# Reverse polarity protection on 5-12V input and output

Can someone help me with reverse polarity protection on 5-12V input and output using a using a P-Channel MOSFET?

How can I calculate which MOSFET, resistor, zener diode I have to use?

I use a 5V 10A power supply.

• What is your load current? Where is the 5-12V coming from? Commented Dec 21, 2019 at 22:29
• @SpehroPefhany From a 5V 10A power supply (a.aliexpress.com/UT2h1TfaF). Commented Dec 21, 2019 at 22:39
• Where is the 12V coming from? What is your maximum load current? Commented Dec 21, 2019 at 22:45
• I don't want to use both at the same time. I only want that there is an option to use reverse polarity protection at 5V and 12V. I use a 12V 10A power supply. Commented Dec 21, 2019 at 22:49

A common rule of thumb for improved long-term reliability is to overrate electronic components by 100%. Examples: For a 12 V system, use a FET rated for 30 V or more. For a peak load current of 3 A, use a FET rated for 6 A or more of continuous drain current.

Because your operating voltage range is relatively low (in FET terms), select a "logic-level" FET. Check the datasheet, but they almost always are rated for 20 V Vgs, so you don't need a zener to protect the gate unless the reverse voltage might be way below -12 V.

A power MOSFET has an internal zener diode between the gate and source. Pay close attention to the orientation of this diode in your circuit.

If your system can tolerate having the MOSFET switch in the GND rather than in the supply, an n-channel MOSFET will cost less and perform better. Plus, the circuit is more simple.

You want a MOSFET that turns on with less than 5V on the gate, so "logic-level", for the zener it should be less than the max Vgs of the MOSFET your MOSFET needs to be able to handle the max current draw of the device - perhaps 10A, perhaps less. the resistor is less critical 1K or 10K or 100K makes little difference.

simulate this circuit – Schematic created using CircuitLab

• you should mention that with a suitable part, expect that it is rated for 50A to run cool at 10A, otherwise a big heatsink is expected to prevent 175'C junctions if it is only rated for 10A Commented Dec 21, 2019 at 23:22
• So would this lcsc.com/product-detail/… and that lcsc.com/product-detail/… and a 10k ohm resistor work well together? Commented Dec 21, 2019 at 23:41
• @TonyStewartSunnyskyguyEE75 , a "big heatsink" on a SOT23 part? Commented Dec 22, 2019 at 2:33
• well, no not that part, that's an N channel mosfet, otherwise good though. it has a built-in zener so the the external zener can be omitted. Commented Dec 22, 2019 at 2:36
• @Jasen yes a SOT23 part needs at least 30x it’s 1x3mm size in copper area on 1 or 2 sides per Watt to Commented Dec 22, 2019 at 2:46

These are LED power supplies. I don't know what application you have , but these power supplies do not need polarity protection.

It is the loads that need it.

Also 12V LEDstrings do not need reverse protection because they can handle it.

I suggest you use polarized connectors rated for 10A.

If you only need 1 , these are pretty common and can be paired and cut in half with soldered pigtails at one end and screwed into terminal block on supply. Plan on using tiny tie wraps to tie holes to prevent stranded wire strain and breakage.

• I'm at designing a pcb that includes a esp8266, microphone, and dellas temperature sensor and pir. The power goes into the pcb and not direct into the LEDs. Reverse polarity would may damage the capacitors and other components. I only want to be sure that nothing bad with reverse polarity can happen. Commented Dec 21, 2019 at 23:52
• Normally you choose polarized plug and PCB receptacle to avoid this , like a 4 pin 10A Molex connector once used for HDD's Commented Dec 21, 2019 at 23:54

I wonder why this has not been suggested, but I think you could simply use a diode bridge (section "rectifier") as you would do to rectify AC voltage.

I would use schottky diodes, because they have a very low forward drop and therefore also low power dissipation in a continuous current situation.

• How would the schematic look like? Commented Dec 22, 2019 at 15:44
• @MarlonOtto I added a link Commented Dec 22, 2019 at 17:51