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Overvoltage circuit protection- simple solution required

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turbok

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I am looking for ideas on a simple way to protect a circuit from overvoltage.

I have a number of circuits that are supplied by 5V DC 4.0A mains to DC transformers.
I have twice plugged in my laptop charger by mistake (unfortunately the connector is the same) and damaged some components. The laptop charger is rated as 19V 4.22A output.

Apart from obviously being more careful which I will try to do, I would like to protect the circuits from this kind of damage - other users will also be using these circuits next to their laptops and I know it's an easy mistake to make!

Can anyone suggest a solution - preferably small component count and possible to implement with SMD parts.
I guess a simple fuse would not work because the current rating for the intended supply is 4.0V which is almost as high as the laptop charger.

Thanks in advance.
 

Nigel Goodwin

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Most Helpful Member
You need a 'crowbar circuit' - essentially a thyristor, zener, resistor, and capacitor - this goes AFTER the input fuse, and blows the fuse if the voltage is too high.
 

dknguyen

Well-Known Member
Most Helpful Member
TVS diodes and varistors are both designed to do this as described by Nigel.
 
Last edited:

turbok

New Member
Thanks very much for the replies.
I have looked up Crowbar Circuits and found this example: Practical Electronics/Crowbar circuit - Wikibooks, collection of open-content textbooks

- If I want to 'allow' up to 4.0A current, then would I replace the 250mA fuse in that example for a 4.0A fuse?
- It looks to me like I should use a 6V Zener (ZD1) to protect a circuit and supply that is rated at 5V. Is that correct?
- Can anyone help me understand how to select values for the Thyristor or Schottky diodes - Q1 and SD1?

If anyone has links to other clearer explanations of how Crowbar Circuits work, then I'd be very grateful - I'm still not completely clear on it.

Alternatively would a Clamp Circuit provide a solution?

Thanks again.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Thanks very much for the replies.
I have looked up Crowbar Circuits and found this example: Practical Electronics/Crowbar circuit - Wikibooks, collection of open-content textbooks

- If I want to 'allow' up to 4.0A current, then would I replace the 250mA fuse in that example for a 4.0A fuse?
- It looks to me like I should use a 6V Zener (ZD1) to protect a circuit and supply that is rated at 5V. Is that correct?
- Can anyone help me understand how to select values for the Thyristor or Schottky diodes - Q1 and SD1?

If anyone has links to other clearer explanations of how Crowbar Circuits work, then I'd be very grateful - I'm still not completely clear on it.
You probably don't require the diode anyway - for the thyristor you simply require one that will easily carry the load - I would suggest a 12A or 15A device to blow a 4A fuse.
 

unclejed613

Well-Known Member
Most Helpful Member
if you know someone who has a bunch of open frame power supplies, quite often they have an "add-on" crowbar board attached to them, and if you get it off a 5V supply, you don't need to change a thing, just wire it up. one word of warning, test it before you use it. quite often i get open frame supplies that have been replaced in equipment simply because the crowbar SCR is shorted.
 

Hero999

Banned
 

MikeMl

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Last edited:

Hero999

Banned
Both.

The 18V zener protects against high speed transistors, the crowbar protects against longer lower voltage transients.

A brief 120V spike will be absorbed by the zener but won't trigger the crowbar due to the RC time delay, a longer duration lower voltage surge will trip the crowbar.
 

Hero999

Banned
Lol, I meant transients of course. :D
 
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