How to protect circuits from reversed voltage polarity!

How to protect circuits from reversed voltage polarity!


In this video I’m going to show you a
couple of ways to protect your circuits from backwards power connections. Let’s say you have a 12 volt power
source like a battery and you want to power something. It doesn’t matter what it is. Let’s say it
draws 2 amperes. When the battery is connected the right
way around everything works fine. But if you’ve had a little too much to
drink and you plug it in backwards… BOOM! Your circuit blows up. The simplest way to protect yourself
here is to use a diode. If the power is connected backwards, no
current can flow through the diode, so your circuit is safe. But there’s a downside to doing this. As you’ll remember from my diode
tutorial, the heat generated in a diode is given by the forward voltage multiplied by the
current. So for example with this 1N5401T diode you’ve got a forward voltage drop of
about 0.85 volts. 0.85 volts multiplied by 2 amperes=
1.7 watts of heat that you have to deal with. And that’s bad. Now let’s try to make that a little more
efficient. Let’s use a schottky diode which will
give us a lower forward voltage. This STPS2L60 should work. At two amperes it has a forward voltage of
about 0.55 volts. The other reason why I chose this diode is
because it has a low reverse leakage current of under 100 microamperes. You have to be careful with schottky
diodes because a lot of them will have high reverse leakage currents of several milliamps, making them a bad
choice for reverse voltage protection. Okay let’s do the math again. 0.55 volts multiplied by
2 amperes=1.1 watts of hear. That’s better than the 1.7 watts of heat
that we had with the regular diode. But we can do even better and not
everyone knows this technique. First, get yourself a P channel MOSFET
like this FQP47P06. Pay very close attention to which pins
are “gate”, “drain” and “source”. Next, put it in your circuit like this. Now when the battery is connected the
right way around the transistor will turn on allowing current to flow through
it. And when the battery is connected
backwards, the transistor turns off, and your circuit gets protected. So you can
be as drunk and irresponsible as you want. This protection circuit is a lot more
efficient and I’ll show you the numbers later in the video. First let’s talk about how it works. Let’s do an analysis when the battery is
connected the right way around. A P channel MOSFET will turn on
when the voltage between the “gate” and the “source” is around -4 volts, or
more negative than that. The exact “gate” threshold voltage will
depend on what MOSFET you’re using. The MOSFET has a parasitic body diode.
So when we first connect the battery, there will be a small voltage drop across
that diode. It will be roughly a 1 volt drop. So 12 volts minus 1 volt means
that the “source” pin is at 11 volts with respect to the circuit’s ground. Now what about the “gate” voltage? Well we connected the “gate” directly to
the circuit’s ground, so the “gate” voltage is 0. So, the voltage between the MOSFET’s “gate” and “source”
is 0 volts minus 11 volts which is -11 volts. And like
I said earlier, since this is a P channel MOSFET if Vgs -4 volts or less, the transistor will turn on. And once it’s on, you don’t have to worry
about that parasitic diode anymore, because the resistance between “drain” and
“source” drops to almost nothing. Now let’s do an analysis where the
battery is connected backwards. Since we’re keeping the ground symbol in
the same place, that means that this node in the circuit
is 12 volts less than ground. So it’s -12 volts. And just like before, the “gate” is at zero
volts. Now, you know the MOSFET is supposed
to be off in this situation because I told you so. But how do we prove it? Well, let’s pretend for a second that the
MOSFET is on. If the MOSFET is on, the resistance
between “drain” and “source” is almost zero. So that means that the voltage at the
“source” is going to be almost the same as the voltage of the “drain”. So we’ve got -12 volts at the
source assuming that the MOSFET is on. Now let’s calculate Vgs again. Vgs is zero volts minus -12 volts so Vgs is 12 volts. Wait a minute… Vgs has to be -4 volts or
less for the MOSFET to turn on and we just calculated Vgs to be
+12 voltswhen the transistor is on. What we’ve got here is an impossible
contradiction. So our initial assumption of the MOSFET
being on has to be incorrect. So what this means is when you’ve got
the battery connected backwards it’s impossible for the P channel MOSFET to turn on
so your circuit is safe. Now let’s talk about the power loss in
the circuit. Earlier I mentioned that when the
transistor is on the resistance between “drain” and “source” is almost nothing. But to be more accurate, the resistance is a parameter called
“Rds(on)” and you can find it out from the transistor’s datasheet. The exact value will vary depending on
the “gate” voltage and temperature but for the sake of simplicity you can
just estimate it with the maximum value they give you. So let’s say it’s 26 milliohms. When you have steady DC flowing
through the MOSFET the power loss is given by current*current*resistance. So 2 amperes multiplied by 2 amperes multiplied by 0.026 ohms
=0.104 watts. That’s seventeen times less than what we
had with the initial diode. So now you can design more efficient killing machines. Finally I want to talk a little more
about how to choose the right P channel MOSFET for your application. The first and most obvious thing is that
the “drain-source voltage rating” has to be higher than your power supply’s
voltage. In this video I used a 12 volt
battery so 60 volts is more than enough. Next, you want “Rds(on)” to be as low as
possible to minimize the resistive power losses. Finally, you need to pay very close
attention to the maximum “gate-source voltage”. One of the reasons why I chose the FQP47P06
for the video is that it has an unusually large “gate-source”
breakdown voltage range. +/- 25 volts. However, other MOSFETs may have a maximum
“gate-source voltage” of only 15 volts or less. So here’s how you can get the P MOSFET to
work with a higher voltage input. Let’s say our input voltage is exactly
30 volts. If we add a resistor and a 10 volt zener
diode to the circuit we can clamp the “gate-source voltage”
to a maximum of -10 volts. So now the transistor is safe. Something like this 1N5240 would do the trick. It doesn’t have to be exactly a 10 volt zener diode…
something in the 6 volt to 12 volt range would probably be fine depending
on the MOSFET. Alright you are now an expert at
reverse voltage protection! Check out my other videos for more stuff
about electronics!

