Why do PIC pins blow up?

[mynameisdan]

Ok, I realize how stupid I sound when I am asking this. It is a serious question I promise!

In an over-voltage condition on an input. I understand the clamp diodes will fry, but if the input is cmos, why are there clamping diodes in the first place? So, if the input is ∞ resistance, why would 12v kill the pin?

Also note, I've ran PICs before with 12v. Apart from a little heat, they turn out fine. So why do the pins die so easily?

 

[hexreader]

I understand the clamp diodes will fry,

If you are lucky, the clamp diodes will fry, subjecting only one I/O circuit to a high voltage. If you are unlucky, the diodes will happily feed 11.4 volts through to the 5 volt circuitry of the PIC. This might just fry the PIC, or it might conceivably damage other components on the 5V line (I am assuming a 5V PIC).

Check out the maximum allowable voltage for input pins, as specified in the datasheet. I suspect that 12V is way outside spec.

why do the pins die so easily?

No idea, since I am not a chip designer, but if you are supplying twice the absolute maximum allowable voltage, why be surprised if your PIC fails?
Don't forget that there is output circuitry on most pins, as well as input circuitry, so not infinite resistance.

I run all of my PIC chips within spec, and have never had a failure.

 

[Pommie]

Ok, I realize how stupid I sound when I am asking this.
Also note, I've ran PICs before with 12v. Apart from a little heat, they turn out fine. So why do the pins die so easily?

You run a part rated for 5V at 12V and wonder why it fails!!!! Yes, you are correct, you are stupid.

Mike.

 

[Jon Wilder]

You run a part rated for 5V at 12V and wonder why it fails!!!! Yes, you are correct, you are stupid.

Mike.

+1. Read the datasheet.

 

[gabeNC]

I've got a question, the redline on my tachometer says 5K, but I've revved my motor to 12K. Why did the motor blow up?

 

[Dr_Doggy]

um, maybe the diode is upside down so it overflows to ground when avalanche value is exceeded, maybe??

also i read that i can be "negatively biased" to bring a value down ??

 

[Nigel Goodwin]

Also note, I've ran PICs before with 12v. Apart from a little heat, they turn out fine. So why do the pins die so easily?

PIC's don't die easily - they are extremely difficult to damage, however it sounds like you pay no attention to their specs, so you'll kill anything doing that.

If you fit correct current limiting resistors on the inputs, then you won't kill the protection diodes, they will work as intended.

 

[mynameisdan]

Please don't assume I am running my PIC chips at 12v. I am simply asking why their inputs blow up so easily. In fairly long times that I have been using these, once or twice accidents happen. A higher than rated voltage is used on some part. I am asking why the PIC will be fine if it's supply voltage is 12v for a short time, but the inputs die straight away.

 

[Nigel Goodwin]

Please don't assume I am running my PIC chips at 12v. I am simply asking why their inputs blow up so easily. In fairly long times that I have been using these, once or twice accidents happen. A higher than rated voltage is used on some part. I am asking why the PIC will be fine if it's supply voltage is 12v for a short time, but the inputs die straight away.

And as I've said - they don't - UNLESS you've not connected it properly.

Try posting the circuit you use.

 

[Jon Wilder]

If you're referring to the programming voltage, the PIC DOES NOT run at 12V during programming. It runs at the normal 5V, while pin RA4/MCLR receives a 12V voltage, which places the PIC in programming mode. The internals of that specific pin are designed to work in programming mode with that voltage on that pin only.