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2019/05/26 17:29:47 (permalink)
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Increase LED Brightness

The schematic shows one row of my 16(row)x32(Col) LED matrix. Each row has 32 LEDs. I'm Scanning rows @ 1/16th rate.I'm using a PIC micro controller. I use 3V LEDs. The problem is LEDs are dim.
How to overcome my problem? I'm about to draw my new PCB, so changes are welcome. 

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#1

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    qhb
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    Re: Increase LED Brightness 2019/05/26 17:40:46 (permalink)
    +2 (2)
    Throw away the ULN2803 and use individual transistors instead.
    (The Darlington configuration is wasting 0.7V that you can't afford.)
    Better still, use FETs rather than bipolar transistors.
     
    How many LEDs are you trying to light at once?
    If it's all 32, if you tried to get 16mA peak through each one (for an average of 1mA), then you'd have 512mA going through the ULN2803.
    The ULN2803 says you will have a 1.5V drop at the current. Subtract the 3V of the LED, and maybe 0.2V across the upper transistor, and you're left with 5 - 0.2 - 3 - 1.5 = 0.3V across the 47R resistor, which is only 6.5mA.
    (That means the ULN drop will be < 1.5V because the current s less than expected, you can do a few calculations to find the actual figure)
    Even with only one LED on, the ULN will drop about 0.75V at 16 mA, => 1.05V across the resistor => 22mA
     
    So bottom line is, you need to massively reduce the size of the series resistor, AND reduce the voltage drop across the bottom transistor to get any reasonable current through the LEDs.
     
    post edited by qhb - 2019/05/26 17:54:57

    Nearly there...
    #2
    InvalidApple
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    Re: Increase LED Brightness 2019/05/26 20:17:24 (permalink)
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    PStechPaul
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    Re: Increase LED Brightness 2019/05/27 00:05:37 (permalink)
    +1 (1)
    A dedicated display driver IC is probably the best solution for this issue. One problem I found with multiplexing 8 digit LED displays was that when processing stopped for some reason (usually debugging), one of the digits would see the full current instead of a 1/8 duty cycle. I attempted to reduce this effect by adding capacitors so that the digit driver would only supply a high current pulse, and then a limited current if the multiplexing was disabled. There are many good display drivers, and I think they are essentially necessary when trying to multiplex more than 8 digits.

     
    #4
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    Re: Increase LED Brightness 2019/05/27 17:07:34 (permalink)
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    qhb
    Throw away the ULN2803 and use individual transistors instead.
    (The Darlington configuration is wasting 0.7V that you can't afford.)
    Better still, use FETs rather than bipolar transistors.
     
    How many LEDs are you trying to light at once?
    If it's all 32, if you tried to get 16mA peak through each one (for an average of 1mA), then you'd have 512mA going through the ULN2803.
    The ULN2803 says you will have a 1.5V drop at the current. Subtract the 3V of the LED, and maybe 0.2V across the upper transistor, and you're left with 5 - 0.2 - 3 - 1.5 = 0.3V across the 47R resistor, which is only 6.5mA.
    (That means the ULN drop will be < 1.5V because the current s less than expected, you can do a few calculations to find the actual figure)
    Even with only one LED on, the ULN will drop about 0.75V at 16 mA, => 1.05V across the resistor => 22mA
     
    So bottom line is, you need to massively reduce the size of the series resistor, AND reduce the voltage drop across the bottom transistor to get any reasonable current through the LEDs.



    Excellant Description.Everything is clear.Eventhough the BJT solution consists with more parts (Base resistor + 2N3904 & the PCB space) the performance is very satisfied than ULNs.Doesn't matter customer complaints that it takes lots of time to solder PCBs' :)
     
    Thank you very much as always.
    #5
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    Re: Increase LED Brightness 2019/05/27 17:08:50 (permalink)
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    Dedicated driver chips not available in my area & I'm aware of SMD parts.I need to stick with commonly available parts.
     
    #6
    acharnley
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    Re: Increase LED Brightness 2019/05/28 06:28:42 (permalink)
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    P-channel fet on the top, get rid of the lower darlingtons and configure the PIC ports for open drain? The newer PIC16/18's can do 50mA per pin so might be able to do the 32 LED's without maxing out the absolute current draw/sink (check datasheet).
    #7
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