Tech Tip #1 : Displays flickering …

In order to start the series, I want to discuss a problem which was really annoying and unpleasant, but found an easy solution.

In my cockpit, All the radios in the pedestal (NAV 1 & 2, COM 1 & 2 and ADF 1 & 2) are made with 7segments displays, operated by IOCARD display cards. All radios have active and standby displays, and it’s a total of 64 digits, requiring 4 display cards in a stack, connected by the same cable to a MasterCard, all located in the pedestal.

Although it basically worked nicely, I had a recurrent and annoying problem : very often, all displays would flicker or blink, which was very unpleasant. I first thought that I had poor wiring or bad contacts, but I soon found out that it was not the case. Then what ? I was plagued by the problem until I had the idea of measuring the supply voltage at the display level. I found 4.6V although the supply unit was delivering 4.95V, very close to the nominal 5V expected. The problem in fact is that all the digits call for a non negligible current, which is not readily measured, because it is pulsed. This in turn resulted in a voltage drop in my wiring, because the wire section was not enough (my 5V cable having a typical length of 3m). At 4.6V, the electronic circuitry in the display card does not perform correctly. I found that using bigger wires solved the problem, the voltage near the display was 4.9V and no flickering anymore !

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