Reverse Voltage Protection

Reverse Voltage Protection

I‘ve long wanted to pull together some reverse polarity protection ideas in one place. Many are found individually on the web, but seldom several in one place. A recent QRP-L thread prompted me to put together this page. I haven‘t tried all of them, and none are original with me. I‘m just pulling together ideas from others. You‘re on your own for circuit and part selection. Click on the pictures for a larger image if needed. Please email if you wish me to add a circuit, have a comment about the page, or if you spot any errors. Thank you es 72 ... WAØITP

Blocking Diode

Super simple, inexpensive, easy to implement. ~.6V voltage drop if use Silicon diode, .2V drop for a Schottky. Some Schottkys may have high reverse current leakage and be unsuitable for this application.

Full Wave Rectifier

Simple, inexpensive, easy to implement. Can reverse input polarity with no effect. Two diode voltage drop ~1.2 for Silicons, ~.4 Schottkys. Some Schottkys may have high reverse current leakage and be unsuitable for this application.

P Channel MOSFET - PMOS

My favorite. Very simple and easy to implement. Little Voltage drop. Functions like a diode in this application. Choose the pmos device based on current and voltage ratings, AND the on resistance Rds(on) - the lower the better, around .025 ohms is good. Vdrop = I2R   Note that the transistor is reversed in this application, normally the Source would be positive relative to the Drain. For more info on using MOSFETS in this application look about 2/3 the way down on this page An NMOS transistor can be used in the negative lead also, but there may be some issues with this approach.

Regulator - PNP or LDO

Regulators with PNP series pass transistors, and some Low Drop Out regulators employing PMOS transistors work well as RVP devices. Look at the LM2490, LM2931 and KA78RXXC families of PNP regulators. Check the data sheets for recommended capacitor values.

Fuse and Diode

No voltage drop. Simple, inexpensive, Not quite as easy to implement as the previous methods. Install fuze in an accessible place. Diode current rating must be greater than the fuse.

Resettable Fuse

Resets automatically, simple, inexpensive, easy to implement. Sometimes referred to as a PPTC device which stands for Polymeric Positive Temperature Coefficient. This a type of thermistor and has an associated voltage drop, current rating, a delay before tripping, and a reverse current spec when tripped. Choose your device based on these parameters. The diode current rating should be greater than the fuse rating.

Relay with NC Contacts

No Voltage drop. Resets automatically. No current drain until tripped. More complex than the above methods. A few milliseconds of delay, typically ~5ms, until the relay pulls in, during which time the circuit is unprotected.

Relay with NO Contacts

Pulls in when plugged in to voltage source. No Voltage drop. Resets automatically. Relay is continously energized until dropout. Continuous current drain until tripped. More complex than the above methods. Some delay, typically ~5ms, until the relay drops out, during which time the circuit is unprotected.

NMOS with Specialized IC

There are some specialized ICs for RVP also. Some are to be used with NMOS transistors, the LTX4365 being one. NMOS devices can be used in the positive lead but the gate voltage must be elevated above the Source voltage. A charge pump is utilized to get it about 8 volts higher so the transistor will turn on. NMOS transistors do have a very low Rds(on) .

Anderson Power Poles

Fail safe if wired correctly. Power poles are ~$1/pr. Assembly instructions are here. I solder rather than crimp them, and use them for nearly all power connectors.

Anderson Power Pole Checker

While the Power Poles are nearly impossible to reverse, here‘s a little gadget someone mentioned seeing. Use it to check the polarity. Green is good, red means fix it before plugging in a rig.

Uploaded 17January2011 ... WAØITP

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