Power & Energy
Reverse polarity protection explained
Reverse polarity protection stops a battery or power supply connected backwards from damaging a circuit. The simplest guard is a diode in series with the supply, which blocks a reversed voltage but costs about 0.7 V. The better one is a MOSFET that the supply itself switches on, which costs only millivolts.
Why backwards is bad
Batteries go in backwards all the time: in the dark, in a hurry, by a child. Everything after them then sees a reversed voltage. An electrolytic capacitor reversed leaks, heats and can burst; chips can be destroyed in an instant. Car electronics face jump leads clipped on the wrong way.
A diode in the way
Put a diode in series with the + wire, anode towards the battery. The right way round it conducts; backwards it blocks, and nothing after it ever sees a negative voltage.
The price is its forward drop, lost whenever current flows:
From a 4.5 V pack, a silicon diode's 0.7 V is a sixth of the supply. A Schottky drops about 0.3 V, better but still something. At 1 A, a silicon diode wastes about 1 W in heat.
Live experiment
Put the pack in backwards
A 4.5 V pack goes in the right way, then backwards, then right again. Guard the gadget with nothing, a diode in the + wire, or a MOSFET in the − wire. The diode keeps 0.66 V for itself; the MOSFET only millivolts. Then fit each one the wrong way round.
“In backwards” is lesson 3 of 6 in Solar Sound, Power & Energy World 3. Free with an account. Lessons unlock in order, so you’ll do the 16 before it first.
A MOSFET that turns itself on
Put an N-channel MOSFET in the ground wire instead:
- source on the circuit's ground
- drain on the battery's −
- gate on the battery's +, usually through a resistor
The right way round, the gate sits several volts above the source, so the MOSFET is fully on: its channel is a fraction of an ohm, and at 22 mA it drops about 4 mV. Backwards, the gate goes negative and the MOSFET is firmly off.
A P-channel MOSFET in the + wire does the same job and keeps the ground wire unbroken. Either way, for supplies above about 10 V, add a zener from gate to source so the gate's thin oxide isn't over-driven.
Other options
- Fuse and diode across the input: a reversed supply drives a big current through the diode and blows the fuse. No drop in normal use, but you replace a fuse each time.
- Bridge rectifier: works either way round, but costs two diode drops.
- Keyed connectors: a plug that only fits one way prevents the mistake altogether.
Laptops, power tools and car modules use the MOSFET trick, because it guards the input while keeping nearly every volt.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“In backwards” is lesson 3 of 6 in Solar Sound, Power & Energy World 3. Free with an account. Lessons unlock in order, so you’ll do the 16 before it first.
Or start from the very first lesson, no account neededPractice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: About 11.2 V. The diode drops about 0.8 V at this load's 110 mA (0.7 V is the usual round figure; more current, more drop). A Schottky would drop about 0.3 and give 11.7.
- Why not “12 V”? A conducting silicon diode always keeps a slice of the voltage, about 0.7 to 0.8 V, so the load can't get all 12 V.
- Why not “6 V”? A diode doesn't share the voltage like a divider. It keeps only its forward drop, under a volt.
- Why not “0 V”? The right way round, the diode conducts. It only blocks when the supply is reversed.
Show the answer
Answer: 0.2 Ω. R = V ÷ I = 0.004 ÷ 0.022 ≈ 0.18 Ω. A silicon diode at the same current looks like 30 Ω.
- Why not “2 Ω”? That's ten times too much. R = V ÷ I = 0.004 V ÷ 0.022 A ≈ 0.18 Ω.
- Why not “20 Ω”? 20 Ω would drop about 0.44 V at 22 mA, a hundred times more than 4 mV.
- Why not “200 Ω”? 200 Ω would drop 4.4 V at 22 mA, almost the whole pack.
Show the answer
Answer: It drops 0.3 V instead of 0.7 V, so more reaches the cell. Both block at night; the Schottky just wastes less of the day.
- Why not “Only a Schottky works”? A silicon diode would work too, just less efficiently. The Schottky is chosen for its 0.3 V drop.
- Why not “It glows when charging”? Neither diode is a light. The Schottky is chosen because it wastes less voltage.
- Why not “It blocks better”? Both kinds block reverse current well enough. The Schottky's advantage is its smaller forward drop.
“In backwards” is lesson 3 of 6 in Solar Sound, Power & Energy World 3. Free with an account. Lessons unlock in order, so you’ll do the 16 before it first.
Questions people ask
What is the simplest reverse polarity protection?
A diode in series with the supply's + wire, anode towards the battery. The right way round it conducts; backwards it blocks. The cost is its forward drop, about 0.7 V for silicon or 0.3 V for a Schottky, lost as heat at every milliamp.
How does MOSFET reverse polarity protection work?
An N-channel MOSFET goes in the ground wire with its source on the circuit's ground, drain on the battery's −, and gate on the battery's +. The right way round, the gate is well above the source and the MOSFET is fully on, dropping only millivolts. Backwards, the gate goes negative and it switches off.
Why does the MOSFET's orientation matter?
Every power MOSFET has a body diode from source to drain. Fitted correctly, it points the way the normal current flows and blocks a reversed battery. Swap drain and source and the body diode lets the reversed battery straight through.
Can I use a P-channel MOSFET instead?
Yes, in the + wire: drain to the battery's +, source to the circuit, gate to ground. It works the same way and keeps the ground wire unbroken. For supplies above the gate's rating, add a zener from gate to source.
Related topics
- How does a diode work?A one-way valve for current: anode, cathode, the 0.7 V forward drop and Schottky diodes.
- MOSFET as a switch, explainedA gate voltage, not a current, opens the channel: threshold, R_DS(on) and the flyback diode.
- How to charge a lithium-ion batteryConstant current, then constant voltage at 4.2 V: the two-stage curve every lithium charger follows.
- Batteries in series vs parallelChain cells for more volts, put them side by side for more hours, and never mix a tired one with a fresh one.