How To Test Large 7-Segment Display

Discussion in 'General Electronics Chat' started by elec_mech, Sep 25, 2011.

  1. elec_mech

    Thread Starter Senior Member

    Nov 12, 2008
    1,513
    193
    Hello All,

    I recently purchased a few large 7-segment displays from Futurelec:

    4": http://www.futurlec.com/LED/7SR40011ES.shtml

    5": http://www.futurlec.com/LED/7SR50011AS.shtml

    I believe this is the manufacturer: http://www.flyingled.com/pid10476843/NFD-50011ABx.htm, but they have even less information on the display than Futurelec.

    I got the 4" to work, no problem except the pinouts don't match.

    I cannot get the 5" to work at all. If the drawing is correct, it uses 4 times the number of LEDs (relative to the 4") to make up a segment, two columns of 10 in parallel.

    The 4" seems to pull 7VDC at 25mA, not the stated 9.25VDC. So, while limiting the current to 20mA, I've tried applying 9, 11, and 14VDC to every pin in the hopes of lighting a segment to pin it out correctly and verify input voltage. Unfortunately, no dice.

    I can try higher voltages, but I don't want to cook the display (by reverse-voltage), especially since I don't trust the pinout given in the diagram.

    Any suggestions?
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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    513
    Have you confirmed the diode forward and reverse directions?

    You should be able to do this with a current meter, limiting resistor and 10V supply.
     
  3. elec_mech

    Thread Starter Senior Member

    Nov 12, 2008
    1,513
    193
    Hi Studiot,

    No, that is part of the problem.

    How would I determine diode direction using these items?

    Put the 10VDC supply through the current meter and a limiting resistor and then what?

    Can elminate the resistor by setting my supply current limit to 20mA?

    Thank you!
     
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
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    513
    No you shouldn't use the power supply in current mode.

    A voltage source, a high value resistor and a current meter in series form a simple ohm meter.

    Connect the setup one way round and the meter should show reasonable current flow.

    Connect in the reverse direction it should show negligable flow.

    Just like testing any diode with a multimeter! on a resistance range!
    However I said 10V to overcome the forward voltage of the diode stacks making up each bar.
     
  5. elec_mech

    Thread Starter Senior Member

    Nov 12, 2008
    1,513
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    Ah, thank you. I ended up limiting the current on my supply to about 20mA and tested at 10VDC. Only by the fact that the comma and decimal point had less LEDs and thus less voltage required did I confirm the datasheet drawing matched once I learned the proper pin number orientation.

    So I found the ground pin and upped the voltage on the remaining segments (which showed no current flow at 10VDC) until something lit up. Turns out the comma and decimal take around 9VDC as the datasheet states, but the remaining segments require 17.6VDC at 20mA! :eek:

    Since this is for a 12VDC application, I'll save these for something else. Thank you for the help!
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
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    513
    You need to be careful using bench power supplies in this way.

    Depending upon the exact nature of the power supply some supplies will destroy circuitry by raising the output voltage to max in an attempt to drive 20 mA through if set to that current value.

    You cannot use both current mode and voltage mode simultaneously.

    Other units are much simpler and do not have a current mode as such they just have an adjustable current limit and will maintain the output voltage at a set value until the load is such as to attempt to draw current above this limit.
    This type is safe to use as you are doing.

    go well
     
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