Protect circuits from reversed voltage

As homebrewers it all happened to use : you power up your creation for the first time and there is a puff of magic smoke because you reversed the supply voltage. 

To prevent that, homebrewers often use a simple diode to protect there creation. 


This method works, but when currents are higher, the diode will be dissipated some power because P= U * I. For a diode with a 0.6 V forward voltage and a current of 2 A there will be 1.2W. Not bad, but it still . . . . 


A other way to project your circuit is using a reverse diode  parallel to you circuit combined with a fuse. The idea behind this is when the supply voltage is reversed, the diode will sink the current (which is high because of the aprx 0.6V forward voltage) and the fuse will blow. 


The second solution is used in many commercial products, I have seen many blown diodes in those products.  So for low current applications (up to 1A) I prefer the first one because collecting blowout fuses isn't a hobby of mine (also replacing the blown diodes isn't).


You can do it better

much, much better.

Years ago I found a video on Youtube showing how to protect circuits from reversed voltage using a P-Channel MOSFET.   


So using a P-Channel MOSFET the dissipation will be P= I^2 * Rds(on).  For a P-Channel MOSFET with a 0.03 Ohm Rds(on) and a current of 2 A there will be a dissipation of only 0.12W. Which is 10 times less then when using a diode with a forward current of 0.6 V with the same 2A (P = U * I = 0.6 * 2 = 1.2 W).

In short : a P-Channel MOSFET can be used as a very efficient low dissipation reverse voltage polarity protection and I have been use that in my circuits ever since. 


Basic schematic P-Channel MOSFET for reverse voltage polarity protection


Symbol : rpp_pfet_symbol


You have to keep a few things in mind :

  • Uds (voltage drain-source) should be higher then the supplied voltage (higher the better) 
  • Rds (on) should be as low as possible (0.1 ohm is nice but 0.02 ohms is much better because for example at 3A you got a power dissipation (I^2 * R) of 0.9W vs 0.18W).
  • Ugs (voltage gate-source) should be higher then the supplied voltage (higher the better).
    When not, you can use a zenerdiode and resistor as shown in the video above (at 6min) and as shown in the schematic below.


Schematic when using a P-MOSFET with Ugs < U, use a zenerdiode with a voltage << Ugs.
For example U = 30V and Ugs = 15V,  use a zenerdiode of 10V.



 Have fun and save you circuits from reverse voltage.

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