OC FLICKERING DISPLAYS WITH LED OUTPUTS ON

[Dawid Szczeblewski]

Hi All!

I have a problem with open cockpits cards which i used to run the overhead, the more LED outputs are activated on the master card the more the 7 segments displays start to flicker. The flickering starts by the time I have 10 annunciators lit, by the time 11 of them are on the displays flicker really bad and by the time 12 annunciators are on the 7 segments displays turn off completely. Has anyone else encounter the same problem ???

I tried different power supplies, I even have a precision laboratory power supply, nothing made a difference.

Every one of my annunciators has 2 LED's in parallel with a current limiting resistor. Every annunciator draws 15mA which is less than the max output of 20mA the IC's on the master card can handle.


Any input would be greatly appreciated.

Thanks!

[Perik]

Dawid,

Have you measured the +5V on the DisplayII card while this is happening?

[Dawid Szczeblewski]

Thanks for a reply Perik, again

I have not, Need to find the schematic as i dont know which pin on the IDC40 is the 5V feed. Will check that now and report.

[Dawid Szczeblewski]

OK so when no LED (annunciators) are activated the 7 segments display card is getting a nice 4.76V and all is good, the more LED outputs I activate the less voltage appears on the displays card feed, with around 10 annunciators ON the voltage on displays card drops to 4.26V. With all LED's "ON" (38 for J2 connector) the voltage on displays card fluctuates between 0V-1V. At the. same time the voltage on masters card drops to 3.6V. Weird indeed.

Tomorrow I will try running a direct 5V connection to displays card but doubt that it will help as all the power supplies I tried had at least 2A max current output, way more than the cards and LED's would ever use. And the problem here seems to lie with the 5V supply.
And so the OC fault finding journey starts again

[Perik]

Dawid,

Really weird. I hope you feed the Mastercard with 5V at J0.
It's the same +5V which end up at pin 1 on IDC connector. (GND is on pin 2).
Check that the internal +5V trace on Mastercard is OK.

[Dawid Szczeblewski]

Sorry disregard that, the voltage on master card and displays card both drop to 3.62V, it was just my multimeter cable playing up.
I checked the trace and its all ok both for master and displays card.
Wonder if the voltage drop down to 3.62 is what causes the displays card to go funny.
I will test it with computer power supply and separate suply for displays card, see if it makes a difference.

[Dawid Szczeblewski]

Have been busy at the airport all day but had a quick chance for another test but still no joy..
I tried with a couple of decent PC power supplies and the voltage drop there was insignificant at few mV, yet the problem with flickering displays persists... also tested the annunciators again and these only draw exactly 20mV each.

[fordgt40]

Hi

Are you sure that your distribution wire is thick enough and short enough to avoid a voltage drop?

David

[deering]

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Every one of my annunciators has 2 LED's in parallel with a current limiting resistor. Every annunciator draws 15mA which is less than the max output of 20mA the IC's on the master card can handle.
..

20mA is the max on any pin. Package max is 50mA.

[Dawid Szczeblewski]

Hi Deeing

Thanks for your reply
Hmm that would leave only 5.5mA per LED per one IC... sounds very little even for a low power LED plus power dissipated in current limiting resistor, really confused now. My problem persists even when the led's are disconnected, the more outputsactivated the worse is the flicker on 7 segment displays