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overvoltage protection

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curious_mind

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I have a fully qualified EMI/EMC power supply that guarantees output of 5V +/-5% when subjected to hostile EFT/surge signals. As a double precaution, I would like to add additional protection device just in case there is a over voltage. Which device or scheme is suitable?. The 5V is connected to my MCU.
1. Ferrite bead with bypass cap
2. Just bidirectional TVS smaj5.0a
3. Combination of TVS and MOV

or any other method, please suggest
 

Ferrite and cap will do nothing about overvoltage.
I think a TVS will give more accurate protection. I think MOVs are better on power lines not regulator outputs.
 

Forget the Ferrite bead, that's for something completely different.

Are you looking for protection of the input (AC line voltage?) or protection of the 5V output?
MOV is probably best for the line voltage but not for low voltages like 5V. A suitable TVS will offer some protection against over-voltage at 5V but is less useful at line voltage. If you want absolute protection for your MCU you need a crowbar circuit but beware of the consequences on the PSU if it operates.

Brian.
 

Hi betwixt

Looking for protection on 5V side. TVS would only suffice or any other combination is required?
 

It really depends on how much current your power supply can produce. If it is up to about 100mA a normal Zener diode will suffice, for more than that a TVS is a better solution. Bear in mind there is no magic component that allows 5V but stops anything higher, they all have tolerances and a gradual conduction beyond knee voltage. Your PSU has a tolerance of 5%, that means it could produce 5.25V so you need a TVS rated slightly higher than that. All these devices work by diverting excess voltage as a current through their bodies, they 'drag' the PSU output down to safe level but in doing so they produce heat and increased load on the PSU itself.

Brian.
 

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The ferrite bead will do nothing. The others will not protect 5V digital circuits from a power supply overvoltage. They don't have a sharp enough "knee". The voltage will be well above 5V before they do much.

A "crowbar" could be a solution, but I suggest one of the following:

1. A low dropout voltage regulator
2. An emitter follower with the base driven from a higher voltage via a properly selected base series resistor (so the base current is guaranteed to be less that the current needed by the circuit).
 

If you need <5% variation in clamping voltage no 2 terminal off the shelf device is going to do it.

They’re not a bad idea, and may be what you want, but understand they don’t have that kind of precision.
 

std_match said:
The ferrite bead will do nothing. The others will not protect 5V digital circuits from a power supply overvoltage. They don't have a sharp enough "knee". The voltage will be well above 5V before they do much.

A "crowbar" could be a solution, but I suggest one of the following:

1. A low dropout voltage regulator
2. An emitter follower with the base driven from a higher voltage via a properly selected base series resistor (so the base current is guaranteed to be less that the current needed by the circuit).
Click to expand...

1) how does a low dropout regulator protect from overvoltage???
2) What? This method will guarantee that you have turned your voltage regulator into a circuit whose output voltage depends on its current, i.e., NOT a regulator.
 

barry said:
1) how does a low dropout regulator protect from overvoltage???
2) What? This method will guarantee that you have turned your voltage regulator into a circuit whose output voltage depends on its current, i.e., NOT a regulator.
Click to expand...

My understanding is that the OP wants an additional protection close to the MCU.

1) Normally 5V in and 5V out for the low dropout regulator. If the 5V from the external power supply has an overvoltage, 5V will still be supplied to the MCU.
2) What I described also needs a 5V6 zener diode from the transistor base to ground.
 

std_match said:
My understanding is that the OP wants an additional protection close to the MCU.

1) Normally 5V in and 5V out for the low dropout regulator. If the 5V from the external power supply has an overvoltage, 5V will still be supplied to the MCU.
2) What I described also needs a 5V6 zener diode from the transistor base to ground.
Click to expand...

1) No. You will NOT get 5v out of a 5V LDO WITH 5v in. ANY regulator will give 5 V out if the input goes higher than 5. That’s what regulators do.

2) As explained by others, the soft knee of the zener is problematic. Adding a transistor does nothing.
 

If you really want to do things right, select a crowbar IC which provides a hard SCR-gate drive, and which has both an accurate voltage threshold and an adjustable time delay to prevent nuisance triggering.

The crowbar IC that I used back in the 1980s is obsolete, but a quick google search shows that the LTC1696 and the MC3423 are still available.
 

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