Using Zener diode as over voltage protection for LED

I following that circuit:


I use an 220 ohm resistor before the LED (not the fuse) and ties the zener diode like the schematic. I use 3V Zener diode.

My goal is to prevent higher voltage than 3V going to LED when i turn up my power Supply over that. But i can't see that the zener does anything. The led gets brighter and brighter when i increase the voltage over 3V.

What am i doing wrong?

Put a resistor between the fuse and the zener.
Even if the zener wants to pull current to manage the voltage, there is no component making the voltage drop.

I have tried that now. Do you mean like that?

This circuit does not work. When i increase voltage on my Power Supply the led just get brighter when i feed it with 3v++

resistor to Zener ,

resistor from zener to LED

I tried that now. I does'nt work. When i increase voltage input over 3V the led just gets brighter.

The first drawing (with the fuse replaced by the current limiting resistor) should have worked. Could you have the Zener backwards? Have you measured the voltage across the Zener? It could be the Zener has failed.

Yes, i can't see what i`m doing wrong. I have tried two zener diodes (3V) and it doesent work.

When i increase voltage ober 3V i see the led bee brighter and if i turn voltage high enough, the led is damage.

If i meassure on each side on zener diode i get 3.2V.

Just to verify, this is your original circuit:


The issue is the 3V zener is trying to hold the input voltage to 3V which it is not capable of.

Consider a car where the driver is holding the gas pedal to the floor (similar to the input voltage) and they have the brakes on (similar to the zener diode).

The engine will overpower the brakes, and eventually the brakes fail (i.e. zener fails)

When i increase voltage ober 3V i see the led bee brighter and if i turn voltage high enough, the led is damage.

Question1: What input voltage is causing the LED to fail?

Question2: Is your goal simply to stop the LED from flashing bright when the power supply comes on? Or are you trying to keep the LED brightness constant.

Question3: What is the range of you expected input voltage?

I think you may need an active regulator instead of a zener.

Unless you have a VERY BIG zener the fuse would not protect it and it would open-circuit. So your zeners are likely broken.

Full explanation here of how to use an LED.

Mostly we dont need an LED to be a specific brightness. So a suitable resistor to match the supply voltage range is all you need - you DONT need to voltage stabilise it.

If the voltage range is too big you will need a voltage regulator.

  1. around 20-25V and the led begins to flicker before it dies.

  2. this is just an test circuit for testing how this works before i implement it in a real circuit. In the real circuit i want to prvevent higher voltage than 24V in to an buck boost converter that has max rating for 24v input.

  3. So my range will be 24V.

A zener diode alone is very unsuitable for protecting the input to something that will draw as much current as a buck boost converter.
If blowing the fuse when the voltage goes above a certain amount is what you want, then look at the crowbar circuit.

What is your goal? If you want the LED to have the same brightness for say 5 V to 24 V the Zener will need to dissipate LOT of power (and the current limiting resistor even more). There are better ways. Such as a linear regulator.

If you want to protect the LED, use a current limiter like an LM317. Set it for whatever current makes your LED bright enough. Then you don't have to worry about the input voltage.

3V zeners are crap anyway. They leak so badly that they won't hold anything near a 3V level; it'll be all over the place in reality.

But let's face it, trying to limit the voltage over an led which is an element that you'd typically current limit (not voltage limit) is kind of silly to begin with. Try a fundamentally different approach instead.

Looking at reply #10, I do not think the OP intends to use this to control an LED in the final circuit, but instead is testing the zener as a means of limiting overvoltage into a buck boost converter, something for which a zener is totally inadequate since it would need to carry more current than the buck boost convert at full load in order to blow the fuse.

Totally inappropriate approach. :astonished:

What is your highest input voltage?

Use a converter rated for this voltage. :+1:

What are you actually needing to do in your "real circuit"?

I think OP is thinking about this topic's circuit...

Yes, thats right.

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