We have discussed about different types of diode in last post. In this post, we’ll learn about reverse current protection using diode. This is basic but significant application of diode.
Concept: Reverse Current
When voltage at output of the system is greater than input, current flows from output to input through the circuit. This current is known as reverse current. It increases power dissipation in circuit. This may damage internal circuitry, power supply circuitry, cables and connectors. The simplest protection against reverse current is a diode in series with the supply.
Let us consider above circuit. When battery is at connected with right polarity, diode will turn ON and there will be normal circuit operation. When battery is connected with reverse polarity, there is not sufficient forward voltage to turn ON diode. In this case, diode acts as open circuit which breaks circuit path. This lead to protection of load from reverse current.
The sudden change in supply current of inductive load (e.g. relay, motor) generates voltage spikes across it. These (negative) voltage spikes results in flyback, which may damage nearby circuit components. To protect component from flyback diode is used. This diode gives negative voltage signal a safe path to discharge (i.e. spike signal flows through inductor and diode again and again till it becomes zero).
Let’s consider a circuit with conventional diode, 12V relay and a transistor acting as a switch. The decrease in current reduces magnetic field as soon as relay is turn OFF. This change in magnetic field induces a current. The induce current generate high voltage at transistor in absence of diode across relay. To protect transistor, a diode is used called as freewheeling diode. It is also called as flyback or snubber diode.
Diodes Limitation to Reverse Current Protection
Using diode in series with a battery is easy and cheap remedy on reverse current protection. But, there is a downside to doing this. The heat generated in diode while protecting circuit from reverse current may be high enough to blow the diode.
For a conventional diode 1N4007 having VF = 1.1 V and assuming load with current of 1.5 Amps.
So, we have to deal with 1.65 watt heat (wasted power). In electronics design we have to limit the power dissipation to lowest possible value. So, let’s do this more efficiently.
Now, consider Schottky diode MBD101 with typical VF = 0.5V in place of conventional diode. Now,
The higher the forward voltage, the more heat generated. Hence Schottky diode is wise choice for reverse current protection. While selecting a Schottky diode, one should take care of its reverse leakage current. The value of reverse leakage current should be as small as possible. Hence decreasing the forward voltage will decrease power dissipation and with Schottky diode power dissipation will be at lowest value (e.g. 0.2 watt or below).
The diode for reverse current protection is good when there is no issue of power dissipation. We will see some other efficient technique for reverse current protection very soon. This is it for this post. Thanks for reading.