9V Power Section Diode Placement

[Cyeos]

Hi all,
After reading this AMZ post http://www.muzique.com/news/reverse-voltage-protection/ ,I am wondering if I've been doing protection diodes wrong for a while.
Is it wrong to just put the 1n400x diode before the series resistor, or does it need to come afterwards as shown by the article above?


Wrong?

I tried asking him in his comments, but he appeared to miss my question.

[disorder]

The diode needs to come after the series resistor. The resistor serves two purposes.

1 - it limits current if you connect incorrect polarity and the diode starts dumping current to ground. You chose a value (often 100ohm) that limits current enough to protect the diode (and possibly protect your power adapter as well, I think). You also choose the value such that it can handle the current without burning up.

2 - if it does burn up, resistors almost always burn up as open circuits, thus protecting everything in your circuit. It's like a fuse!

[Ben Lyman]

I've been doing it with a 1n5817 like this after some expert advice.

You just have to accept a very slight voltage drop but apparently nothing much can go wrong with this set up. I use a super bright LED so I can use a big (47k) resistor with it. The 47ohm/100uF filters noise above 34Hz

[Keppy]

There's no "need to" here. Lots of circuits don't even have R1. In this setup, the diode will survive brief reversals (like a battery momentarily touched to the wrong leads).

However, Disorder is right about the current limiting benefits. It will help keep the diode from burning up if the polarity is reversed for any length of time, like if you plug the wrong adapter in and don't notice until trying to play. Probably.

In my own circuits, I do what Ben posted. The protection is surer, though like he says you live with the diode drop on the power supply voltage. An added benefit is that the LED current spike gets smoothed out by the power filtering.

[Fast Pistoleros]

real important I think for digital etc that only likes to see 5volts ..always a good idea to protect one of those 4-5 dollars chips !

[ubersam]

This is what I've been doing for reverse polarity protection: http://www.geofex.com/article_folders/mosswitch/mosswitch.htm - it's actually a mosfet used as a switch that closes (turns on) when the polarity is correct but remains open if the polarity is reversed.

[Ben Lyman]

...In my own circuits, I do what Ben posted...

That there is a very reassuring endorsement for this method 

[amz-fx]

You can put the diode before the resistor, which is part of the R-C power supply noise filter.

With it before, the diode will carry all of the current in a reverse voltage situation. If the power supply can provide more than 1A, the diode will get hot very quickly and burn out. Usually it opens up. In that case, the diode is open and the reverse voltage then flows on to the circuit that it was intended to protect, with possibly damage to the circuit.

The series diode, as shown in the article, is a better idea.  It's what I use in the pre-assembled effect modules.

http://www.muzique.com/news/reverse-voltage-protection/

regards, Jack

[SuzukiScottie]

I use a series 5817 too. Interesting article about the mosfet switch. Worth a try too.

[Cyeos]

Thanks Jack!

What is the general result of the voltage drop from the 1N5817 diode in series that is suggested, since everything will probably still power up just fine?
Less headroom?

[Kipper4]

Here's a data sheet

Posted a bad link thanks ,

[bluebunny]

1N5817 diode

Try this one, Rich:

http://www.diodes.com/_files/datasheets/ds23001.pdf