Simple and practical circuit designed using diode forward voltage drop

The forward voltage drop of low-power silicon diodes such as 1N4148 and 1N4001 is generally around 0.6~0.7V. Ingenious use of the forward voltage drop of the diode in circuit design can make the circuit simpler and reduce costs. The following introduces two simple and practical circuits designed using the forward voltage drop of the diode.

Two Examples of Designing Circuits for Diode Forward Voltage Drop

1. Battery voltage indication circuit using diode as reference voltage

Figure 1: Battery voltage indication circuit

The forward voltage drop of a silicon diode is generally between 0.6 and 0.7 V (the specific size is related to the forward current flowing through the tube), and it is very stable and changes very little with current, so it can be used as a reference voltage for a general comparator in the circuit.

In the figure above, two 1N4148 diodes are connected in series to form a reference voltage with a forward voltage drop of about 1.2V (it is difficult to buy a voltage regulator with a voltage value lower than 2V). The emitter of the transistor VT is connected to the reference voltage of about 1.2V. Resistors R1 and RP divide the battery voltage and add it to the base of VT. When the battery voltage is high, the VT base voltage is greater than its emitter voltage. At this time, VT is saturated and turned on, and the LED indicator lights up. When the battery voltage drops to a certain value, the VT base voltage is less than its emitter voltage. At this time, VT is cut off and the LED indicator goes out, indicating that the battery should be charged.

This voltage detection circuit cleverly uses the forward voltage drop of the diode as a reference voltage, so that ordinary triodes can also be used as voltage detectors. Although the accuracy is not as good as the voltage detector made by LM393, the cost is very low and can meet general needs. In the figure above, by adjusting the resistance value of RP, the LED indicator light can be turned off when the battery voltage is lower than a certain value. Resistor R2 is the current limiting resistor of the LED.

2. Simple buck circuit using diode forward voltage drop

Figure 2: Simple buck circuit consisting of diode forward voltage drop

By connecting multiple diodes in series, the DC voltage can be reduced to the required voltage. The output voltage of this voltage reduction method is very stable and the heat generated is much smaller than using resistors to reduce the voltage.

The circuit in the figure above cleverly uses the forward voltage drop of the diode to power a 5V cooling fan. The input voltage of the circuit is a 6V power adapter. VD1 is a silicon rectifier diode with a forward voltage drop of about 0.7V, and VD2 is a Schottky diode with a forward voltage drop of about 0.25~0.3V. The two tubes are used in series to reduce the 6V voltage by 1V. Since the rectifier currents of VD1 and VD2 are both 1A, the forward voltage drop across the two diodes in this circuit is basically stable at 1V regardless of whether the working current of the motor is 0.1A or 0.8A. If a resistor is used to reduce the voltage, the voltage across the resistor will also change when the load current is different, which will cause the voltage across the motor to be unstable.