Power Protection Circuits

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.

 

none

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.

 

series diode

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.

 

crowbar

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.

 

Bridge Rectifier

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.

 

 

P-Channel MOSFET

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.

 

 

LTC4365


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.


Posted: 2 years 7 months ago by Jon Chandler #9738
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This is a topic you may not think about, but some simple protection against reverse polarity can save you from a moment of in-attention and destroyed parts.

After the ho-hum response about my post about the new LTC-4356 protection chip from Linear Technology, a review seemed in order.
Posted: 2 years 7 months ago by Jon Chandler #9740
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I want to emphasize that some voltage regulators do include reverse polarity protection, but most do not. Don't assume a regulator provides that protection unless it's specifically spelled out.

Here is a example that does, the Micrel MIC5225.

This feature is clearly pointed out on the data sheet.

Posted: 2 years 7 months ago by hop #9743
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Thank you Jon for addressing a very important topic when it comes to our hard earned hardware in the field. Awesome contribution and I added to my circuit notes. YOU ROCK!
Posted: 2 years 7 months ago by Jon Chandler #9773
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Adding protection against excessive current flow may be a good idea in some cases. A typical 5 volt wall wart can supply anywhere from 700 mA to as much as a couple amps, which is far in excess of what most of our PIC circuits require.

If your dev board is on the bench, it may be shorted out if it touches a tool on the bench or if a clipped-off lead lands in the wrong place. Over-current protection will ensure that shorts don't cause excessive damage to your board of circuit.

Some wall warts are self protecting to some extent. If you try to draw more current than the supply can handle, the output voltage drops. But this means drawing in excess of 700 mA, which can be much greater than a simple circuit will draw unless there is a dead short.

A fuse is the usual way to protect against excessive current draw. Fuses are cheap and effective...and a pain to replace if they blow!

For a circuit operating on 5 volts or less and drawing less than 50 or 100 mA in normal operation, Fairchild Semiconductor has an interesting possibility in the FPF2001 - FPF2007 Full Function Load Switches. These chips are designed to be used as controllable load switches. Depending on the model, if the current exceeds 50 mA or 100 mA, the switch shuts off the load. Some versions of the chip will remain off until power is removed. Others will reset after a short delay. An electronic fuse. Pretty nifty.

Data Sheet

There are a couple limitations. The parts will handle a maximum voltage of 5.5 volts, so they can only be used with a regulated supply of 5 volts or less. The other limitation is that the trip point is fixed at 50 mA or 100 mA depending on the model. This will be a nice range for many PIC circuits.

These Fairchild parts don't protect against reverse polarity or over voltage. It would be a good addition following one of the polarity protection methods or the LTC-4356 for the ultimate in circuit protection.
Posted: 2 years 6 months ago by bitfogav #9861
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Jon what about the cheap resettable fuses as were on the subject of circuit protection, like the ones you can find on some of the development boards like Amicus18 and Arduino, the resettable fuses can come in many different current values.

Posted: 2 years 6 months ago by Jon Chandler #9862
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Resettable fuses are definitely an improvement in the crowbar circuit! You don't need to come up with a new fuse if a mistake is made. There are still limitations in this method as I outline in Power Protection Circuits - A Closer Look at the Crowbar Circuit, namely that the power supply must have enough current available to blow the fuse. If it can't blow the fuse, the circuit will be reverse-biased at Vf of the diode. This may be large enough to damage some components.

If I can find that tube of P-Channel MOSFETs I know is here somewhere, I'll expand on it next.
Posted: 2 years 6 months ago by bitfogav #9863
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These type of resettable fuses "above" can be there for over current protection in some cases then?, used to protect your USB circuit on your PC for example.

At my place of work we use automotive resettable fuses - they are a good tool for fault finding


Posted: 2 years 6 months ago by LightningPhil #9888
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Great introduction to the circuit. Had not considered using anything other than diodes. Have always added capacitors and sometimes a bit of inductance to discourage noise though.

Excellent work!
Posted: 2 years 6 months ago by Jon Chandler #9889
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From the comments at Hack-A-Day, here's a circuit using a PNP and an NPN transistor. It's probably not quite as good as the P-channel MOSFET, but it will be better than the series diode circuit and possibly built from parts you have available.

Two Transistor Reverse Polarity Protection Circuit
Posted: 2 years 6 months ago by techmonkey #9894
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What about over current protection? I have been having a hard time a circuit that will clamp the current down like the more expensive lab supplies. My power-fu is very weak.

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