Protection Diodes

[e44-72]

Hello

I once burned out a cmoss 4066 ic due to me putting the battery on backwards and so I thought to stop this happeneing again I should protect my circuit with a diode. Now I was wondering whether it is best to put the diode on the positive side of the circuit or facing back to the battery on the negative side.

I was thinking this because if I have an IC in my circuit and normally when battery is the right way roung positive is gioing to the positive pin and negative is going to the negative pin of the IC. If the diode is on the positive side and I reverse the polarity, nothing can go back to negative as the diode is stopping it but the positive of the battery is giong into the ground pin of the IC

However if the diode is on the negative side when I reverse the polarity nothing at all can get to the IC as the diode is stopping the positive getting to the ground Pin. Do you see what I am saying?, I have attached a basic circuit diagram to illustrate this.

What do you say is best putting the diode on the positve or negative side?

Thank you for reading and any replies.

 

[Sceadwian]

If you put it on the negative side the ground of the chip will be one diode drop above the true ground (relative to any part of the circuit outside of the diode)
If you put it on the positive side, the +VCC line will be one diode drop lower than the actual VCC (affecting all I/O pins in both cases)

I'd lean towards the + side to avoid the biased ground myself, for the sake of interfacing with the rest of the world at least. An extra diode drop to the VL of the logic might be high enough for some external components not to read as low, and will produce a small additional bias current.

 

[e44-72]

Thank you for your reply Sceadwian.

So if I put my protection diode on the positive of my circuit and somone where to accidently put the battery on the wrong way would my IC's be protected or would they be dead?

 

[audioguru]

If the protection diode is in series with the positive of your circuit and the battery is connected backwards then NOTHING will happen.

 

[Sceadwian]

Either way will work e44, the specific location does not matter in the slightest for it's protection value with battery reversal. It does however effect the perceived voltage of the chip relative to any external components/circuits that is using the same ground or VCC line WITHOUT the diode. One of two things will happen, the voltage low of the output will be raised one diode drop relative to the typical voltage drop above ground for logic (this is generally not a good idea) or the voltage high of the output will be lowered by one voltage drop (this is generally tolerated well)

Be forewarned that even if this protection diode is in place that the chip can still be supplied with a fatal current during battery reversal from something OUTSIDE of this protection element, so it's only effective if every device that is also plugged into this circuit is fed from the same power supply AFTER the protection diode. If you have multiple power supplies and signal lines from different sources they all have to be looked at together.

 

[e44-72]

OK, thank you for your replies and saving my IC's

 

[e44-72]

Thank you Sceadwian, I understand what your saying.

So it would probably be better for me to put the diode on the positive and have the possibility of the high of the outputs going down one diode drop rather than the low's coming up one diode drop. I will probably test this out on breadboard first before any soldering is done.

Thanks again

 

[OlPhart]

Explore the voltage drop difference between standard diodes and schottky versions. Sometimes using 1N400x or 1N540x is appropriate (feeding 12V to a 7805), but if you're low on difference, I'm a fan of 1N5820s.... <<<)))

 

[audioguru]

Guess why a "polarity protection" diode is called an idiot diode.

 

[Sceadwian]

AG, this is why good power plugs are physically engineered to be polarized. Not wall warts or the like. True polarity based plugs.

 

[kinarfi]

Another idea is to put the diode in parallel with the ic, but reverse biased so it blows the fuse and limits the reverse voltage across the ic
Kinarfi

 

[e44-72]

Thank you all for your replies