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Recently, I was introduced to the idea of using a full-wave rectifier, in order to protect against reverse polarity damage in DC devices.

I hadn't even considered using a rectifier in an already DC circuit, but now that I think about it, why doesn't every device that has the potential to be damaged by backwards power and ground connections use this idea?

I can't wrap my head around why something that could easily protect the circuit whilst simplifying the setup wouldn't be included?

Jay Greco
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  • Might be it is just cheaper to use 1 protective diode than 4 diodes for full rectifier? I think that it suffices the exceptional cases of wrong polarity. Though, cost-efficiency and potential losses analysis may reject this intuitive hypotheis. – Val Jun 05 '13 at 10:43
  • If the equipment requires 2 batteries and the people have the two positive leads facing, then the Bridge won't compensate for that! – skyler Jun 05 '13 at 19:15
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    @skyler it won't, but putting two batteries also results in a voltage across the outer terminals of zero. Might as well say the bridge wouldn't protect against having _no_ batteries. – Jay Greco Jun 05 '13 at 22:44
  • @jay Yeah I was just saying if people are dumb enough to put them in backwards, they have a likely chance of not being consistent with there reverse-polarity-stuff – skyler Jun 06 '13 at 03:22

5 Answers5

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One other answer that nobody else has mentioned is this: If you power a circuit through a bridge rectifier, then the entire circuit must be allowed to "float" with respect to any other "ground" in the system. If you were to connect the negative output of the bridge to "system ground" and then hook up the "AC" inputs the wrong way, you'd short out the power supply.

This would be a problem, for example, in a modular car audio system, where each component would be connected to the 12 V power bus, but would also share a ground with each other component in order to connect the audio signals.

Dave Tweed
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    Good one; and furthermore, "ground" is always a diode drop away from the real ground. Currents from different circuits in the device all return through the diode, and its drop varies with the current, creating ground interactions. – Kaz Jun 05 '13 at 06:29
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There is no reason why a DC polarity reversal should take place, and the warranty can basically blame it on the user.

If the device is battery powered, the use of a standard, convention-adhering battery holder with clear markings should prevent such a thing from ever happening. Even users who don't look at markings are trained to put the flat part of an AA battery against the spring, and slide the nub against the leaf contact. 9V batteries have gendered connectors; no way to screw up short of deliberately making a temporary wrong-way contact while the power switch is on. The 99.999% of the users who are able to engage two brain cells cells together when installing a battery don't want to sacrifice battery life for the sake of the remaining 0.001%.

If the device has an AC adapter, then a polarity reversal can never happen if the original AC adapter is used. If a different AC adapter is used, which has a compatible DC barrel jack, but which puts out opposite polarity, or perhaps AC, that's the user's responsibility. Chances are that by the time users have lost the original AC adapter, the item is out of warranty. Possibly, they are not even the first owners, and so do not have the original receipt. So the chances are low of the company having to replace the item or provide a free warranty repair because of damage caused by a wrong polarity (or voltage, for that matter) aftermarket AC adapter.

Internal DC power supplies that run strictly on AC via an AC power cord obviously have no need for DC polarity reversal handling; the only way it could happen is that someone assembles the circuit wrong.

Kaz
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    Why is this down-voted? Each paragraph is a legitimate reason not to use a bridge rectifier, and a direct answer to the question. – Dave Tweed Jun 05 '13 at 05:01
  • In the FIRST Robotics Competition, motor controllers such as [these](http://robomatter.com/images/ifi/217-3367-jaguar-motor-controller_01.jpg) are often mistakenly powered in reverse by new team members. Adding in a rectifier would have saved at least a few of the Jag's I saw fry. Of course I understand there would be some extra cost and power loss associated with a solution for a problem that is easy to avoid, but I think it's a case worth noting. – lakechfoma Jun 05 '13 at 21:15
  • Two notes, one, it is possible for polarity reversal to happen with the original ac adaptor. probable? Maybe not, but definitely possible. Two, most places are not going to test for reverse polarity issues in warranty replacements unless it's a high end/cost product, or there is physical signs of the jack being forced. And they rarely require you to send the ac adaptor out. Essentially, the manufacturer figures X number of returns/warranty replacements cost Y amount. Is Y greater than the cost of reverse polarity protection? No, then screw the protection, just factor the returns in to the cost – Passerby Jun 11 '13 at 10:20
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    Improper AC adapter insertion doesn't always imply that the adapter was lost. It's not uncommon for someone to have a bunch of AC adapter cords coming up behind a desk or in some other situation where identifying which cord goes with which plug may be difficult. I don't think I've not seen devices which were intended for DC operation use full-wave rectifiers, but I've seen some with a simple series blocker diode, or sometimes a fuse or PTC fuse followed by a reverse-biased diode. – supercat Jun 11 '13 at 15:37
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    I'm not trying to refute this; it's mostly valid. But I did want to point out that, by definition, 50% of all humans have an IQ less that 100. 15.8% have an IQ less than 85. Thinking like this is elitist, not elite. Just put in the one diode already. –  May 13 '14 at 07:05
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The main reason is efficiency. The diodes have a fixed voltage drop, that is a property of semiconductors. As the current flowing through the diodes starts to increase, an proportional amount of power is wasted as heat. See this answer for a more efficient reverse polarity protection scheme using a P channel MOSFET.

Community
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Matt Young
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    ...and [here's](http://electronics.stackexchange.com/questions/39600/voltage-fuse-or-better) yet another method (although that posts is mainly about over-voltage) – Nick Alexeev Jun 05 '13 at 03:50
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    The math is pretty easy, too. A Schottky 10A diode will have 0.6V or so of drop. If you have two of those, that's 1.2V drop. Multiply by 10A, and you get 12 Watts of power burning a hole in your PCB. For higher-power motor controllers, the numbers quickly get worse. The P-channel protector is much better, but low-loss P-channel devices with high voltage tolerance and high current tolerance are $2 or more, even in large quantities. – Jon Watte Jun 14 '13 at 20:46
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  1. cost

  2. power lost over the diode voltage drop

Applications that really need this protection are more likely to use a series MOSFET or a dedicated power management IC that will protect against reverse voltage, and also monitor current, supply reset lines for brownout conditions, etc.

Phil Frost
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I just thought of another answer:

If the device has a fuse, or is expected to be powered by something that has a fuse, then you can use a power diode biased the wrong way between plus and minus. If the device is hooked up wrong, the diode will conduct, and blow the fuse. This is known as "crowbaring."

This works extra well if the diode in question is a 5 kW TVS diode; it can protect against over-voltage one way, and reverse protection the other way. This idea still suffers from some loss in the fuse, though, so it's not a good idea for circuits where very large power is drawn (such as motor controllers.) For those circuits, you the user are supposed to be engineering capable, as they are "parts" not "products" (and thus, also not FCC certified, covered by consumer protection, etc.)

Jon Watte
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