Many circuits can benefit from protection against accidental reverse polarity.While most can be protected by polarized connectors to the power source, many hobbyist circuits and kits can be powered by jumper wires, thus eliminating this simple type of polarity protection. In these cases, a reverse polarity circuit would be a very useful addition to your circuit.
In this guide, we will explore three simple methods for adding this protection to your projects. This overview will only cover protection circuits on the high (positive) side of the circuit. Each of the protection methods can also be applied to the low (ground) side. The low side versions of these circuits offer the benefit of using NPN & N-channel devices instead of their PNP or P-channel equivalents, the former which are often cheaper, more readily available and sometimes higher performance. However, low side circuits change the voltage level of the ground path which could cause issues for some circuits.
If you wish to study low side approaches, App Note AN636 from Maxim is a good one to consult. I think for most circuits, the high side versions work very well and will prevent any possible problems with a low side approach. Therefore, we will only be covering the high side circuits. I just wanted to mention the low side in case your particular needs might require something different.
The first circuit we’ll consider is the ordinary rectifier diode. Simply using a diode as shown is often a good approach. Its advantages are simplicity and low cost. It contains only one component costing only pennies.
One of the biggest disadvantages is a substantial voltage drop. Since rectifier diodes typically drop around 0.8 volts, your resulting Vcc will be lower by 0.8 volts. That voltage could also vary depending on the diode, the temperature, and the load.
Another factor to consider is the extra power consumption for circuits with high current loads. Simply multiply the diode’s forward voltage (Vf) by the current you expect to draw, and you can see how much extra power this diode will use. For currents greater than 500mA, you will even need to use a larger power diode.
You can improve this circuit somewhat by using a Schottky diode instead of a rectifier diode. It has a lower voltage drop – usually about 0.6 volts, but you can get some that are even lower than that. There is one potential problem with using Schottky’s though. They have more reverse current leakage, so they may not offer sufficient protection. If you want to try a Schottky diode, you will need to examine its leakage current and your circuit to see if it can handle it without damage. Making such a determination is not very easy, and as we’ll see, there are much better approaches. Therefore, I would shy away from using Schottky diodes in most cases.
A greatly improved protection circuit to a blocking diode can be provided by using a pnp transistor as a high-side switch as shown. The saturated voltage drop across the transistor is much lower than it is with diodes and the part cost is still very modest.
In normal operation, the base is at a lower voltage than the emitter so the transistor turns on. When the circuit is reversed, the transistor is reverse biased and it effectively shuts down the rest of the circuit.
The limitations of this approach is the fact that there is some power loss from the base current, and that loss is constant regardless of the circuit’s current power draw. In circuits where a very low quiescent current is typical, this approach could greatly increase its level.
Also, like the diode circuit, there is still some voltage drop (maybe a couple of tenths), and for higher power circuits the transistor will not be able to handle the high current loads. For circuits which are usually active in their power usage and that draw modest amounts of current, this simple type of protection is hard to beat.
To choose the bias resistor, estimate your circuit’s maximum current and divide by the transistor’s minimum gain. Provide a little margin, and calculate your resistor accordingly. For example, if you expect a maximum current of 100mA for my circuit, and the typical minimum hFE of your transistor is 50, then the base current should be at least 2mA. Let’s use 4mA for the base current to provide some margin. If the supply voltage is 5 volts, then the base bias resistor should be 5v / 4mA = 1.25k or something thereabouts.
For the ultimate in low voltage drop and high current capability, replacing the PNP transistor with a P-channel MOSFET as shown in this circuit, can’t be beat. Please note that the FET is actually installed in the reverse orientation as it would normally – the drain and source are reversed. This orientation is necessary so that the slight leakage current through the FET’s intrinsic body diode will bias the FET on when the polarity is correct and block current when reversed, thus shutting off the FET. Here is a real nice video tutorial of how the magic works.
If the supply voltage is less than the FETs maximum gate to source voltage (Vgs), you only need the FET, without the diode or resistor. Just connect the gate directly to ground. I have found that most smaller FETs maximum Vgs is 12 volts or less, which can be a problem for 12 volt (or higher) supplies. If after checking your FET’s spec sheet, you find that Vcc could exceed the maximum Vgs, then you must drop the voltage between the gate and the source.
The circuit shown does exactly that by a very clever means. By inserting a zener diode with a voltage less than the maximum Vgs, it limits the voltage to a safe level between the gate and the source. You will need to calculate the resistor value so that it will provide enough current to properly bias the zener diode chosen. The zener diode’s spec sheet will provide the minimum current required to achieve the zener breakdown voltage, and you can then calculate your resistor value from that.
Choosing the Best Circuit
Each of these circuits offer a different set of advantages and disadvantages. I have listed them in order of increasing complexity and cost. In choosing what is best for your circuit, examine what your voltage and power needs are. Then match it with the simplest circuit that will suffice for those needs.
For example, if your circuit can handle the voltage drop from a diode, and your circuit is low current, just use a blocking diode. Don’t think that just because the FET circuit is the best in terms of performance, it is the best choice. That performance also comes at with a greatly increased cost and complexity.
Good engineering tries to minimize both of those factors. Choose the approach that meets your design requirements the best.
Please contribute your thoughts and experiences in the comments below.