Having trouble with switch regulator!

Discussion in 'General Electronics Chat' started by danielh85, Mar 9, 2008.

  1. danielh85

    Thread Starter New Member

    Mar 9, 2008
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    Is anyone familiar with the LM2577 step-up voltage regulator? I am trying to make a scrolling LED sign. The regulator is being fed via 74146 shift register. I'm trying to light 20 LED's from the regulator but the LED's are very dim. Have tried a few things but nothing works.
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    Posting a schematic would be helpful.

    hgmjr
     
  3. danielh85

    Thread Starter New Member

    Mar 9, 2008
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  4. hgmjr

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    Jan 28, 2005
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    Can you post the schematic of the circuitry that includes the circuitry in addition to that of the switching regulator?

    What we need to see is the way you have connected up the LEDs and the other components that make up your circuit.

    hgmjr
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    The old 74146 has an output current of about 5mA at about 3V.
    The switching regulator needs an input current of hundreds of mA at 3.5V or more.
     
  6. Papabravo

    Expert

    Feb 24, 2006
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    That's a real kneeslapper -- using a shift register to "feed" a switching regulator is a very vague concept. I think you need to explain what you are doing in much greater detail if you expect any constructive assistance. Just because two things can be connected together does not mean that they should be connected together.
     
  7. danielh85

    Thread Starter New Member

    Mar 9, 2008
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    i am trying to make a scrolling effect with the shift-register but I needed a way to supply enough power to 6 sets of 20 LEDs. sounds like a little much huh? i was unaware that the switch regulator required that much current.
     
  8. hgmjr

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    Jan 28, 2005
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    It sure would help matters if you could post a schematic.

    hgmjr
     
  9. Papabravo

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    Feb 24, 2006
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    Let me be a bit more blunt. You have explained virtually nothing useful about what you are trying to do. With your choice of words you have created the impression, at least im my mind, that the output of the shift register is connected to the input of the switching regulator. This is not a rational thing to do unless you are a grat deal more clever than I give you credit for.

    It is not outside the relm of possibility to design a switching regulator that will have enough output power to supply a string of 20 LEDs. 20 * 10 mA/LED sounds like 200 mA. How much input power do you have available ( Voltage and Current)? At what voltage do you expect your 200 mA to be consumed? This will at least give us a clue as to the switching regulator topology. Next question is what kind of inductor did you choose. The proper inductor makes a HUGE, let me say that again, HUGE difference in the performance of a switching regulator.

    I'm probably wasting my breath until you can manage a schematic and a bill of material.
     
  10. danielh85

    Thread Starter New Member

    Mar 9, 2008
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    Here is a basic schematic of what i am working with. Everything is basically the same except that the my package type is DIP so the pins will be different. And also i am not using the voltage divider network comprised of 17.4k resistor and 2k resistor. I am only using the 2k resistor.

    I tested the regulator part of the circuit with a 555 timer before using the shift register and it worked fine. I am getting approx. 8.5mA @ 3.83V output from shift register, but I am getting 3.5mA @ 4.41 V from 555.

    Also, I am using a 100uH molded axial inductor
     
  11. beenthere

    Retired Moderator

    Apr 20, 2004
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    I don't think you can get that kind of power out of a TTL output. The switched can step up voltage, but those LED's still need on the order of 200 ma. The 74146 can probably do about 16 ma.

    You have not read the LM2577 data sheet with any attention. It has the oscillator built in. You just have to supply it with the external components and a 5 volt source capable of up to 3 amps. A TTL output does not come close to this requirement.

    Also, my TTL manuals do not show a 74146. Is that a good part number?
     
  12. danielh85

    Thread Starter New Member

    Mar 9, 2008
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    I get 105mA to the LEDs from the regulator with the LE555 output to the regulator but not even 1mA output from regulator with shift-register hooked up to the regulator.

    this makes no sense since the shift-register is putting out more current than the 555.

    ohhh, the shift-register is 74164.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    Audioguru nailed it ;)

    A 555 timer (bipolar) can sink or source up to 200mA.

    A 74164 cannot source nor sink even 1/10 of that.

    Your design needs work.
     
  14. Papabravo

    Expert

    Feb 24, 2006
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    What you are doing appears to me to be completely nonsensical. You have not provided a complete schematic. I have no idea where the 74164 VCC pin is connected. I have no idea what you think the switching regulator is supposed to do with the output from a TTL shift register. You must have got the idea to eliminate the one of the resistors in the feedback network from somewhere, but I'm pretty sure the part won't do anything remotely useful without it.
     
  15. danielh85

    Thread Starter New Member

    Mar 9, 2008
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    Have you an idea of what part of the circuit could I supplement but still get the same result that I need.

    What I am trying to do with the LEDs is spell out a word with one of the 20 LED strings representing one letter of the word. Then I want the word to scroll one letter at a time in sequence, then shut off in sequence. The first thing that came to mind was a shift-register, but apparently that will not work.:eek:

    Is there another shift-register that can output more current?
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    You need to use an interface IC, such as a ULN2x03 or ULN2x04. The x03's are used for TTL-level signals, x04's for CMOS. x=0 is Seven Darlington pairs, x=8 is Eight Darlington Pairs. Keep your switching boost regulator powered from your Vcc/Vdd, and use the ULN2x0n to sink the current from the cathodes of your LEDs. You must use either current regulation, or current-limiting resistors to supply your LEDs.

    You really shouldn't try to source the switching supply from anything but your power supply - which I'm assuming is quite low in voltage.
     
  17. danielh85

    Thread Starter New Member

    Mar 9, 2008
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    According to the data-sheet of the ULN200x, the shift-register should output enough current to drive the ULN200x ?
     
  18. danielh85

    Thread Starter New Member

    Mar 9, 2008
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    According to the data-sheet of the ULN200x, the shift-register should output enough current to drive the ULN200x ?

    So I should use the boost regulator as the source and the ULN200x as the sink?
    That sounds right!
     
  19. SgtWookie

    Expert

    Jul 17, 2007
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    Absolutely - you're understanding how to do this much better now. ;)

    Look at the ULN2804 - that should do the trick for you.
     
  20. danielh85

    Thread Starter New Member

    Mar 9, 2008
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    Thanks a lot. Appreciate the help.

    Had a moment there. Don't know what I was thinking trying to supply the boost regulator from shift-register!:eek:

    Another few questions. I supplied an attachment schematic photo of the boost regulator circuit. Should the 100uH inductor be a toroid type?
     
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