Simple circuit protection

edmunds

Senior Member
Dear all,

I have seen these little devices that look like a capacitor, but are actually circuit protection devices that prevent circuit damage on reverse polarity and over-voltage events. Or so I think. I cannot find much information on how to implement them online, probably because I'm using some wrong keywords. Has anyone used such a device and can give me some leads?


Thank you all,

Edmunds
 

nick12ab

Senior Member
Based on your description of it looking like a capacitor, the device I think you're thinking of is a resettable fuse, polyfuse or PTC. Comprehensive information on those can be found on the internet if you search using those terms.

However, those won't necessarily protect the circuit from reverse polarity or over-voltage. If the current flow required to damage one component in the circuit is less than the rating of the fuse, the circuit can still be damaged. A crowbar circuit can be used to protect against over-voltage, and a simple series diode will protect against reverse voltage at the expense of a small voltage drop.
 

premelec

Senior Member
@Edmunds - a lot depends on how much current and voltage you have in play - for simple reverse voltage protection a shunt diode [that conducts on wrong polarity] and series PTC fuse from power line to diode work quite well - if you can take a little voltage drop then a series diode to power works fine... A zener diode can help with overvoltage. How much over voltage can you take? That determines a lot about the circuitry.
 

Adiman

Member
What you are probably talking about is a MOV (metal oxide varistor) they are basically very similar to a Bidirectional TVS diode and behave mostly the same. They are only good for clamping spikes above their rated clamping voltage, but will do nothing in reverse polarity situations as they are bi-directional.

As others have said for reverse battery protection an in-line (series) schottky diode would be best if you don't mind losing 0.5-1v to the input of your circuit from the battery. This is the best reverse voltage protection you can get, as it will simply nothing will happen if you get the battery the wrong way around.
 

edmunds

Senior Member
Dear all,

Sorry for coming back late to this, but these are all great replies and exactly what I was looking for. Thank you!

Edmunds
 
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