100 Replies to “How to protect circuits from reversed voltage polarity!”

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  2. I have question related to audio signal in speakers.
    I have two amps, one is a tube amp with mono speaker out and other one is an IC amp with stereo speaker out (Both has around 5-15 W and 4ohm). And I am planning to connect both into one pair of speakers. Then I realize it won’t like that. Now I am thinking to put a 2 way (250v 10A) switch in the main voltage of the amps (250v). So that, it won’t make any flow back current. So, my question is, do you think audio signal will pass through the speakers to the other amp if one amp is not working and make a flow back current to the IC or the tubes even I put a 2 way switch? Do you have solution for that, if it going to happened? Or should I put a 6p DPDT toggle switch in the speaker outs instead or together.
    Hoping for a response, thanks in advance.

  3. Nicely presented, easy to understand and not at all confusing. Very pleasant voice too – good all around!! Now for a question… Is there any way I can add a circuit to a device I am building which can not just protect it from revere polarity, but which would automatically 'correct' the polarity. That way it wouldn't matter whether the tip was + or -. I considered using a bridge rectifier, but saw the problem with that with the heat generated.

  4. The 100k resistor is just limiting the current through the zener diode to protect him from a large current that could burn the zener diode ?

  5. Only one person, (Cybersmythe), in this list of 645 comments has suggested the standard, simple and obvious alternative to this over complex solution to reverse polarity protection. A FUSE in the feed and a reverse connected diode across the circuit .

  6. Thank you for the awesome tutorial. I already fried a Sabertooth 2×25 dual motor driver and I wasn’t even drunk! I will try to build a idiot proof protector base on your video and can you recommend the P mosfet for my project? Th power source will be two 12 volt, 35 mAh each. The motor driver I am trying to protect is the Sabertooth 2×60 motor driver. Thanks in advance for your advice.

  7. While its a good video I think the best protection is a physical one. Just make your power source unable to plug in backwards. Also the diode is more cost effective and fine for most DIY projects if the user is worried but its great to know other options as well and thats where this comes in.

