42 votes
Accepted

What is an "XY" rated safety capacitor, exactly?

Safety capacitors are classified by X and Y ratings. Let's properly define everything, and then it should become clear how those capacitors can be rated for both X and Y at the same time. Class X ...
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  • 26.4k
12 votes
Accepted

How does this overvoltage protection work?

It's pretty simple: The Zener diode voltage is at (or slightly above) the normal Vcc voltage. For example, a 5.6V Zener for a 5V Vcc. When Vcc is below the Zener voltage, no current flows through the ...
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  • 59.9k
11 votes

How does this overvoltage protection work?

Basically it's a power zener diode circuit. The zener begins turning on the BJT when sufficient voltage is across the line. The transistor takes current and the more that the line voltage tries to ...
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  • 390k
10 votes
Accepted

FPGA over-voltage protection for input trigger signal

It depends on how fast you want it... Zener diodes have pretty high capacitance, so you'll need a low value series resistor, which means it will draw a lot of current from the signal source. With ...
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  • 62k
9 votes

I²C overvoltage protection fail

How can I modify the design/components to have almost 3.3V on the GPIO pin (when it's high) and a precise voltage clamping ? Zener diodes are not as perfect as you might probably think. Even below ...
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  • 390k
9 votes

I²C overvoltage protection fail

Zener diodes below about 5v are really terrible and pass significant current below their zener voltage. This chart for a typical small zener diode (BZX84) shows how soft the "knee" voltage ...
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  • 29.8k
9 votes
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Is the overvoltage protection used in this circuit a normal way of doing it?

Is the overvoltage protection used in this circuit a normal way of doing it? I've seen it done this way. It's basically equivalent to a programmable zener diode i.e. nothing is shorted out; if the ...
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  • 390k
8 votes
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Long wires causing Raspberry Pi to die?

The wire has inductance and, the longer the wire is the more inductance it has. An inductance likes to maintain the current flowing through itself so, when your switch opens, the small stored energy ...
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  • 390k
8 votes

"shark teeth"-like PCB feature in high voltage section of modern HDTV

You can get rough calculations of the breakdown voltage from calculators. This paper has some measurements from an etched 0.55mm tooth gap: Wan, F., Pilla, V., Li, J., Pommerenke, D., Shumiya, H., &...
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7 votes

Zener Diode + fuse overvoltage protection for LEDs with constant current driver- fast or slow blow fuse?

The current through the zener will be determined by the source impedance of your 12/24 volt supply and the associated wiring. Most supplies will have a low enough internal impedance that the zener ...
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  • 5,451
7 votes
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Overvoltage protection with zener - How do you get a sharp knee clamp for analog input?

Here is one method: simulate this circuit – Schematic created using CircuitLab If the errant user leaves the +12 on there the resistor will see almost half a watt and the transistor a few ...
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7 votes
Accepted

Analog input over-voltage protection using Schottky and Zener Diodes. Can it work?

Yes, that would work fine, but you can make this even more simple: simulate this circuit – Schematic created using CircuitLab Zener diodes behave more or less as a "normal" diode when in ...
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7 votes

Is the overvoltage protection used in this circuit a normal way of doing it?

Bad, but could be marginally acceptable depending on what you are trying to protect from. The reference voltage from the 4 diodes will change a lot over temperature. If the overvoltage is constant, ...
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  • 10.3k
7 votes

Is the overvoltage protection used in this circuit a normal way of doing it?

Besides the shunt regulation addressed in other answers, there is one more point of bother. This only works well if the caps have the same capacitance. If they don't, they charge and different rates ...
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  • 11.9k
6 votes

What is an "XY" rated safety capacitor, exactly?

No, X caps should fail open to prevent the risk of fire. However, there have been some parts that have caught fire after a number of years in the field. It's common wisdom that paper dielectric caps ...
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  • 2,187
6 votes
Accepted

Voltage monitor circuit (reverse polarity protection with high voltage cut-off)

Your circuit has issues and I would not try to "patch" these as there are simpler, more reliable solutions! I would use a poly fuse which is a self-recovering fuse. After it blows it will recover (...
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6 votes

FPGA over-voltage protection for input trigger signal

Any accountable credits of this answer have to go to @bobflux . bobflux's answer points out all the aspects to be considered. I just would add a variation of bobblux's. simulate this circuit – ...
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  • 3,661
5 votes

Zener over voltage protection from DC hand crank generator

A linear regulator is inappropriate here. Use one of the many many buck regulator chips out there. Pick one that can handle a bit more than the max possible output after the full wave bridge, and ...
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5 votes
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Designing an effective power supply for embedded products

The polarity protection works correctly as explained in Mosfet in reverse polarity protection. The rest is the Typical Applications given by Microchip in the MCP16301/H datasheet. So, I don't see any ...
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  • 10.5k
5 votes
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Overvoltage and reverse polarity protection

Most likely, no. When the SCR fires on an overvoltage, the crowbar current that is needed to blow the fuse also passes through the FET. Since the FET's source is then dragged down to near the negative ...
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  • 5,501
5 votes
Accepted

Type-C Non-Negotiated Power and 20V protection

USB charging has become very complex but also very powerful. There are several official and de-facto standards used concurrently: USB 1.0 / 2.0 / 3.0 Apple chargers Qualcomm Quick Charge 1.0 / 2.0 / ...
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  • 1,766
5 votes

"shark teeth"-like PCB feature in high voltage section of modern HDTV

In effect it's an over voltage spark gap. If a line surge occurs, it will cause breakdown across the shark's teeth and (hopefully) offer some over-voltage protection to other parts of the circuit.
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  • 390k
5 votes
Accepted

Is this TL431-based overvoltage limiter circuit alright?

TL431 will work fine in this application, with a few caveats: When VREF<2.5V it is off but it will still draw a small cathode current to power the internal circuitry. When VREF=2.5V, in order to ...
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  • 62k
5 votes
Accepted

Is this a correct way to divide and 'clamp' a noisy 10 V signal to 3.3 V?

Most logic inputs will have a specified maximum current rating. This is the allowable maximum current that can be "forced" into the input without causing problems such as latch up (for ...
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  • 390k
4 votes

overshoot and undershoot on 3 volt pulse

You can probably clean this up by using a series resistor at the FPGA end. This will slow the rise and fall times slightly but should eliminate the overshoot and ringing. You will probably have to ...
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  • 3,293
4 votes
Accepted

ESD/overvoltage protection with DM1231-02SO TVS array

Your interpretation is correct (note that the datasheet under Features (page 1) indicates a 12kV rating on the OUT pins vs 4kV on the IN pins which makes the in/out characteristic non-symmetric. The ...
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  • 1,996
4 votes
Accepted

Crowbar circuit fuse placement

Your circuit with F1 overlooks how a crowbar circuit is designed to operate--the SCR is intended to conduct a very large current, forcibly blowing the supply fuse and isolating your circuit from the ...
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  • 14.5k
4 votes
Accepted

Resistor placement for Zener diode for over voltage protection

The first circuit you show, with the resistor in-line with the load current, is what you require. That way, the Zener diode will limit the voltage, and the excess voltage will be dropped across the ...
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4 votes
Accepted

ADC protection best practice

simulate this circuit – Schematic created using CircuitLab If you have need a resistive divider, it can limit current instead of an explicit current limiting resistor. The diode rail clamps ...
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  • 48.3k

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