Add overvoltage protection to save voltage regulator

[matrixofdynamism]

So there is a linear voltage regulator that I intend to use. However, like everything else, this component also has an absolute maximum voltage level of its input. What is the correct way to protect input of voltage regulator from overvoltage? This shall be connected to a bench powersupply so wouldn't want the circuit to be destroyed.

Now I think, we should be able to add a zener diode that has maybe a breakdown voltage that is just close to the absolute maximum of the linear voltage regulator. However, if this condition was reached, the diode will probably be overheated and destroyed unless there is current limit also applied somehow. I guess in this case, we could add a zener diode to high side, with resistor in series on the low side (to GND) to limit the current. Is this the correct way to create an over voltage protection circuit?

The thing is, I want to add reverse polarity protection along with overvoltage protection. Therefore, if I add zener diode, there is still the issue of what will happen when the polarity is reversed. The reverse polarity can be handled using some FET transistor circuit but I haven't gone far with that  idea yet.

Please let me know if this question has already been asked here and fully discussed.

[Jwillis]

 An in line diode is the simplest polarity protection but you'll get a voltage drop from the diode . Or you could use a P Channel Mosfet for better results.

The over voltage protection will follow the polarity protection .

These are ones I've used but I'm sure there are others that may be better.
 

[barshatriplee]

Place a series resistor between the input voltage and the voltage regulator's input pin.
Connect a zener diode in parallel with the voltage regulator's input pin.
Choose a zener diode with a breakdown voltage slightly higher than the maximum input voltage of the regulator.
In normal operation, the voltage drop across the series resistor is small, and the zener diode remains in the off state. However, if the input voltage exceeds the breakdown voltage of the zener diode, it starts conducting and clamps the voltage across the regulator's input pin.

[G-son]

Zener diode plus fuse perhaps? If input voltage goes above zener voltage the zener will allow a large current to go straight to ground, blowing the fuse.

[matrixofdynamism]

Using fuse means that it will be blown and needs to be replaced.

During my search I came across some very phenomenal ICs like one called efuse. Maybe that could be a good option. Adding protection is important since we don't know how the end user will connect the PCB.

[matrixofdynamism]

You gave two circuits, the second one uses BJTs. I thought we should avoid these since they will dissipate more power and FETs are better i.e more efficient and robust. I am not sure if this approach is correct.

So what you have showed is, reverse polarity protection followed by over-voltage protection. Do I understand correct?

By "in-line diode" I assume you are talking about the use of schottkey diode since it has a lesser voltage drop and power dissipation.

Lets add one more feature here. Suppose I need to turn on an LED for scenario where over-voltage condition occurs. Where the LED go? In between the Zener and the resistor (cathode to ground) or somewhere else?

[srb1954]

You gave two circuits, the second one uses BJTs. I thought we should avoid these since they will dissipate more power and FETs are better i.e more efficient and robust.

It is an incorrect assumption that FETs are more efficient than BJTs. It all depends on the circuit configuration and where they are operated in a linear mode such as this circuit the power dissipations and efficiencies of the the two types of devices will be comparable.

[matrixofdynamism]

I see.

The FET circuit shows reverse polarity protection followed by over-voltage protection. Do I understand correct?

By "in-line diode" I assume you are talking about the use of schottkey diode since it has a lesser voltage drop and thus lower power dissipation.

Now, lets add one more feature here. Suppose I need to turn on an LED for scenario where over-voltage condition occurs. Where the LED go? In between the Zener and the resistor (cathode to ground) or somewhere else?

[Jwillis]

I see.

The FET circuit shows reverse polarity protection followed by over-voltage protection. Do I understand correct?

By "in-line diode" I assume you are talking about the use of schottkey diode since it has a lesser voltage drop and thus lower power dissipation.

Now, lets add one more feature here. Suppose I need to turn on an LED for scenario where over-voltage condition occurs. Where the LED go? In between the Zener and the resistor (cathode to ground) or somewhere else?

An LED between Zener and resistor would cause a voltage drop between 1.8 and 3.3 V depending on colour of LED. A LED from zener with its own limiting resistor to ground would work. You may not even need a limiting resistor for LED depending on the current allowed from the 2,2K resistor. Not certain what voltage you'll be using here. Keep in mind the total current requirements from zener as to not exceed it's power rating. 10mA for the LED is more than enough for reasonable brightness.

A schottkey barrier diode would work fine in series on positive rail.

Yes put the polarity protection before over voltage protection then your regulator.

[Kim Christensen]

Whatever protection circuit you choose, even that will have a maximum voltage it can withstand. So it really depends on how stupid you think the user will be.
So determine the maximum voltage you want to protect against and THEN design the protection circuit.

Your solution might be as simple as choosing a more robust voltage regulator IC. Take a look at voltage regulator ICs designed for automotive use. They have quite a few additional protection features over plain old regulator ICs...

[JoeyG]

How about this? https://youtu.be/QZ0WXQWND-I

[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.