Technology: Engineers share the design of series-adjusted voltage-stabilized power supply circuit

Figure 1 is a voltage- regulated power supply with adjustable output voltage using a transistor . This circuit adjusts the output voltage by changing the voltage drop of the high-power transistor Tr1 connected in series with the load. The output voltage Vout is determined by the voltage at point A, i.e., Vref+VBE2.

Where Vref is the voltage of the Zener diode (5.1V), VBE2 is the voltage between the base and emitter of transistor Tr2 (0.65V>, and VR1 is a variable resistor. Since the resistance value of VR1 varies from 0 to 5kΩ, the output voltage varies from 7.6 to 12.8V. When the sliding part of VR1 has poor contact, the output voltage will become the minimum voltage.

The maximum power consumption of the adjustment tube Tr1 is 3A×(15V-8V)=21W, so it should be installed on a heat sink below 4℃/W. Since VBE2 will change with temperature and IC2, the voltage regulation circuit and voltage regulation characteristics are not very good.

In the circuit of Figure 1, the circuit does not have an overcurrent protection function. When a short circuit occurs at the output end, the input 15V voltage will be fully applied to Tr1, causing Tr1 to burn out instantly. Figure 2(a) is an overcurrent protection circuit. By adding a series resistor Rs and a small power transistor Tr3 to the emitter circuit of Tr1, the current flowing through Tr1 can be limited when a short circuit occurs at the output end of the circuit. In normal operation, Tr3 is cut off because the voltage drop on the Rs resistor is very small. When the output current of the circuit increases, the voltage drop on the RS resistor also increases. When the voltage drop on the RS resistor (VBE3 of Tr3) exceeds 0.65V, Tr3 is turned on and the VCE of Tr3 decreases. The VBE1 of Tr1 also decreases, so the output current flowing through Tr1 will be limited to a certain set value. In this circuit, the current limit value is 0.65V/Rs≈2A. In addition, the circuit of Figure 2(a) can also be used as a constant current source circuit for light-emitting diodes. When applied, the output end of the circuit in Figure 2(a) is grounded, and the collector of Tr1 is connected to the light-emitting diode. Figure 2(b) shows the current limiting characteristics of the circuit.