Over Voltage Protection

ESPHome
1

I’m trying to protect my ESP32 control panel from accidental over voltage. It is being powered by a POE splitter that has switchable voltage output 5V, 9V, 12V, and 24V. Seems the easiest solution is a zener diode and a resistor but I’m confused at what to use.

This explains it but I’m not sure I have my calculations correct: Over voltage with Zener

Voltage is: 5.6V
Current is: 690mA

5.6V x 0.7 = 3.9 Watts
So, I would need a 1N4734A 4 Watt zener?

For the resistor:
24 - 5.6 = 18.4/.7=26R resistor
Does this also mean it needs to be a 4 watt resistor?

2

I didn’t study your setup, but what about LDO that can handle those voltages?

3

As this is for accidental over-voltages you could use a crowbar circuit instead of a simple zener regulator.

https://en.wikibooks.org/wiki/Practical_Electronics/Crowbar_circuit

4

Do you want it to be able to operate on higher voltages? If so you would want to use a switching regulator like the Oki 7805SR. It will reduce the higher voltage (won’t be able to use the 5V setting) to 5V safely and with little additional power consumption.

If you just want protection, the best option is an over voltage protection IC (aka e-fuse) which will disconnect when the voltage exceeds the threshold.

The Zener and resistor idea is outdated, and has issues with power dissipation, as you have discovered.

5

Do I only need to adjust the zener and thyristor values in this diagram? Also, it appears the schottky can be omitted. Please confirm.

6

I just need protection. Can you link an example?

7

Adjust F1 for 1.5x you normal max load current.
Adjust Z1 for the voltage you want it to trip at.
Select Q1 for the rated short circuit current for the time it takes F1 to blow.
You may need to adjust R1 depending on Q1’s trigger voltage.

8

You can use Google to search the net given that you know the keywords.

Here’s one I found:

https://www.aliexpress.com/item/1005001855053619.html

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9

Just to confirm, Q1 thyristor is an SCR, correct?

10

Correct, but honestly I’d go with Clyde’s suggestion. Cheaper and easier.

11

Personally I’d just use a switching regulator - then it just works. And there are more of those to choose from than OVP chips.

12

I use This in many of my projects. It’s small, it’s switching, you can set desired output voltage. Since you say you have POE i guess your module is (far) away. In this case it’s better to power you module with higher voltage, not with 5V, because using higher voltages lowers power losses. Also, if wire is long you’ll have 5V at the beginning, but who knows how much at the end (depending on wire length and module consumption)…

13

It took some doing, but I was able to get a test setup working well. I used a 0.5A fuse, 0.8A thyristor, and a 5.6V zener. It reliably triggered at 6.4V which I suppose is safe for my 5V test rig, or I could move down to a 5.1 volt zener. Either way should work well.

My question is how to increase the current rating for my panel. My total current using a 1.5 factor is 3A. I am thinking of using a 4A thyristor and a 3A fuse. Does this sound right?

14

Here is my final design. It functions well, though I did have to reduce the fuse to 1A because the original 3A fuse wouldn’t blow as intended and instead resulted in a short.

IMG_2938 - Copy - Copy
IMG_2938 - Copy - Copy1920×2560 284 KB

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