5V over voltage protection

Thread Starter


Joined Mar 16, 2009
I have a circuit that operates on 5VDC.
I am looking for a way to protect the circuit if 12VDC is accidently applied to the DC voltage inputs.
Is there a simple device(s) out there that will protect the circuit from a continous 12VDC input?

Anyone have any ideas for me?


Joined Jul 17, 2007
A 7805 regulator will do that.
However, you will then need at least 7v to supply your circuit, and at least a 0.1uF cap on the output.

See the attached for a simple 5v regulator using just a 220 Ohm resistor, an NPN transistor, a 5.6v Zener diode and an output filter capacitor. Current through the Zener will get rather high at 12v (30mA), but you wanted simple.


Last edited:


Joined Mar 24, 2008
There is another type of circuit out there called a crowbar. It is so called because it throws a "crowbar" across a power supply to blow a fuse rather than fry valuable electronics, much as you have described. I'm not sure about this circuit, I'm pulling it from memory.




Joined Aug 5, 2008
I’ve used a circuit like this.

I use it for OCP though sensing the out put of a HS current sense translating it to a voltage and flipping the fet open when the threshold is exceeded disconnecting the faulty load.

The divider senses the rail voltage at the input of the board (R21,R22)When it exceeds in this case about 5.5V the 431 conducts turning on Q6 lifting the Fet’s gate up to it’s source, turning it off. The component sizes will depend on your load. An LED could be placed between R24 and ground for a visible indicator to accommodate for the idiot factor.

If you have a high power load you could always use a real comparator and a buffer to drive the FET. There are lots of decent P-FETS at that voltage range for higher power;

AOD417 VDS (V) = -30V; ID = -25A; RDS(ON) < 34mohm; Total Gate Charge (10V) typical 16.2 nC


For the simulation I’m using a pulse source with a 1mS rise time this simulates a source with impedance charging up a bulk cap at the board’s entry terminals. You have to adjust component values FET etc it was just a quick sim for an example. You have to consider your load current as well, mainly if you have a high inrush be sure the fet can take it. The scr crowbar is also good this is just another option. As to witch is more cost effective depends?

The best solution would probably be a big sign “IF YOU BREAK IT YOU PAY FOR IT”,or unique connectors.


Joined Jul 17, 2007
From a PM:
Thank you for the suggestion.

As my circuit is operating on 5V if i am going to use this circuit i need minmum input voltage as 6.0V.
so i am going to use use (9-12)V as my input and this circuit so that i can get 5V.
Sorry, I don't help via PM. It defeats the purpose of the board (which is to share information) and deprives you of other valuable input, along with jamming up my PM inbox.

If you're going to be regularly operating at 9v to 12v, you may wish to increase R1 to 330 Ohms.

You haven't said anything about your load requirements. The 2N2222 will only support up to 100mA in the load when Vin=11.5v. Power dissipation in the 2N2222 increases as Vin increases or Iout increases.

If you need higher output, you could look at a Darlington transistor, such as a TIP120. They come in a TO-220 case. You can use a heat sink. This will also enable you to use a somewhat larger value for R1.