Hi all, I'm working to bring an old LED traffic light back to life and make it fully functional again. But I have a question regarding the schematics because some things have to be modified and don't want to make a mistake. This traffic light natively runs on 48VAC. Each individual aspect (red, ye…

[image] TimewaveZero: Surge/over-voltage protection (so the LED string is never exposed to a higher voltage than the Zener is rated for). Probably, but it depends on the actual Zener rating.

That is weird because the reverse breakdown voltage works similarly to the forward voltage of the LEDs. We don't seem to know the forward LED voltage or the Zener voltage... My only guess is that it helps to divert some current during the voltage peak (about 66V).

I agree with @jremington @DVDdoug . The Zener diode maybe to prevent voltage spikes and protect the LEDs because Zeners start to conduct current in reverse direction when the reverse breakdown voltage is reached. The 2nd explanation you provided also seems appropriate to me. I had a suggestion: Yo…

[image] TimewaveZero: Surge/over-voltage protection (so the LED string is never exposed to a higher voltage than the Zener is rated for). Probably this. It's a rather crude way of powering leds - but it works. [image] Devanshu_Acharya: You require a Capacitor in parallel with a Full bri…

Thank you all @DVDdoug @jremington @Devanshu_Acharya for your thoughts! Much appreciated. I just saw that the pic I uploaded in my first post was not the final version. Pardon me. Here is the circuit with all the data (the Zener circuit is unchanged): [TL_zener_data] The Zener voltage is in all …

[image] TimewaveZero: But how about explanation 1 and 2? I think you got those covered already, more or less. [image] TimewaveZero: Reverse polarity protection (but why, since a rectifier is already present in the circuit?); There you go; there's no reverse polarity risk due to the br…

#1 makes little sense. #2 is possible. Ask the designer what they were thinking.

It is a clever way to get better utilization of the LEDs: Without the Zener you must size the resistors to limit the LED current in the worst case conditions (tolerance of resistors, transformer ratio, mains voltage). With the Zener you can use sightly smaller resistors and in case there is too muc…

Thank you all! Makes sense indeed. [image] anon35827816: Depends on how you define 'good'. If the leds need to be flicker-free in this application, you'd have a point. They don't, apparently, so the cap is completely unnecessary and only adds costs and bulk. Flickering is not an issue as the…

Another thing that surprises me from my measurements is that the 32 yellow LEDs draw more than 2.5 times as much current (58.2mA, 2.37W) when compared to red and green ones (each ~22mA, 1W). I was expecting all 3 colors to be in the same ballpark as they are all of the same 5mm clear LED type in equ…

What is the role of this Zener diode in a LED string? (Old traffic light restoration)

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jremington September 10, 2021, 2:24pm 2

Probably, but it depends on the actual Zener rating.

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What is the role of this Zener diode in a LED string? (Old traffic light restoration)

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