Non-Lexical Vocables PhotoThe real world can be a harsh environment for your project when it's ready to leave the work bench.  Circuits with an external power supply, whether it's batteries, the 12-volt supply in a vehicle or an AC-DC converter, can be exposed to reversed polarity or voltages outside the expected range.  Many of us have made the mistake of reversing power supply polarity with usually fatal consequences.  Adding some protection to your circuit need not be complicated or expensive and is a worthwhile addition when circuits may be used by others.  This article presents a few methods of circuit protection.  When your circuit is going to be used by others, adding some protection will help to ensure reliable operation.

 

Reverse Polarity Protection	None
Over Voltage Protection	None *
Under Voltage Protection	None *

This is a typical circuit power supply connection.  the power supply could be a battery, a vehicle power supply or an AC adapter.  The circuit may operate directly from the power supply voltage which is the case when using a regulated AC adapter or when operating from raw battery voltage.  Alternatively, the circuit may include an on-board voltage regulator.  In this case, the supply voltage must to greater than the regulated voltage to allow for regulator headroom.

With a circuit of the first type, there is no protection against reverse polarity, over voltage or under voltage.  Reverse polarity is usually deadly to a circuit, as is anything beyond a small over voltage condition.  Under voltage conditions can lead to intermittent and erroneous circuit operation and may cause over-current conditions that can destroy some types of chips.

With a circuit of the second type with a voltage regulator, there is usually no protection against reverse polarity although some regulators do provide this feature.  Within the operating range of the regulator (which does not always greatly exceed the output voltage), a circuit is somewhat protected against over voltage conditions although regulator current power dissipation may lead to overheating in the event of an extended over voltage condition (if the wrong AC adapter is connected for example).  Most regulators offer no safeguards against under voltage conditions.

 

Reverse Polarity Protection	Yes
Over Voltage Protection	None *
Under Voltage Protection	None *
Pros	Simple to implement, low cost
Cons	Diode Voltage Drop, power dissipation

The addition of a simple power diode provides protection against reverse polarity.  The downside of this method is the diode voltage drop of up to 0.7 volts, which will reduce the voltage supplied to the circuit.  This voltage drop may not be tolerable with externally regulated supplies or if the supply voltage does not included adequate headroom for regulator dropout voltage.  The diode also dissipated power equal to the supply current times the voltage drop.  Shotkey diodes will result in a lower voltage drop.

 

Reverse Polarity Protection	Yes
Over Voltage Protection	None *
Under Voltage Protection	None *
Pros	Simple to implement, no voltage drop or power dissipation
Cons	Fuse blows if power is reversed

This method also uses a power diode, this time in parallel with the power supply.  In normal operation, the diode is revesre-biased so no current flows.  If the power supply is connected backwards, the diode conducts, causing the fuse to blow and isolating the circuit.  In normal operation, there is no voltage drop, eliminating the limitations of the series diode method.  The down-side is a blown fuse if the supply is reversed.  A fuse must be used with this method or the power supply will be shorted.  A Polyfuse may be used which will automatically reset when power is correctly connected.

 

Reverse Polarity Protection	Yes
Over Voltage Protection	None *
Under Voltage Protection	None *
Pros	Polarity doesn't matter
Cons	2 x Diode voltage drop and 2 x power dissipation

Using a bridge rectifier allows power to be connected either way - it will be correctly routed to the circuit.  The price of this ease of use is twice the voltage drop of single diode.  If an externally regulated power supply is used, this voltage drop will interfere with circuit operation.  If an on-board regulator is used, enough headroom must be allowed for the voltage regulator dropout voltage.

 

 

Reverse Polarity Protection	Yes
Over Voltage Protection	None *
Under Voltage Protection	None *
Pros	Extremely small voltage drop
Cons	MOSFET must be carefully selected

This circuit uses a P-channel MOSFET in an usual configuration.  When power is correctly connected, the body diode inherent in MOSFETs conducts with a very low on resistance and voltage drop - in the mV range if an appropriate MOSFET is selected.  If the power is reversed, the positive voltage on the gate turns the MOSFET off.  The circuit has minimal impact on a properly connected circuit.  The MOSFET must be selected for low on resistance and should have a logic-level gate.

More information may be found in this Sparkfun forum thread and in Advanced Power Switching and Polarity Protection for Effects by R.G. Keen.  Mr. Keen's circuit adds a resistor but I have tested the above circuit with good results.

 

 

Reverse Polarity Protection	Yes
Over Voltage Protection	Yes
Under Voltage Protection	Yes
Pros	Protection against reverse polarity, OV and UV
Cons	Slightly more expensive than other alternatives

A slight increase in complexity can provide nearly bulletproof protection.  The LTC4365 from Linear Technology not only provides reverse polarity protection. it adds over and under voltage protection.  If the supply voltage is outside the range set by a resistor voltage divider, the output is shut down to protect the circuit.  The circuit won't be harmed by over voltage or provide erroneous outputs because of low voltage.  The -40 - 65 volt working range of the LTC4365 will protect your circuits from the extremes that can occur in "12 volt" automotive systems.  Because the power has to be stable for a period of time before being restored, the LTC4365 also protects against the connection of non-rectified AC adapters or un-filtered DC adapters.  Where reliability is a must, the ~$4 cost of this addition is cheap insurance.

Design Note: Protect Sensitive Circuits from Overvoltage and Reverse Supply Connections

LTC4365 UV, OV and Reverse Supply Protection Controller Product Page

Note: typos in the part number have been corrected.

Conclusion

We may not think about the need for circuit protection until it's too late.  It's all-to-easy to get wires connected backwards or to grab the wrong AC adapter and destroy all your efforts in the blink of an eye.  This is not an exhaustive collection of methods but it's a good start on working methods.

 

* If the circuit has a voltage regulator, some protection against over voltage is provided within the range of the regulator.  Operation at higher-than-expected voltages may cause excessive regulator power dissipation and overheating.  Some regulators may provide protection against under voltage but most seem to pass voltage when the supply voltage is lower than regulated voltage + regulator overhead.  A select few regulators provide protection against reverse polarity.