Several test power supplies

1. Soft start power supply

It is mainly used in occasions where the power supply impact is required to be small. The circuit diagram is shown in Figure 1. Many amplifier designs, whether DIY or commercial circuits, will generate strong impact noise when the power is turned on. This is usually caused by the power supply rising too fast. If it is a high-power amplifier, it is easy to burn the speaker. Of course, general amplifiers have added a delay to turn on the speaker circuit. The solution to this problem is to use a soft-start power supply with a slow start characteristic. In the circuit of Figure 1, the AC power supply rectifies D, filters C1, and the voltage regulator diode DW provides a reference voltage. The minimum output voltage is 0.6V. According to the output voltage requirements, two voltage regulator diodes can be connected in series, and the total voltage drop of the voltage regulator diode can be selected between 28V and 63V. Switch S1 (linked with the main switch) controls the power supply on and off. When S1 is closed, the voltage on capacitor C2 takes about 1s to rise to the working voltage, and the output voltage follows the slow rise. When the voltage-rising voltage regulator diode breaks down, the output voltage is stable. When S1 is disconnected, capacitor C2 discharges through the base of T1, and the voltage drop takes about 5s. If the amplifier design is not very critical to power-off, switch S1 can be omitted.

The voltage on capacitor C2 cannot exceed 80V. If it needs to exceed, it needs to be replaced with a capacitor with a higher voltage resistance. The tube voltage drop of T3 must meet the requirements. It cannot be too large to avoid wasting power and generating heat. Ideally, the voltage drop caused by the tube voltage drop of T3 after the load is connected plus the fluctuation value of the power supply voltage plus 2V. When no load is connected, the tube voltage drop of T3 is allowed to be as large as 10V. T3 must be equipped with a heat sink. It is also best to add a heat sink to T2.

2. Four balanced power supplies

It is mainly used in situations where two sets of positive and negative symmetrical power supplies are required. The circuit diagram is shown in Figure 2. In addition to providing a set of positive and negative symmetrical power supplies (±1), the circuit also has the ability to provide another set of positive and negative power supplies (±2). The voltage of this set of power supplies is higher than that of the (±1) set of power supplies, and the output current is also smaller. As can be seen from the schematic diagram, it is possible to obtain an auxiliary power supply using the main power supply winding.

Assuming the transformer output voltage is 2V and the tube voltage drop of the rectifier diode is negligible, the voltage of the main power supply (±1) is ±√2 v1. In the positive half cycle, C1 charges capacitor C3 through D2. In the negative half cycle, capacitor C4 is charged through D3. Correspondingly, the voltage at points ±2 is ±2√2 v1. The rectifier tube in this circuit uses the IN400 series. According to the amplitude of the output voltage, the value of capacitors C1~C6 can be 100~680μF.

3. One winding can output positive and negative power at the same time

The circuit is shown in Figure 3. The positive power supply circuit is a traditional bridge rectifier circuit. For the negative power supply part, when the diode D3 is turned on, the potential of the positive end of the capacitor C2 is approximately zero. The power supply charges C3 through D3, C1, C2, and D2. In order to ensure that the voltages on C1 and C3 are almost equal, D3 must be always turned on. This requires that the output current of the positive power supply circuit must be greater than the output current of the negative power supply circuit. If the two are equal, a bulb can be added to the positive power supply circuit to serve as an indicator light.