A question about reverse polarity protection

[chromesphere]

Gday!
I saw a thread about polarity protection, and i thought i would ask a question.

So my meager understand is that in the typical 1n4001 diode configuration, if the power is connected in reverse, the diode acts as a short between the plus and minus protecting the circuit beyond.

Does the power supply only provide 0.6v (for the diode) to the circuit?  I'm just thinking, if it provides 9v's with a decent amount of current, wouldnt the power supply burn out / heat up over time?  Or do "most" power supplies have a protection mechanism inside to prevent this from happening? (i know that they usually have thermal detection to shut off...but still seems unsafe to me?)

It just occured to me, you know, shorts from plus to minus usually arent healthy for the power supply and if thats whats happening with a reverse polarity protection diode, and the power was left on for an extended amount of time, well, you know, could cause a dead psu or worst case scenario, a fire.

And as Sweet Brown might say "nobody aint got time for that".

Cheers,
Paul

[R.G.]

So my meager understand is that in the typical 1n4001 diode configuration, if the power is connected in reverse, the diode acts as a short between the plus and minus protecting the circuit beyond.

That is correct. Well, it acts as a few-ohms resistor above 0.6V. Very much the same thing to a 9V power supply.

Does the power supply only provide 0.6v (for the diode) to the circuit?  I'm just thinking, if it provides 9v's with a decent amount of current, wouldnt the power supply burn out / heat up over time?  Or do "most" power supplies have a protection mechanism inside to prevent this from happening? (i know that they usually have thermal detection to shut off...but still seems unsafe to me?)

What happens is that the power supply and the diode fight it out. A 1N4000 series diode can conduct 30A for several milliseconds before being damaged. Most modern power supplies have seen the results of inept humans connecting them up and have some kind of current limiting. Current limiting was actually developed and installed before thermal limiting. But the power supply does its level best to force current into the 0.6V until it hits current limit.

More sophisticated power supplies do better. Current limiting is not enough by itself. The power supply is still eating the difference between its desired output voltage and what it can actually do at full current limit, so the power supply heats up, as well as the diode. Better circuits implement some kind of so-called "fold-back" circuit so that when the current limit is reached, it internally does some kind of shutting down towards zero volts and zero current.

Even more modern ones in effect note that Something Bad is happening, and do some more-clever shutdown and perhaps restarting in a while, when maybe things are better on the load side.

It just occured to me, you know, shorts from plus to minus usually arent healthy for the power supply and if thats whats happening with a reverse polarity protection diode, and the power was left on for an extended amount of time, well, you know, could cause a dead psu or worst case scenario, a fire.

You're right. Most of the power supplies you hook up to pedals are fine with short times (few minutes, etc) of being clamped with a diode. All of the normal three-terminal regulators have built in current limiting and thermal limiting. Sometimes the transformers making low voltage out of the AC power line voltage can't deal with this very well after a while, though. Poorly designed wall warts will overheat and -- well, do something. The better ones shut down nondestructively. All of them are supposed to have passed safety certification and may melt down, but are supposed to do that in a way that does not start fires.

This stuff is near and dear to me, as I started technical life as a power supply designer. The power supply my company makes will go up to its overcurrent/over power threshold, then shut down for a second or so, then try to restart. I've hooked them up to dead shorts and single diodes overnight to see what happens. They were merrily ticking in restart the next day. So it depends on what the power supply designers put in for you, and whether they were experienced enough and clever enough to expect and provide for shorts and near-shorts, which are worse in many cases.

What is really deadly is a dumb wall wart which provides not 9Vdc, but 9V**AC**. In practice, this burns out the protection diode into a short, then burns it open, then fries all the electronics that the dearly departed diode can no longer watch over.

[chromesphere]

Thanks RG i was hoping you would respond   Thanks for the insightful information, you have definitely cleared that up for me.

Cheers!
Paul