Power mosfet reverse polarity

Discussion in 'The Projects Forum' started by liteace, Jun 18, 2013.

  1. liteace

    Thread Starter Member

    Mar 7, 2012
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    Hi All, another question for the experts out there, I have here a psmn017-30pl, I have

    D: negative switched side
    G: control from 555 timer
    S: ground

    If the 555 circuit was not powered would it cause any damage to psmn017-30pl if the polarity was reversed

    Thanks
     
  2. John P

    AAC Fanatic!

    Oct 14, 2008
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    I assume "negative switched side" means that the terminal (drain) can sink current when the transistor is turned on.

    No, negative voltage won't harm the transistor provided that you don't exceed the maximum gate to source voltage (+/- 20). In fact you can't drive the drain more than a tiny amount lower than the source: the inherent diode in the MOSFET will be forward biased and will conduct current. Maybe that's a problem for your circuit, but it's harmless to the MOSFET, unless the current is enormous.
     
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  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Its an N-fet and has a diode across the D/S terminals for back emf protection( D should be positive S the negative supply), so it would conduct it you reversed the supply on the D/S terminals.
     
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  4. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    As far as damaging the mosfet physically no. But if plus voltage is put on the source of a Nmos and negative on the drain, it will conduct. Due to the 'intrinsic' diode that's part of all mosfets. These diodes are part of the manufacturing of the mosfet and not to be considered as a replacement for a diode needed in the circuit.

    When asking a question about a circuit always include a schematic.

    edit: you guys type faster:)
     
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  5. liteace

    Thread Starter Member

    Mar 7, 2012
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    I call it a switched earth, I dont know if that is the correct way of putting it, it has

    Power > load > D > S > earth

    so if no power was supplied to G would D & S be open circuit

    Thanks
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    For the fet to conduct a voltage between G/S has to be present, then it conducts between D/S....
     
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  7. liteace

    Thread Starter Member

    Mar 7, 2012
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    Will this work, the rev pol is the reverse polarity protection relay and the heater is nichrome wire, I dont think the 40 amp relay is in the correct place or is it

    Thanks
     
  8. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Don't understand why:
    1.Why reverse the polarity of a heater? It makes no difference to a resistor/nichrome wire which way the polarity is.

    2. If your using a mosfet, why do you need a relay? Chose the correct values for the mosfet and the relay is not needed. A mosfet is used in many circuits instead of a relay, both are electronically high current switches, controlled by a smaller signal.
     
  9. liteace

    Thread Starter Member

    Mar 7, 2012
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    The reverse polarity protection relay is there only to protect the heater timer circuit, I will need some sort of relay as this will draw about 12-15 amp at start-up and that is a little to much for the small switch, is it better to remove the relay and do it like this

    Thanks
     
    Last edited: Jun 19, 2013
  10. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Your reverse polarity function can be as simple as a diode. There are many diodes that can take that amperage. I'm not trying to put you down, but, maybe if you explain what your wanting to do, you can get help to do it simpler. Like all things in life, in electronics simpler is usually better. And there are many here that are willing to help.
     
  11. liteace

    Thread Starter Member

    Mar 7, 2012
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    Thanks but I read somewhere that you get a slight voltage drop through a diode, is that correct or not, if not as you say I could use a diode on the heater and the reverse polarity protection relay on the heater timer control, would that work ?

    Thanks
     
  12. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Yes, there is a slight voltage drop. It varies depending on the type of diode. A schottky diode is the type to use, ~0.4V to 1V drop. Is this a circuit that will be disconnected a lot? That would be the only reason to worry about reverse voltage protection, that I can see.

    You are getting generic answers to generic questions. Knowing what your trying to do would get better answers, but it's your call.
     
  13. liteace

    Thread Starter Member

    Mar 7, 2012
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    I dont understand the part "Knowing what your trying to do would get better answers" Im sorry if Im not coming across to clear. What Im trying to do is protect a small heater control circuit that I built using a 555 timer from getting fried if its connected the wrong way round, as its has a nichrome wire as the heater & the circuit to control the heater I would like it to run on 12 volts and not 11 volts like it would do if there was a diode in there
     
  14. Meixner

    Member

    Sep 26, 2011
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    Place the protection diode in the line that powers only the 555.
     
