# Reverse Polarity Protection for Your Circuit, Without the Diode Voltage Drop.

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## Introduction: Reverse Polarity Protection for Your Circuit, Without the Diode Voltage Drop.

Ever blow up a circuit because you reversed the polarity of a battery?  Or got one of those pesky center-negative AC power bricks?  Or even carefully connected your circuit to a bench supply, and still got the leads reversed?

Well, I have.  It can ruin your day.

The easiest way to protect your system from a reversed battery is seen in the first image.  Simply put a diode in one leg or the other of your circuit (usually the + side).  If you don't care about your system's efficiency, and you have at least 0.7 V of margin when your DC supply is at its lowest possible value, then this will work fine.  RLOAD represents whatever the heck you're running -- a microcontroller, a coffee warmer, whatever.

However, if you have less than 0.7V of margin in your system, you may replace the diode with a Schottky diode.  This brings your drop down to 0.3V.  Better, but still not great.  For example, if you're running a system from 2 AA cells, you still can't afford the  ~0.3V drop of a Schottky because you may have a chip that requires 2.0V to run, and NiMH cells bottom out at about 1.0V/cell.

So, what's a guy (or gal) to do?

The third image shows a simple trick for reverse polarity protection that has a minimal voltage drop across the protection device.  In this case, you use an N-channel MOSFET on the return (negative) side of your project.

When the DC voltage is connected properly, the MOSFET's gate is pulled high relative to its source, which turns it ON.  In this case, the FET behaves like a low-value resistor.  When the DC voltage is reversed, the gate is pulled LOW relative to its source, and the FET turns off.  Simple as that.  Mostly.

You must choose a MOSFET with a low enough turn on voltage that it's FULLY turned on at your minimum operating voltage, and has a low enough RDSon at your operating voltage that your voltage drop will be low enough for your system to operate.  I show a Si4838DY that will fit the bill for many projects.  At Vgs = 2.0V, it has an ON resistance of only 0.003 ohms! And it can handle up to 60 Amps, so it's effective for low and high current applications.  It's expensive, at over \$3.00 each though (\$1.50 in quantity).   For less demanding applications, perhaps a IRLML6344TRPBF will better fit the bill.  It's only \$0.50 (\$0.13 in quantity).

As your operating voltage goes up, a whole plethora of transistors open up, that may be better suited for your application, and cheaper.  Just be sure to look at the RDSon vs. Vgs chart on your proposed transistor.  Once you learn to peruse the DigiKey search engine and scan the data sheets, you can find transistors pretty quickly.

By the way, carefully check to make sure that you don't reverse the Source and Drain of the transistor!  If you do, the body diode of the FET will conduct during reverse polarity connection, and you'll fry your circuit.

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## Questions

​Hi, thank you very much for all you people that cares about instructions, project and helping others... :)

I have a little circuit ( latching relay coil with NE555), and for maintaining the voltage at 5,5V I use a LM2596 Step Down DC converter. ( that it has diode to prevent current inversion, but I prefer to use also this solution... :)

I use a laboratory power supply that can provide up to 30V.

The latching circuit consume about 40/50 mA with 10 Volts power in.

What mosfet can I use to ensure that I still have protection at 30V with correct polarity and also reverse polarity??? ( I didn't understand well the parameter Vgs, that, with correct polarity, should be equal to V1, right???)

Thank you very much!!! ;)

hey,

I'm a little confused, when I simulate the circuit and inverse the source and drain terminals on the mosfet, it doesn't switch on. I would have thought since the device is bipolar, and the gate voltage is greater than the source, it would have turned on?

They aren't bipolar

I dont quite understand how this works. In my expierence when using n channel mosfets to use switches with low power ratings in high current applications if you place the battwries in backwards it completes the circuit without pressing the button and the mosfet gets uber hot. So whats stopping this from happening in this application?

I'm not sure I understand your question. There is no button in this case. In this case, if you put the battery in backward, pin 1 of J1 is +, whereas pin 3 is -. which means the body diode of the MOSFET between V2 and V1 is reverse biased and won't conduct. In addition, the gate of the mosfet is LOW with respect to V2, which turns the mosfet off. Therefore, no current flow, and no heat.

Caleb, the voltage drop across an ordinary silcon diode, like a 1N4001 [or a large power diode], is dependent [to a degree] on how much current you're pulling through it, so it can easily be more [like 1.1V-1.2V] than just 0.7V that is dropped across it. Like you've said, it all depends on whatever the heck sort of load you're running.

Heh, yes of course. I forgot to mention that in the original post. Thanks. -Caleb

hello, very nice tutorial. i would like your help though just to make sure my choice is the best one. i have a board that works between 3.2 volts and 4.3volts at a maximum current of 12 amps. it would be nice to have the smallest voltage drop possible. should i grab a mosfet? if yes should it be a pnp or an npn? (i didn't understand the "n channel" thing.) thanks in advance, Themistoklis.

5 replies

mosfets come in 'n-channel' and 'p-channel' varieties. BJTs (aka bipolar junction transistors) come in the NPN and PNP types. This circuit definitely requires a MOSFET. At 12 amps, with small (let's say 0.1V) drop, you need a transistor with an RON of R=V/I = 0.1/12 = 0.008 ohms. That's really pretty low, you'll need really thick connectors in your system to keep the overall voltage drop low. But... the mosted I showed in the article, the Si4838dy is only 0.003 ohms when turned on, so that should do quite well for you.

oh and last question. does the gate connect to positive and S/D to the grounds?

BTW, here is a through-hole MOSFET that may be easier to work with: http://www.digikey.com/product-detail/en/BUK951R9-40E,127/568-9860-5-ND/3672467, and should perform as well.

There are 3 pins, the GATE, SOURCE, and DRAIN, labeled G, S and D. you connect the

* GATE to the + input voltage (pin 3 of the power jack in my diagram).

* DRAIN to the - input voltage (pin 1 of the power jack in my diagram)

* SOURCE to the GND of your circuit.

On the transistor, the GATE is pin 4, source is pins 1,2,3, and DRAIN is pins 5,6,7,8. Since this is a high current SMT part, there are multiple pins for the S and D, connect them all for lowest on resistance.

-Caleb

so with a Si4838dy i will be good... thank you very much man... really appreciate it..

Heh, there are two main types of transistor, bipolar junction transistor (BJT), and Field effect transistor (FET). There are many subtypes of each.

The one you linked to is a BJT, subtype NPN. BJTs also come in PNP type (and a few other types as well). http://en.wikipedia.org/wiki/Bipolar_junction_transistor.

The one in this circuit must be an N-channel MOSFET (http://en.wikipedia.org/wiki/Mosfet).

They have different symbols, but often come in the same package, so you need to read the data sheet. If you look at the data-sheet of either transistor I linked to in the article, you'll see the package drawing, and the pinout.

-Caleb

Thanks, guess I'll have to do some more reading up before I tackle that one then.

One of my circuits in particular that I would use this on is my lm386n-1 mini guitar amp. It runs off 9V and the chip can run off between 4V-12V. So for this project the shottky diode or even the silicone diode would be adequate wouldn't it?

Cheers
Matthew

Yes, exactly, sounds like in your circuit, the silicon or schottky diode will do just fine -- you only go from about 9V to about 8.3 in the worst case. This trick us really useful when you really need to scrape every few millivolts.

-Caleb

Good info, thanks.