of overlooked that. So driving the 2803 in series AFTER the LEDs and grounding them within the 2803?? Or just removing them....

    Man, I think you're onto something.
     
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