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Reverse polarity protection circuit. Help with component values?

  1. Mar 16, 2013 #1
    EE is not my forte. Although this little circuit is so simple that I could get myself 90 percent there, it performs such an important function that I would like to get it more like 99 percent correct.

    I want to add the circuit to this board

    I already fried one board. Another is on order and I'm "bench testing" it with a number of SMPS ranging from 15vdc@3.0amps to 20vdc@5.0amps.

    Below is the source of the protection circuit I want to add.
    Is it possible to calculate the correct value/capacity for diode/resistor/pmos fet from the description I've given of the power sources and amplifier used?
  2. jcsd
  3. Mar 19, 2013 #2
    Guess that's a "no" then?

    What if I take a less permanent approach? I'm really only using this for the testing phase, it won't be part of the final project since it will be hard plumbed to an internal xformer.

    So would this temporary circuit protection circuit work ok?

    The concern about diode overheating at 5.0 amps and voltage wasted on the diode wouldn't matter since this module would only be temporary.

    Just a little board between the amplifier and the power source with two terminals in and two terminals out. Will this work to protect against temporary reverse polarity? Would the LED light up to indicate the wrong condition?
  4. Mar 19, 2013 #3


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    Just curious why you don't just use one of these?

  5. Mar 19, 2013 #4
    It adds one more possibility for error between the power source and the amplifier. I mentioned I'm bench testing. A few amplifier modules and half a dozen various SMPS.
    Last edited: Mar 19, 2013
  6. Mar 19, 2013 #5
    Why not just a full wave diode bridge? 1.4V Drop - but pretty much fool proof.
  7. Mar 19, 2013 #6


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    A reversed power diode and a fuse would probably suit you. (Or lots of red and black paint on all your connections and the right coloured wires. lol)
  8. Mar 20, 2013 #7


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    You think the OP would reconsider polarized connectors if they were red and black? :biggrin:

  9. Mar 21, 2013 #8
    Screw it. This turned out to work perfectly as a temporary "bench-testing" failsafe on a separate little board. Correct polarity... all's good. Reverse polarity.... led warning and nothing touches the board (amplifier).

    Once I get everything sorted out and tested on the bench and have decided on connections, power supplies, physical layouts, etc... then the board is no longer necessary and I just hardwire. Set the little reverse polarity protection module aside for the next project.

    Thanks for all the, ahem, help here.
    Last edited: Mar 21, 2013
  10. Mar 22, 2013 #9


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    ahhhh the awesome Anderson Powepole connectors. They are used extensively in my radio work and for general use around the workshop
    "Best thing since sliced bread" ;)

  11. Mar 22, 2013 #10


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    Caveat emptor.
    What exactly were you after, in any case?
  12. Mar 22, 2013 #11
    "quia si nunquam petere"
    (if you must ask you may never know)

    Sophie, do you honestly misapprehend what I've been "after"? Have you, or anyone else responding to this thread taken a moment to Google "reverse polarity protection" to see "what I'm after"? It's not that obscure or esoteric. DC voltage plus and minus into an amplifier board. The board has no protection against reversed polarity input whatsoever. Some boards incorporate this protection via discrete circuitry or within the pre-opamp. The boards I'm testing have neither. One little "oops" and it's bricked.

    I merely want to eliminate the possibility, via the implementation of a single 1.50 cent IC, of encountering one of those "oops" moments while bench testing various power supplies and destroying a board that took two and a half weeks to get here from China.

    What don't you understand Sophie? Have you bothered to absorb the whole thread? How could you have misapprehended my need?

    Bench testing, over the course of time, wide varieties of power supplies and boards. It is ridiculously impractical as seems to have been suggested here), to utilize one polarized type of inline plug set for all circumstances.

    Aw hell. Never mind. We're getting nowhere here really.

