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

[TimewaveZero]

Great input everyone! Much appreciated.

@anon35827816

that does mean the leds flicker at 120Hz (or 100Hz in Europe). For traffic lights I would consider it undesirable as it can be very distracting in people's peripheral vision.

Ah yes, I see what you mean there. That could be undesirable indeed but for it's new life as a strictly decorative item it's not of concern.

@JohnRob

I'm assuming Red, Yellow & Green are not on at the same time.

You're correct. Only one color/aspect lights at any given time (it's an old Benelux type of traffic light):

• Red: stop
• Yellow: caution
• Green: pedal to the metal

@chrisknightley

Predicting that the 48VAC of the OP's circuit will come from a transformer connected to commercial power, its frequency will be significantly lower than that of a switch mode power supply.
If you design an switch mode power supply by assume that you are switching at such frequencies, the size of the capacitors will be quite large.

Yes this traffic light was operated from a transformer connected to (I believe it was) a regular 220VAC street line (Europe). After I'm done rewiring this thing it will ideally be powered from a 24V or 30V switching power supply hooked up to a regular 220VAC household plug. 24V I'm still debating since the green LEDs already draw 23.7V per 8 LED string. So that might be too close for comfort?

@Grumpy_Mike

I think the thing you are all forgetting is that the safety requirement for UL, and other codes, is that the failure of any one component, open or short, should not affect the safety of the circuit. This is why the current limiting resistors are split into two , and there are two resistors in series with the diodes in the bridge diodes.

Good thinking! I wasn't aware of that. I just have a lot more soldering to do it looks like.