Standard practice reverse polarity protection 12V wall wart - Page 1
Forum section: electronics / beginners

author: mkiijam
author_rank: Regular Contributor
timestamp: May 07, 2023, 04:29:07 am
content: Hello, I'm design a simple little relay box with some TRS jacks for use in the audio world. I have a DC in jack and would like to add a diode to protect against somebody in the future jamming in a wallwart with the wrong polarity.My first thought was just a diode on the 12v, which I was pretty sure how it was done, but then I started thinking that would just put the 12v on the "ground" part of the DC plug and thereby sending 12V into all the shared grounds through the whole system.Does that make sense? What am I missing?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 07, 2023, 04:47:38 am
content: A diode in series is a very common and viable solution. The 12 V from the wall wart are relative to its terminals, not an abolute thing. So with the diode one one side blocking current flow there is no problem with the positive side of the wall wart connected to the circuits ground. With the negative side effectively isolated (by the diode) it is free to move. Instead of 0 and +12 V one could see the wall wart also as -12 V and 0 V - it only depends on where you put the reference potential to measure voltages too.The main downside of the simple diode solution is that some 0.7 V of the supply voltage is lost. This is often OK with 12 V, but could be an issue with smaller voltages like 5 V.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: May 07, 2023, 05:46:59 am
content: A popular solution is a high side P mosfet, this is how:https://components101.com/articles/design-guide-pmos-mosfet-for-reverse-voltage-polarity-protectionIs eliminates a voltage drop of the diode.

author: Karel
author_rank: Super Contributor
timestamp: May 07, 2023, 07:00:35 am
content: Alternatively, you can add a diode (in reverse direction) parallel with the powerlines and add a fuse in series with the power input.The diode needs to be able to withstand the peak shortcircuit current when the fuse blows.

author: Gyro
author_rank: Super Contributor
timestamp: May 07, 2023, 12:17:46 pm
content: ... and the fuse would need to be small enough that it will blow under the short circuit current of the wall wart (depending how beefy it is - some may go into overcurrent shutdown / limiting). An alternative would be a PTC thermistor to save the hassle of fuse replacement.

author: MrAl
author_rank: Super Contributor
timestamp: May 07, 2023, 01:00:18 pm
content: You can also try a Schottky diode just watch out for the leakage current if the load is very light, maybe add a parallel resistor 5k if not a problem. Those diodes have less voltage drop some as low as 0.2 volts or so.

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 07, 2023, 01:58:10 pm
content: I think most Walwart rated for 12VDC would put out more than 12VDC any way so losing 0.7V for the diode wouldn't be a problem.

author: Zero999
author_rank: Super Contributor
timestamp: May 07, 2023, 06:27:06 pm
content: Quote from: mkiijam on May 07, 2023, 04:29:07 amWhy do you need reverse polarity protection for relays?The only reason would be the back-EMF protection diodes passing too much current, but you could use RC networks instead.

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 07, 2023, 06:30:57 pm
content: Quote from: Zero999 on May 07, 2023, 06:27:06 pm

author: Zero999
author_rank: Super Contributor
timestamp: May 07, 2023, 08:37:05 pm
content: I'm not psychic.Are you referring to another topic, or is it a common project, I'm completely ignorant of?The original post doesn't mention microcontrollers or anything else, just relays. I thought it was some sort of multiplexer/switch box.If there's a microcontroller in there, just put the diode in series with the regulator. Use 12V relays with zeners across the driver transistors. That way, if the power is reversed, the relays will just turn on. No harm done. The good thing about zeners is they absorb more energy, than a plain old diode, turning off the relays more quickly, which reduces arcing on the contacts.

author: radiolistener
author_rank: Super Contributor
timestamp: May 08, 2023, 02:00:17 am
content: here is reverse polarity and overvoltage protection for 12V:

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 08, 2023, 11:24:01 am
content: If you worry about someone use a wallwart that has wrong polarity they shouldn't you also worry that they use a wallwart of different voltage?

author: SmallCog
author_rank: Frequent Contributor
timestamp: May 08, 2023, 10:50:22 pm
content: Another option, elegant in some ways and not in others, is a bridge rectifierUse the AC terminals for your input, the +ve and -ve terminals for your circuitThe device will then run off of either polarity of plugpack (the elegant part) but there'll be voltage drop from the diodes (the un-elegant part)

author: radiolistener
author_rank: Super Contributor
timestamp: May 09, 2023, 12:09:21 am
content: bridge rectifiers and diodes leads to voltage drop, it is very critical if current is high, because it will leads to a high power loss, diode overheating and low power voltage, which may cause to some issues in the powered device.Using relay which enable power from a proper polarity is the best solution.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 09, 2023, 09:47:27 am
content: A bridge rectifier has twice the diode drop. So it is not such an attractive solution.The relay solution needs extra power - so it depends if this is a good or bad solution.The planed relay circuit would very likely only use a more moderate power and also work with 0.7 V lower supply.The main solutions available are:1) diode in series :+ cheap and reliable- loose some 300-700 mV2) MOSFET (can also be N channel in the neg side) as a kind of ideal diode:+ low voltage loss- main let transiensts trough3) relay:+ low voltage loss- high costs and quite some power drain4) diode in reverse + fuse+ low dop, fuse may be useful anyway- may damage supply or need fuse replacement