Smooth turning on of the power amplifier. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Smooth turning on of the power amplifier. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches

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This simple device improves the reliability of your radio and reduces network interference when you turn it on.

Any radio power supply contains rectifier diodes and high-capacity capacitors. At the initial moment of turning on the mains supply, a pulse current jump occurs - while the filter capacitances are being charged. The amplitude of the current pulse depends on the value of the capacitance and the voltage at the output of the rectifier. So, at a voltage of 45 V and a capacitance of 10000 microfarads, the charging current of such a capacitor can be 12 A. In this case, the transformer and rectifier diodes briefly operate in short circuit mode.

To eliminate the risk of failure of these elements by reducing the inrush current at the time of initial switching on, the diagram shown in Fig. 1.7 scheme. It also allows you to lighten the modes and other elements in the amplifier for the duration of the transients.

Rice. 1.7 (click to enlarge)

At the initial moment, when power is applied, capacitors C2 and C3 will be charged through resistors R2 and R3 - they limit the current to a value that is safe for the rectifier parts.

After 1 ... 2 seconds, after the capacitor C1 is charged and the voltage on the relay K1 increases to a value at which it will operate and with its contacts K1.1 and K1.2 shunt the limiting resistors R2, R3.

In the device, you can use any relay with a response voltage lower than that acting at the output of the rectifier, and the resistor R1 is selected so that the "excess" voltage drops across it. The relay contacts must be rated for the maximum current in the power supply circuits of the amplifier. The circuit uses a relay RES47 RF4.500.407-00 (RF4.500.407-07 or others) with a rated operating voltage of 27 V (winding resistance 650 Ohm; current switched by contacts can be up to 3 A). In fact, the relay is already activated at 16 ... 17 V, and the resistor R1 is selected with a value of 1 kOhm, while the voltage across the relay will be 19 ... 20 V.

Capacitor C1 type K50-29-25V or K50-35-25V. Resistors R1 type MLT-2, R2 and R3 type C5-35V-10 (PEV-10) or similar. The value of the resistors R2, R3 depends on the load current, and their resistance can be significantly reduced.

Rice. 1.8 (click to enlarge)

The second scheme shown in fig. 1.8 performs the same task, but allows you to reduce the size of the device by using a smaller time-setting capacitor C1. Transistor VT1 turns on relay K1 with a delay after capacitor C1 is charged (type K53-1A). The circuit also allows, instead of switching the secondary circuits, to provide a stepped voltage supply to the primary winding. In this case, you can use a relay with only one group of contacts.

The resistance value R1 (PEV-25) depends on the load power and is chosen such that the voltage in the secondary winding of the transformer is 70 percent of the nominal value with the resistor on (47 ... 300 Ohm).

Setting up the circuit consists in setting the delay time for turning on the relay by selecting the value of the resistor R2, as well as choosing R1.

The above schemes can be used in the manufacture of a new amplifier or in the modernization of existing ones, including industrial ones.

Compared to similar devices for two-stage supply voltage supply, given in various magazines, those described here are the simplest.

Publication: cxem.net

See other articles Section Clocks, timers, relays, load switches.

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