Electronics > Beginners

Add overvoltage protection to save voltage regulator

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JoeyG:

Whales:
Polyfuses (self-resetting fuses) are a cheap improvement for the fuse + reverse-diode route.  They only suit some situations but might be OK here.

golden_labels:
Let’s take an approach known from security: define your threat model.

Overvoltage is the mechanism, through which damage occurs. But why do you think there will be an overvoltage condition in the first place? How is it going to happen? Any specific cases? Why do you think it’s worth including protection for a regulator worth a dollar? Protection, which itself is sacrificial or has limited capabilities. Why do you find it so important to protect against injecting higher voltage at this specific point of the circuit, but not any other? What is the expected range of voltages and powers you wish to defend against?

This should help answering your question better. Possibly by indicating that the entire protection is not needed.

matrixofdynamism:
I am going to design my first PCB that shall contain CPLDs. I want to add circuit protection only as a learning exercise for this DIY hobby project.

There are going to be two power sorces, USB and DC input. The DC input has range 5V to 12V. It connects into linear regulator. The current draw will be a few hundred mA at most. Once I get confidence by creating a working PCB, I shall make this design more complex overtime.

Adding reverse polarity and short circuit protection is aimed to learn how to do this. Since I have never ever done this ever, it has not even been clear how to spec a circuit of this nature.

golden_labels:
Doing this as a learning exercise is a reasonable explanation of what motivates you. It does not describe the source of the danger against which this would protect.

Imagine I asked: how can I protect garden from intruders? Could you answer this? Not really. Somebody entering my garden is the mechanism through which actual harm happens. But it does not tell much. Do I want to protect my grass from neighbour’s dog pooping? Is this against local youngsters drinking alcohol at night? Is this against thieves? Is this against S.W.A.T. team seizing the property? For dogs either a post with “no pooping here” or a foot-tall row of flowers is sufficient. It will be on the front. For teenagers it will be automatic lights, mounted on the house. For thieves I need to protect the entire area, from all sides. For the final scenario I need proper legislation.

Similarly: overvoltage is the mechanism, but the right choice for protection depends on the nature of the overvoltage. It’s not the same if somebody connects a 30 V power supply to your 12 V circuit, if there are short voltage spikes in your car 12 V installation, there is a short to mains, if a lightning strikes, or if you cause an ESD event on the logic pins.

As for the current picture: if it’s a linear regulator and you are dropping from 12 V at half amp, this will be a few watts to dissipate. Be mindful of thermal limitations of the regulator.