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I am Nikhil from India, working as an embedded programmer. I made half inch 7segment display based digital clock. But after few months few 7segment displays got damaged. I got same complaints in many boards. Please help me to debug the error.

5v dc power supply is used.Pic16f877a is used for controlling.74hc164 is the shift register ic used.

Bd140 transistor is used for on/off 7segment display.

5v is connected to the emitter of bd140.

Base is connected to pic microcontroller via resistor.Collector is connected to common anode of 7segment display.Cathode pins are connected to 74hc164 via current limiting resistors.

I think this is the recommended connecting method. But around 1/10th of the total segments used are getting damaged within few months.

I used 560e as current limiting resistor and increased its value upto 2k2, but no change only brightness is getting down.

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  • \$\begingroup\$ Are you sure that the damage isn't mechanical, rather than electrical? Perhaps you have bad solder joints or poor quality PCBs whose traces are cracking. \$\endgroup\$
    – Dave Tweed
    Jan 19, 2016 at 12:32
  • \$\begingroup\$ I already used the same design for 1inch 7segment display, and its works fine. \$\endgroup\$
    – Nikhil
    Jan 19, 2016 at 12:36
  • \$\begingroup\$ Whatever current limiting resistors used, the forward voltage drop of 7segment is above 2v. So for reducing the forward voltage drop, i am redesigning my circuit which uses 3v3 power supply for 7segment operation and 5v for pic microcontroller. I am not sure how much successful it will be. \$\endgroup\$
    – Nikhil
    Jan 19, 2016 at 12:41
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    \$\begingroup\$ Is it the same brand/model of 7-segment in each case? Circuit schematic and 7-segment datasheets would be useful. \$\endgroup\$
    – CharlieHanson
    Jan 19, 2016 at 12:41
  • \$\begingroup\$ Have you seen any patterns in the damaged segments? \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Jan 19, 2016 at 12:45

1 Answer 1

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Years ago one of my designs once experienced similar unexplained failures- it turned out that the assembly house was using resistor packs marked 470 which were actually 47\$\Omega\$! The excess current caused segment failures, especially in hot industrial environments. Naturally this was also a wake-up call for their inspection and testing.

Failing the above, I would suggest bad quality displays, but that's fairly rare.

Edit: If they are failing just as often with 2.2K resistors (and your PCBs are not faulty, clean, dry and uncontaminated) then the displays are faulty, no way around it. Make sure you're getting first quality parts. One possibility is that the internal double-sided PCBs have faulty plated through holes which are cracking at the edges. I've seen that in bare PCBs when the PCB manufacturer has poor control over their process. The display manufacturer (many of which in China are smallish companies) would have known this (high fallout rate) and may have shipped the parts anyway to a customer who demanded the very lowest price (your supplier?). Just a thought.

Edit2: Since you say you've tried several brands, then we're kind of left with some problem with your PCBs or the environment they are in. LEDs are not very fragile electrically and can withstand a lot of short-term abuse without failure, and your descriptions do not indicate anything remotely dubious electrically.

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  • \$\begingroup\$ @PeterJ Yes, might not be, but I've done enough field service to know how much to weight such comments. My favorite was the instrument that "just stopped working". The fact it had a fork-lift tine run halfway through the housing wasn't mentioned. \$\endgroup\$
    – Spehro Pefhany
    Jan 19, 2016 at 12:47
  • \$\begingroup\$ I used 2k2 resistor of carbon film type and metal film type, but result is same. Theoretically 2k2 resistor is too high for a 7segment display to work in 5v supply rails. Also working environment is not industrial, just normal room temperature. \$\endgroup\$
    – Nikhil
    Jan 19, 2016 at 12:48
  • \$\begingroup\$ @Nikhil Please see my edit above, expanding on my previous last point. No resistor is really "too high" (just too dim maybe) but 2.2K is really conservative. \$\endgroup\$
    – Spehro Pefhany
    Jan 19, 2016 at 12:53
  • \$\begingroup\$ Display failure means its not completely off. Some segments will glow dimmer compared to other segments. And some segments will glow even if its common anode pin is low. \$\endgroup\$
    – Nikhil
    Jan 19, 2016 at 12:55
  • \$\begingroup\$ @Nikhil Could also be something to do with contamination or wire bonding. Anyway, it's not our business to figure out the problems for the LED guys. Have you actually examined the instruments for failure analysis to ensure they're not getting wet or something silly like that? I've also seen similar weird failures when contaminated water is run over PCBs. The traces get eaten away and conductive residue is deposited. \$\endgroup\$
    – Spehro Pefhany
    Jan 19, 2016 at 12:58

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