  15. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Well block diagrams are just that, block diagrams. They are not schematics. If you were to look at what you posted, (not having designed it yourself), would you know what the person was trying to do?

    As far as a heater or for that matter a resistor, which is what a "heater" is, it doesn't make a difference whether it is 12V or 11V. A heater works on amps not volts.

    And once a circuit is wired correctly to it's supply, why is reverse voltage a problem? Things that are/need correct polarity and need to be disconnected from a supply usually have polarized plugs on them, not loose wires that can be connected incorrectly.

    As to this thread have you ever given the total amount of information, that you have given in this quoted post? This is the internet, and people can't read your mind. Or see your work bench. Did you notice that people stopped responding to you once you stopped giving information? Except dummies like me?
     
  16. wayneh

    Expert

    Sep 9, 2010
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    Well, it makes no practical difference and your point is correct, but it does make a small difference. With the diode in place, the heater would dissipate about 95% as much power as without it, and the diode the remaining 5% (0.6 out of 12V).

    A schematic or even a photo would shorten this thread to a post or two. As shortbus notes, it gets hard to invest much effort in a thread when the OP can't be bothered to communicate clearly. That usually means a supplying a schematic or a picture, both are truly worth the thousand words.
     
  17. liteace

    Thread Starter Member

    Mar 7, 2012
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    Sorry guys if Ive upset you in any way, I still cant understand why Im getting beaten up here. I have built this:

    http://forum.allaboutcircuits.com/showpost.php?p=591596&postcount=19

    This will be connected every now and again to a 12 volt vehicle battery, at some stage either me or someone will connect it the wrong way round even know its got 1 big red & 1 big black croc clips on the end of the wires, so all im looking to do is protect is some way without any voltage drop
     
  18. BillO

    Well-Known Member

    Nov 24, 2008
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    I don't think anyone is trying to beat you up. The schematic that you supplied will make this much easier.

    You only need the diode between the B+ coming from J3 to the rest of the electronic circuit elements (the chips). That will protect all the electronics from a reverse polarity hook-up. I would suggest a Schottky diode of about 500ma, but a regular silicon rectifier like a 1N4001 should work just fine. I would also put a switch between J3 and J1 to keep power off the heater until you verify the correct connection otherwise the MOSFET diode will get toasted. Add a small LED between J3 and GND with a 1K resistor as a polarity indicator.

    Let me know if you understand this or need a little circuit diagram.


    BTW, a '12V' vehicle battery supplies nominally about 13.2V. This whole circuit is not really voltage sensitive and I can see it working effectively over a a fairly wide range. Don't worry too much about what the actual voltage is. Of course, the lower the voltage, the longer the heater will take to get to the desired temperature, and vice-verse, but it will otherwise work just dandy from about 11V to 14V. So putting a 30 amp rectifier in series with the whole shebang would not unduly affect operation and might be the simplest thing to do. That rectifier would need a heat sink though.
     
    Last edited: Jun 22, 2013
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  19. liteace

    Thread Starter Member

    Mar 7, 2012
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    Thank you very much, yes a little circuit diagram would be great if poss.

    Thanks again
     
  20. BillO

    Well-Known Member

    Nov 24, 2008
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    Okay, have a look at the schematic.

    Each scenario is a modification to the circuit you showed. The terminals, J1 to J4 are identified and the added components only are shown. Everything else remains the same.

    In Scenario 1 the full voltage is applied to the heater. SW1 is not closed until proper connection to the battery has been verified. The Green LED will light if the connection is correct, the RED LED will light if it is not correct.

    In Scenario 2, the 30A rectifier will prevent any damage if the thing is connected incorrectly, but it will rob about 1V from going through the heater. The rectifier will need to be mounted to a heat sink that will allow it to operate while dissipating about 15W.
     
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