  8. Hi, I have a question. Can I use the pmos technique in any converter(ex: buck,boost) to replace the diode? But it usually has an output capacitor to stable my output voltage. Then when the main switch of the converter is off, the output capacitor will give a voltage to pmos's source-drain. Then it seems the pmos will be on at anytime in the converter. Did I get anything misunderstood? Or isn't use under the situation I'm talking about? Thanks.

  9. I just put together a circuit to be used to test automotive power door lock actuators. They are 12vdc reversible brushed motors. I used a DPDT monentary switch with jumpers across the outside terminals and get reversed current when actuating the switch, which powers the small motors just fine.
    I decided to add diodes to each output terminal of my project box to light up when the + switched was to either terminal. It works fine by itself, but when connected to a functioning motor get a flash of the negative side LED either way I switch my current? I've tried adding diodes in each side of the circuit, first just to the LED connections and in the main lines running to the motor. I don't know enough about circuits to figure this out. Is there any info available that might show a circuit like this?

  10. it's weird. when i run it in simulation I get a 0.8V voltage drop over the mosfet with a 300mA current. Ron is 0.034ohms acording to ltspice specifications, Cannot make sense of it

  11. To complete the analysis, you can show that the power lost by the 100Kῼ and diode is far less than the 1.7W across a forward-biased diode; power dissipated by 10V across a 100Kῼ = 0.4 mW? That's around a 3 decade improvement! Excellent by any standard.But at that voltage, the 100Kῼ only allows 200uAmps through the Zener. That might not constitute a high enough keep-alive current for the diode to work. At any rate, that was nicely done. Thanks for the crystal clear sequence of analysis.

  12. Thank you for the tip!
    I’ve just used these bad boys for some projects I’m working on and they work like a charm : https://es.farnell.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10001&langId=-5&urlRequestType=Base&partNumber=2317617&storeId=10176

  13. Really awesome video! I've been bingewatching your channel lately and like it lots!
    Question: one could also use bridge rectifiers to do the work when you deliberately want to let current flow with either polarity as DC, right?

  14. Honestly, the only thing I really understood was at the very very end when u said "You are now hereby granted with powers of reverse voltage protection!" 😬👍

  15. Out of curiosity, if the positive end of batteries is marked on schematics as the flat side, with the button on the negative end, why are all batteries the other way around? Also, using this same deal, you can use a cheap low amp switch between gate and ground and solve two problems for the price of one. Adding a switch to a high current load without using a bulky switch, and provide reverse battery protection at the same time.

  16. How about when using this with a L7805 voltage regulator? There is a capacitor on Vo that would potentially still be charged upon disconnection and reconnection. I have read that using a bleed resistor across Vo and Gnd would work, but if you could better explain that would be very helpful

  17. better way : FBR ("FoolBridgeRectifier." lol..). works in either way its input being connected to a source voltage. or an efficient LowDrop-DC regulator (which also protects a circuit from being reversed finally, at the cost of needing a sufficiently higher input voltage to clamp a supply voltage down to an intended value for the circuit)

  18. Great video, very good teacher. Do you have one on limiting power to load. The trend in my field is to limit power to just what the load needs to operate so no fuses are required in that line. Say a 24vdc solenoid at the end drawing .385 amp(60-90 ohms). In this case if a short to ground should happen it doesn't burn or heat anything up bc it's not getting the full 16g wires current flow. Thanks again for your great videos and your insight.

  19. One of my neighbour has bypassed from my inverter connection. They got main,neutral and inverter connection. Is it due to reverse diode in the socket. How can I isolate it. Pls help……

  20. I am thinking if we could use a relay here. If we deal with tens of Amps – relays are more efficient

  21. Awesome tutorial, very clear and definitely not a waste of time, like many others out there! Thanks a million!

  22. I would like to see some explanation of why you call a voltage negative in the video. Depending on how you hook up the meter, the voltage will appear positive or negative; however, I am sure that there is some basic assumption going on here that is not mentioned in the video but expected to already be understood by the viewers. Then again, if I knew what the assumptions were and what they applied, I probably would not need to watch this video in the first place. So for me it was interesting and informative in a way; but essentially impractical.

