problem with kemo electronic seven segment displays

Discussion in 'General Electronics Chat' started by Dawud Beale, Dec 11, 2012.

  1. Dawud Beale

    Thread Starter Member

    Feb 10, 2012
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    I just purchased this:

    http://www.kemo-electronic.de/en/Components/Elements/LCD/S043-LED-LCD-Displays.php

    The product came with the following "datasheet":

    http://www.kemo-electronic.de/datasheets/s043.pdf

    I cant see the LED forward current or the LED forward voltage drop on there and I need to use the type D LED which has no part number so I cant find a datasheet elsewhere.

    Doesnt this make using the LED 7 segment display near impossible? How would i decide what limiting resistor to use etc without that information?
     
  2. tshuck

    Well-Known Member

    Oct 18, 2012
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    You can measure the forward voltage of the LEDs using a multimeter set to diode test.

    Also, if your DMM doesn't have a diode test, you really should get a new one, you can put a 1K resistor between one LED and the supply(assuming 5V). Then you can measure the forward voltage there.

    If you are too lazy, you can assume 1.7V forward voltage(provided they aren't high power LEDs) and calculate for that.

    Most LEDs can handle 20-30mA without bursting into flames, so this is a decent current to shoot for, just remember to use a margin of safety...
     
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  3. Dawud Beale

    Thread Starter Member

    Feb 10, 2012
    275
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    excellent info, thanks very much.

    any basic tutorials available on testing an LED, wiring it, calculating the voltage source value, resistor value etc?

    I need a very basic tutorial on 7 segment displays but Im searching through youtube, google, here etc and I keep getting arduino and PIC projects which Im not read for yet.

    I'll have a look at my multimeter when i get home, i got a fairly expsensive one from MAPLINS so Ill check if it has those capabilities

    Thanks for the response
     
  4. tshuck

    Well-Known Member

    Oct 18, 2012
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    The book on this site is pretty useful...
     
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  5. Dawud Beale

    Thread Starter Member

    Feb 10, 2012
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    Ah lovely thanks but is there anything for how to wire up an LED to a battery or voltage source and just test each segment, calculate the required voltage source and resistor, just a basic intro to seven segment LED's that explains the basics?
     
    Last edited: Dec 11, 2012
  6. takao21203

    Distinguished Member

    Apr 28, 2012
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    10 to 20 mA work for most if not all smaller 7 Segs.
     
  7. tshuck

    Well-Known Member

    Oct 18, 2012
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    not to my knowledge, though I'm sure something exists...

    This may help you understand more about them.

    Usually, all of the LEDs are pretty much the same in a seven-segment display. If you want to test each LED, though, you can. It looks like the people that made you "datasheet" skimped on a lot, they didn't even label anode or cathode!

    If you connect the cathode and anode incorrectly, you shouldn't have a problem as the LED will be reversed biased, not allowing current to flow through it(unless you are using a fairly high voltage. I'm assuming you are using 5V or less...). You should try to connect the anode to +5V and attach the cathode to ground through a 1kΩ resistor.

    What this means for you, to test 'a', is that you should connect pin 3 and/or pin 8 to +5V and pin 7 through a 1kΩ resistor. You should see 'a' light up. If not, then it may be a common cathode device, meaning you'll need to connect pin 3 and/or pin 8 to ground and pin 7 to +5V through a 1kΩ resistor. Once the LED is lit, measure the forward voltage of the diode. Then you can calculate the resistance needed to get your desired amperage...
     
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  8. Dawud Beale

    Thread Starter Member

    Feb 10, 2012
    275
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    Ok great thanks Ill give it a go when I get home and see what happens. That "datasheet" can't even be called a real datasheet, not sure what they were thinking. All the other parts have serial numbers so at least you can search for a real datasheet but the one i need doesnt even have a serial number which is weird.

    Btw regarding anodes and cathodes, as pins 3 and 8 are common annodes I assumed the rest of the pins were common cathodes, is this not correct?
     
  9. tshuck

    Well-Known Member

    Oct 18, 2012
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    I put that disclaimer because the datasheet says A(C) which, to me, means that it may be a common anode or a common cathode pin.

    The fact that there are other pins to the LEDs, means that they can't be common anything... There will usually be a common cathode, to ground, or common anode, to Vdd. This reduces the number of pins you need to hookup the device....
     
  10. Dawud Beale

    Thread Starter Member

    Feb 10, 2012
    275
    7
    ahh I see. Well anyways I'll try what you mentioned and see where I get to from there.

    Thanks for all your advice, help and resources, really useful.

    If yourself or anyone else has additional resources or advice please feel free to add it in.
     
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