My idea is to have an intermediate cheap component that will blow-up before my uC if the voltage goes above 5v from 7805 (or even some fuse, but I am not sure if a fuse will blow-up with over-voltage like it blows-up with over-current). I have lost many Atmega328P chips because of 7805 accidentally supplying 12v to them. I have heard of SCR - is it the component I am looking for which blows-up with voltage?

Also, please tell me how to use a zener with 7805.

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    \$\begingroup\$ There are no such things as voltage fuses that blow when the voltage is too high. But there are components that will try their best to clamp the voltage below a certain level before too overheat and fry. You connect them between the line being clamped and GND. Zeners and TVS diodes. But I think you ought to just fix the origin of the problem: the reason why your ground keeps getting disconnected so easily. That's indicative of your construction methods being faulty and trying to fix it with a protective device is like making sure everyone wears hard hats indoors instead of fixing the ceiling. \$\endgroup\$ – DKNguyen Jan 14 at 5:16
  • \$\begingroup\$ Any component except an insulator will go to 12v when you disconnect it from ground. Including the zener. Though you would have to disconnect both the zener and the 7805 for that to become a problem. \$\endgroup\$ – piojo Jan 14 at 5:17
  • \$\begingroup\$ DKNguyen - thanks. I was wondering how to place my 5.1v Zener diode, so now I know that I need to connect it between 5v output of 7805 and GND. But I am still not sure about polarity. Where should I connect cathode of Zener? \$\endgroup\$ – J J Jan 14 at 5:23
  • \$\begingroup\$ Piojo - that is a wonderful info about how components will pass 12v to my ICs. The reason they keep doing it is because I was testing it on breadboard and they werent soldered-in. Do you mean that both zener and 7805 will have to be disconnected form ground to pass 12v to my uC? If so, that sounds like placing a zener to 7805 having grounds connected separately is beneficial then \$\endgroup\$ – J J Jan 14 at 5:25
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    \$\begingroup\$ I second DKNguyen feedback, fix the cause, not the symptom. Consider soldering the regulator to a adapter board and stick that into your bread board. A pin header will make more reliable contact than the leads of the component. You might even solder the caps to the adapter board also. A cut prototyping board may suffice. There are probably even proper TO92 adapter commercially available. \$\endgroup\$ – try-catch-finally Jan 14 at 5:29

One simple way is to use a fuse in conjunction with a relatively big Zener diode.

This is not perfect but cheap and simple and will work in most cases.

If the input voltage goes above the Zener voltage (in this case 5.4V) the Zener will short and the fuse will blow.

For the fuse, you can use a PTC resettable fuse


simulate this circuit – Schematic created using CircuitLab

A better way is to use a voltage clamp circuit instead of a zener.

Another solution is to use an LDO (or other types of regulator) for the input but you need to consider the voltage drop caused.

Overvoltage protection circuit (note this circuit will cause a voltage drop, LDO IMO is simpler): enter image description here https://circuitdigest.com/electronic-circuits/overvoltage-protection-circuit

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  • \$\begingroup\$ Thanks for answer, Damien. I am interested to know about how to use a voltage clamp? I already use an LDO 7805, but since it was on breadboard, it kept disconnecting from ground and frying my uC. People assured me it wont happen on my commercial pcb product, btu now I am paranoid. I do have 500mA fuses. Are they fine? \$\endgroup\$ – J J Jan 14 at 5:27
  • \$\begingroup\$ Also, am I right that zener is placed in reverse-biased setting in your above diagram which will start conducting only when the breakdown voltage occurs and draw much current to blow the fuse? \$\endgroup\$ – J J Jan 14 at 5:29
  • \$\begingroup\$ It is correct. The fuse has to be designed so that it will hold at the nominal current use of your circuit. On a PCB it is unlikely a component would get unsoldered, if it does, then there is a manufacturing issue and anyway the rest your board probably won't work. \$\endgroup\$ – Damien Jan 14 at 5:34
  • \$\begingroup\$ Thanks a lot Damien. I do have a zener of 5.1 rating and a fuse of 500mA. Do I need a lesser value fuse? I think if zener will draw infinite curent then 500 mA should suffice because my other devices in circuit (an OLED display, an RGB led, a buzzer, two 0.2 A fans) can draw upto 500mA. But I still need an opinion on this \$\endgroup\$ – J J Jan 14 at 5:38
  • \$\begingroup\$ Just make sure the Zener can draw more current than the fuse can hold (for a short time). 500mA at 5V is 2.5W. \$\endgroup\$ – Damien Jan 14 at 5:44

Simplest way is to add another regulator, but it has to be an LDO regulator, not a 7805.

For example, the AP2210N-5.0TRG1 has only a few hundred mV drop at 100-200mA and can handle up to +15V in. It's advertised as being able to withstand negative inputs as well, but I don't see a specifically stated voltage limit on that. Be sure to use the recommended input/output capacitors or it can go unstable.

If you do supply +12V the regulator will likely go into thermal limiting, which is not good for the part, but it will protect the chip it's connected to.

That's to answer your question, but really you need to fix the problem at the source as DKNguyen says, for example, by soldering the 7805 into a bit of perf board so the GND pin is solidly connected.

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