  23. One of my neighbour has bypassed connection from my inverter… He is running water motor and induction heater through when there is power supply and else they use inverter to run fan and bulbs when there is a power cut off. Pls help how can I resolve it….

  24. Great video! Can I use this as reverse polarity protection for 5v or 3.3v? I'd also consider adding a Zener diode / resistor, although I don't see myself using more than 5V on the circuit. What would the resistor values be for a 5V max circuit? Thanks!

  25. Hi there! This is an awesome video really. But I think there is a mistake that the body diode direction should be from source to drain i.e, anode should be connected to source and cathode to drain. Justify this?

  26. Good stuff. I've seen a few of your videos like this one. You don't overload them with information the average hobbyist doesn't need, but you don't leave it so vague that a part timer might still get it wrong.

  27. Does the zener diode protecting the PFET have negative effect of leakage current? Even if the polarity is correct. This may have big impact when battery capacity is small such as Li-poly battery, or hybrid vehicle that concerns leakage.

  28. Thanks for this. I've been trying to figure this out and this video put all I have learn in place. Thanks for all the hard work.

  29. Don't even imagine any different use of such transistor. Just imagine if in such circuit you make PWM on gate. In this case, when the MOSFET closes his internal diode still conduct current…

  30. One question, at 3:00min if we have 11V already on S, does that mean the mosfet is already conducting(independent of gate voltage)? It is confusing, because we need to apply negative voltage on gate in respect to S to turn on the mosfet right?

  31. Can you place the schottky on the VDC return side? I always see it on the supply side when schematics are drawn, but I can't think of any reason it wouldn't work on the return side also of say a simple 12 VDC circuit running a few LEDs and a resistor.

  32. Great video, no god-awful background music (which is always too loud), or sound effects. Thanks Afrotech, you do good work.

  33. Like the video very much. It explains in clear and understandable English hot to do it and the cons of every other possibilities. Thanks for the video

  34. I love coming to your channel and videos to "wrap up" my understanding of various topics. I'm newbie and just getting back into using p-channel mosfets. I had an ok understanding of how to read the p-channel data sheet to choose the correct transistor for my app however after listening to this it makes complete sense – I wish all my teachers were like you "back in the day" (or at least I wish they had youtube when I was in college lol) – thank you!

  35. Great tutorial… usually, most diodes have a 0.6v turn on value and that's from reverse leakage … my favorite is LM 317, will regulate from 3.5 to 35 Volts as long as there is 1.5v across the drain and the source. :o)

  36. Great video Afrotechmods! I also love your videos on udemy! Just awesome!thanks for sharing!
    A qst please is there a smart way to filter the best diode or mostfet for one's needs? Besides going through endless listing of specs?

  37. Thanks for the video.

    I'm a designer, but not trained or versed in electronics. I am looking for an electronics engineer in my area (Sarasota, FL) to hire for an IC design, so I can then build a working demo circuit. Having a tough time finding reliable, trustworthy candidates.

    Do you know of a forum online for electronics engineers?

  38. Am I glad I stumbled on this tutorial! What a great trick. I never thought about that one. Furthermore, one could do the same with an N type MOSFET by just connecting the drain on the minus side of the power source.

  39. Very intuitive explanation. You make excellent videos!! Please make more! One of the channels on YouTube that does make sense and is very educational down to the point you want. Lots of good examples and case scenarios.

  40. I would like to know which circuit (the most efficient) to raise the voltage at the gate of a mosfet, type 4v to 10 to 12V, because this voltage would be ideal (according to their datasheet) for its acceptable and satisfactory performance. So I would like to know how to create a CIRCUIT DRIVER preference with TRANSISTORS AND NOT WITH CI´S, so are you doing this? you could either do a video lesson about it or explain to me or post some picture of some circuit that would do it (preferably with transistors) BUT if you have no way, it could be with CI´S DRIVERES EVEN …. I will be waiting for your answer , thank you

    ENDEL NEIVA

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