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Voltage Levels

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Lord_Nikon, Mar 7, 2018.

  1. Lord_Nikon

    Lord_Nikon New Member

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    So, I decided to start an 8x8 LED matrix project using a MAX7219 and PIC18F4550. My issue I've run into is that my the LED matrix I found is a max 2.7 forward voltage drop. The MAX7219 outputs around V+ -1.The minimum amount for V+ is 4V so this will put me near the 2.7V maximum of the matrix (4V -1 = 3V) My question is, is this an okay design? or should I just buy an LED matrix that can tolerate higher voltages.

    Also, what is a good way to generate 4V? I was thinking of just using an LM7805 and putting a voltage divider to create 4V . Are there better ways? or is this a decent design?

    Thanks in advanced!

    https://datasheets.maximintegrated.com/en/ds/MAX7219-MAX7221.pdf
     
  2. crutschow

    crutschow Well-Known Member Most Helpful Member

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    The MAX7219 can operate at up to 5.5V, so I see no problem operating it at 5V. That should give you plenty of overhead for your LEDs.
    Why did you want to use 4V?

    So just use the LM7805 to provide 5V for its power.
    (Note: You never want to use a voltage divider to generate a lower voltage if the load draws any significant current.)
    Offhand, it looks like an okay design to me.
     
  3. Lord_Nikon

    Lord_Nikon New Member

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    Thanks for the reply! The reason I want to use 4V is that the matrix I bought has a max rating of 2.7V. I tested it with 5 and that blew the LEDS. So my thought is that if I supply the PIC and MAX7219 with 4V then the output of the max7219 to drive the LEDs will be roughly 3V. The highest I could get the LEDs on without blowing is about 3.5V. I should have checked the voltage rating before buying all my parts. Noob mistake.

    Also why isn't it a good idea to use a voltage divider to create a lower voltage? Just curious.
     
  4. dave miyares

    Dave New Member

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  5. JonSea

    JonSea Well-Known Member

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    You have a few issues of understanding.

    First, the MAX7219 is a constant current LED driver. As the first paragraph of the data sheet says, segment current is set by a single resistor for all LED segments. You will probably want to select a resistor value to provide about 10mA.

    Second, what you are calling "maximum LED voltage" is the LED's forward voltage. When you provide 10 or 20 mA current to the LED, the voltage across it will equal tge forward voltage.

    Look up a discussion on setting LED resistors to understand why the forward voltage is important and where it comes into play.
     
    Last edited: Mar 7, 2018
  6. JonSea

    JonSea Well-Known Member

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    Ok, 3 areas of misunderstanding!

    You can't connect a voltage source directly across an LED. An LED must have a series resistor wgen operating from a fixed voltage source. The resistor value is calculated to allow 10 - 20 mA to flow. With a 5 volt source, the series resistor should be at least 331 ohms, but anything higher is ok...it just makes the LED dimmer.

    Note: the series resistor when operating from a voltage source is completely different than the resistor used to set the current with the MAX7219.
     
  7. JonSea

    JonSea Well-Known Member

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  8. dave miyares

    Dave New Member

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  9. Lord_Nikon

    Lord_Nikon New Member

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    Just watched a 3 minute video. Thank you! I understand now.
     
  10. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Because the voltage will vary with the load attached to the divider, due to the resistance of the divider.
    You always want to use a voltage regulator for a load of any significant current.
     

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