# Best method for reverse polarity protection and without power loss?

Discussion in 'General Electronics Chat' started by Fuji, Dec 22, 2014.

1. ### Fuji Thread Starter Member

Nov 8, 2014
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1
My power source is 9v for the entire circuit. As I undertand, the common way to prevent damages to a circuit would be to use a diode. Another method is a bridge rectifier However, I get voltage drops and lose power with a diode, and twice the loss of power with a bridge rectifier.

Taking to consideration for voltage drops with circuit protection, my question is if there is a possible way to make a reverse polarity protection without any loss of power at all? Or will there always be losses of power no matter what for this type of feature in any circuit?

Last edited: Dec 22, 2014

Jul 18, 2013
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What device are you offering protection for?
You could always increase the supply to compensate.
A single diode the polarity has to be correct, with a bridge it does not matter.
Max.

3. ### Fuji Thread Starter Member

Nov 8, 2014
100
1
Thanks for the reply. It is not a product that I bought. I am building a circuit myself that is powered with 9v. It hit my mind that I wanted to protect the circuits. Right after the power source of 9v, I have a buck booster. I thought maybe protection before the booster would be beneficial.

So what your saying is, maybe increase the voltage by say...1.5volts, which then = to 10.5v just to compensate 9v? Sounds like a good idea.

For 9v power, is a 1A diode or Schottky diode (I assume is better for lower voltage drop) good enough?

Last edited: Dec 22, 2014
4. ### crutschow Expert

Mar 14, 2008
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If you don't want to have to increase the supply voltage to compensate for a diode drop, here's a low forward-drop, reverse-protection circuit using a MOSFET.

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5. ### ronv AAC Fanatic!

Nov 12, 2008
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Or NFET of the same circuit.

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Jul 18, 2013
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The advantage of the bridge the circuit still operates with the wrong polarity applied.
Max.

7. ### LDC3 Active Member

Apr 27, 2013
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Yes, it would be difficult to get more than 0.5A from one 9V battery for any length of time.

8. ### cmartinez AAC Fanatic!

Jan 17, 2007
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Crutschow's right, a p-mosfet is the best way to protect for reverse polarity with minimal power loss. Watch the explanation on this video.

9. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
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Diodes are often fitted in parallel with the supply and after the input fuse to protect against reverse supply polarity. There will be no voltage drop but supply reversal will pop the fuse every time.

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10. ### #12 Expert

Nov 30, 2010
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Many manufacturers use polarized connectors so you can not connect the battery backwards.

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11. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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Or use a schottkey diode in series with the battery

12. ### BReeves Member

Nov 24, 2012
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This is what I would do, cheap, easy and as stated no voltage drop.

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13. ### MikeML AAC Fanatic!

Oct 2, 2009
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Every mobile HF and VHF transceiver (Ham and Commercial) I have seen has a 30A reverse polarity rectifier across the 12Vdc power input the radio with a 8 to 25A in-line fuse in the +12V line. Some also put a fuse in the -12V line too, but that doesn't involve the reverse rectifier.

If the installer does the dumb thing of connecting the power backwards, he would instantly smoke the radio if it were not for the reversed rectifier, which typically has a peak surge rating of ≥~500A, so that it can vaporize the fuse...

Jul 18, 2013
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The bridge is very common on 24vdc Hydraulic solenoids where there is a BEMF diode and a LED indicator used, allows connection without regard to polarity mistakes.
Max.

15. ### ErnieM AAC Fanatic!

Apr 24, 2011
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As Figi is asking for a zero power loss protection circuit then any and all schemes employing a diode are missing the point. A MOSFET will only loose power due to the drain to source resistance, which can be very very small for modern devices.

A MOSFET is a most useful way of dealing with things AS LONG AS THE INPUT VOLTAGE IS LIMITED. If the input can exceed the gate to source voltage then either the method is not valid, or some voltage limiting is necessary. A resistor and zener can work well there in many cases, though it may loose some power to run the zener for circuits constantly running over the Vgs limit.

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16. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
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How is a parallel-connected reverse-biased diode not lossless ?

17. ### crutschow Expert

Mar 14, 2008
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I'm not sure that the cost of a diode, fuse and fuse holder is less then the cost of a MOSFET and a resistor. And if the voltage is accidentally reverse connected with the diode, then you will need to find a replacement fuse (and of course we all have those on hand).

18. ### ErnieM AAC Fanatic!

Apr 24, 2011
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Sorry, I was assuming that failure was not an option.

19. ### LDC3 Active Member

Apr 27, 2013
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You misunderstand how the circuit is put together. The diode is parallel to the load and (usually) reversed bias. When the power is connected correctly, there is no current through the diode. If the power is connected incorrectly, current flows through the diode and blows the fuse.

20. ### ErnieM AAC Fanatic!

Apr 24, 2011
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On the contrary it is you who miss the implications of your proposal.

Connect power in reverse, fuse blows, the rest of the circuit is intact.

Now connect power in proper fashion. Circuit does not operate as there is a bad fuse that needs to be replaced.

Thus this circuit accepts failure as an option.