Advice needed - multiplexing 7 segment displays.


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  1. #1
    Senior Member Mellbreak's Avatar
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    Default Advice needed - multiplexing 7 segment displays.

    I'm trying to build a 5-digit 7-segment display. The LEDs I'm using are 2.3" (approx 6cm) high type FJS23101AH. Unfortunately, I can't find a datasheet on these devices although I know that each segment requires 7.5v at 15-20mA to drive it.

    Not being an EE, I'm not sure what current I should drive the segments at if they are being multiplexed. Given that there are five digits is it safe to drive the segments at 75mA for 1/5 of the time?

    Thanks,

    Bob

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    Member GAMBOA's Avatar
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    Default Re: Advice needed - multiplexing 7 segment displays.

    Bob,

    you need to know the peak current that LED diodes can withstand. In a design I did a few years ago I used discrete diodes and the peak current that the diodes supported was 100mA. In my design I adjusted a maximum current to 80mA. It was also a multiplexed system. The calculation you have made is correct. If you lower the excitation time you have to proportionally raise the intensity but only up to the maximum peak current that the LED supports.

    Regards,
    Gamboa
    Last edited by GAMBOA; 28th December 2018 at 11:21.

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    Senior Member Mellbreak's Avatar
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    Default Re: Advice needed - multiplexing 7 segment displays.

    Thanks Gamboa, that's very helpful. I'll have to see if I can find a value for the peak current from somewhere.


    Bob

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    Senior Member Mellbreak's Avatar
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    Default Re: Advice needed - multiplexing 7 segment displays.

    Having had a good look around the Internet, I still haven't found any information on the peak segment current for this display. In general terms the peak current seems to be about 5 times the average current. Would this be a reasonable assumption to make?

    Bob

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    Member GAMBOA's Avatar
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    Default Re: Advice needed - multiplexing 7 segment displays.

    Bob,

    I have used the TOS-40101AMRL-B (Taiwan Oasis) and the technical data in the data sheet says:

    Peak Forward Current per segment: 80mA
    Continuous Forward Current per segment: 20mA

    Regards,
    Gamboa

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    Default Re: Advice needed - multiplexing 7 segment displays.

    Thanks again Gamboa, I'll start by using something around 60mA and see how that goes.

    Bob

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    Fanatical Contributor fanie's Avatar
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    Default Re: Advice needed - multiplexing 7 segment displays.

    The very first program I wrote was a 3 digit multiplexed display, and I'm still using the same today for a basic volt and amp meter. There however is a problem, and that is it limits the pic to do other things which will disrupt the timing.
    Hence, I use some 4094's to drive the 7 segments and the pic only write to the display when it is suitable which can be any amount of segments, (I tested up to around 30 x 4 digit displays which changed the 5V regulator to double as a finger toaster).
    I have found this to be a lot more practical and am using it in almost all projects requiring a display. It just makes life so much easier. My displays are 4 digit each but can be added as many as the supply can handle. Another advantage is you use only 3 output pins of the pic for clock, data and strobe, a small pic can drive a lot of displays without using more pins, and if you are out of pins, you can even share the clock and data pins.
    The software is also easy, I use the same call routine, just change the amount of digits to write to.
    Last edited by fanie; 7th January 2019 at 23:37.
    Fanie

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    Default Re: Advice needed - multiplexing 7 segment displays.

    Hi, i have build a few displays using HC 595.https://www.google.com/url?sa=t&sour...fS84xBi0Wr_0D3
    Works great! I you want I can share my programs on the forum.

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    Default Re: Advice needed - multiplexing 7 segment displays.

    That's it, the same functionality. The 4094 is a bit cheaper here than the 74 595 and unfortunately they are not pin compatible.
    Fanie

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    Prolific Poster joesaliba's Avatar
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    Default Re: Advice needed - multiplexing 7 segment displays.

    A few years ago I took Fanie circuit diagram and made a PCB for it. If someone needs it I can share EAGLE files.
    Regards

    Joseph

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