    End of thread.
    Last edited: Mar 22, 2013
  13. Mar 22, 2013 #12


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    What I don't understand is how you can get upset that your question didn't receive the rapt attention you obviously expected. You seemed to be complaining that no one happened to be as interested in it as you were or that no one took it very seriously.
    If the safety of your boards is that important to you then write yourself a check list to follow every time you connect them up.
    Is a simple protection diode not sufficient for you? Just feed them using a bridge rectifier on every power rail.
    What do all the thousands of other circuit builders do?
  14. Mar 22, 2013 #13


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    I brought up polarized connectors in a question thinking that maybe you hadn't considered them; as lots of designers use them in all sorts of equipment. However I'll bite. Here's the first hit from your suggested Google "reverse polarity protection" search.


    And the first two sentences;

    Isn't that what sophiecentaur suggested in post #6?
  15. Mar 22, 2013 #14
    Look folks, before you get your panties in a knot, know I consider myself a b- level electrical engineer. I post to a forum, a forum called "Physics forums" with the expectation of fielding advice from something more than b minus level enthusiasts. Maybe I'm just expecting too much. Series diodes and polarized plugs are high school level stuff and I really must apologize if I misapprehended the level of expertise here.

    I was well beyond such suggestions before posting. I wanted advice on implementing a Pmos fet or dedicated reverse polarity IC protection device. I was pretty explicit about that. With all respect, and with apologies if I appear to be insulting anyone by suggesting so, I was hoping for more than the color-coded, polki-yoked, approach. There are many very good reasons for wanting a more integrated and fail-safed approach, or a company like Fairchild Semiconductor, a company that is arguably smarter than the combined lot of us here, would not have decided to invest R&D and tooling into manufacturing such a dedicated IC.

    So they're discussing the need for this chip based on consumer response and field/failure data and you're sitting at the conference table and raise your hand and say "what about just using a damn diode? Or a color coded plugs?"

    And the tumbleweeds roll slowly across the conference table in utter silence from the gallery as all eyes turn toward the most likely to see a layoff in the next crunch.

    Why don't I ever learn. They end up in forums like this with breakfast coffee and a danish, sitting in their sweatpants, still trying to "help".
  16. Mar 23, 2013 #15


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    That's just offensive and quite uncalled for.
    You get out what you put into a forum.You put nothing in (not even civility) and you got nothing out.
  17. Mar 23, 2013 #16

    jim hardy

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    Well, maybe we didn't answer your original question.

    which was:
    I saw your post but honestly I thought it was one of those asking everything be done for you.
    And it seemed overkill for simple reverse polarity protection.
    So I didn't reply.

    The smart-a** reply would have been: "yes".

    I think just a few more words in your request for help would have averted all this unpleasantry.
    You asked the readers to go back and parse out for themselves what were your several questions.
    And to guess why you want to introduce such sophistication into such a simple task.
    Then you blast them for suggesting simpler alternatives.


    The notes below the diagram in your first post completely describe how and why the circuit works.
    I don't think there is anything about circuit values unanswered.

    If you wanted help with picking a Mosfet, which is not calculating values,
    here's IR's selector guide

    clicking "P channel" takes me here
    https://ec.irf.com/v6/en/US/adirect/ir?cmd=eneNavigation&N=0+4294841671 [Broken]

    clicking on IRF9520N , the only one with TO220 because that's a handy package for breadboarding, takes me here
    http://ec.irf.com/v6/en/US/adirect/ir?cmd=catSearchFrame&domSendTo=byID&domProductQueryName=IRF9520N [Broken]

    and the top one is only one still available.
    Clicking its "datasheet" link takes me here

    and we see that it has adequate voltage and current capability for the supplies you mentioned.
    But its Vgs is only twenty volts which your supply can exceed.
    So you'll need to include the zener and resistor as described.


    "A question well phrased is half answered"
    and it wasn't clear just what help you wanted.

    This is generally a very civil forum. When I fail to get something I am after, I ask myself "what should I have done different ? "

    I hoped typing all this would make me feel better, but it hasn't.
    What should I have done different?

    Just typed "Yes".
    Now I feel better.

    So here's an appnote with advice on using mosfets.

    regards, and lighten up

    old jim
    Last edited by a moderator: May 6, 2017
  18. Mar 23, 2013 #17


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    Well said Jim.
    You should work for the UN!
  19. Mar 23, 2013 #18

    jim hardy

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    Thanks sophie.

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