Power amplifier cooling fan control device. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Audio equipment

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The principle of controlling the UMZCH forced cooling fan with a small heat sink is that the airflow turns on when the signal level at the amplifier output is exceeded, so the fan noise at low power is practically inaudible. A device with a fan can also be recommended for installation in amplifiers of a conventional design (with natural convective cooling) that are in difficult operating conditions.

When designing audio frequency power amplifiers with an output power of more than 20 W, forced heat removal from powerful transistors and UMZCH microcircuits is now often chosen, which can significantly reduce the cooling surface area. To do this, use fans from computer power supplies and cooling fans for system units of personal computers. These fans have relatively small dimensions (80x80x25 mm), low cost and are always available in any computer store.

Typically, cooling fans can either be on all the time, or turn on when a certain heat sink temperature threshold is exceeded. Both methods of inclusion have their drawbacks. In the first of these, continuous fan noise inevitably interferes with work. At a sufficiently high volume, this noise is inaudible, but at a low volume and during pauses during playback, it is heard very clearly. Over time, as a result of the wear of the fan bearings, the level of noise generated by it only increases. Turning on the fan, which depends on the temperature of the heat sink, also has a drawback - at high output power, the heat sinks heat up and the cooling system turns on, but when the volume and, accordingly, the power decrease, the noise of the fans will be heard, although forced cooling is no longer required, natural air circulation is enough.

The described device is free from these shortcomings and turns on the cooling fans when the set threshold of the amplifier output power is exceeded.

Fig. 1

The scheme of the device is shown in fig. one.

Chip DA1 contains two independent comparators. On the first of them, a node is assembled that determines that the output power of the amplifier exceeds a certain threshold level, and on the second - a node for delaying turning off the fan.

The signal from the output of the power amplifier is fed to the inverting input of the comparator DA1.1 through the resistor R1. The zener diode VD2 protects the input of the comparator from the negative voltage coming from the power amplifier when amplifying the negative half-cycles of the signal. On the elements R2 and VD1, a parametric stabilizer is assembled, which sets the threshold for the operation of the comparator. Resistor R3 serves as the load of the output stage DA1.1, made according to the open collector scheme. Capacitor C1 and resistor R4 set the delay time for turning off the fan. Diode VD3 is necessary to prevent the discharge of capacitor C1 through resistor R3. The delay allows you to keep the voltage on the fan for some time to remove the energy released on the heat sink. The tuning resistor R5 can be used to adjust the turn-off delay time. The signal from the output of the comparator DA1.2 controls the transistor VT1, which turns on the cooling fan.

Consider the operation of the device when the signal level changes.

If the voltage at the output of the power amplifier is less than at the zener diode VD1, a high level operates at the output of the comparator DA1.1. When the voltage at the input of the device exceeds the voltage at the zener diode VD1, a low level will appear at the output of the comparator DA1.1 and the capacitor C1 will begin to charge through the diode VD3 to the supply voltage. While the voltage at the non-inverting input of the comparator DA1.2 is less than at the inverting one, the voltage at its output is low, the transistor VT1 is open and the fan is turned on.

As soon as, after the signal level decreases and the voltage capacitor is discharged at the inputs of the second comparator, it is switched, a high level appears at pin 7, the transistor VT1 closes and the fan turns off. The output power of the power amplifier at which the device will operate can be calculated by the formula

Рout = (Uvd1)²/Rn

where Uvd1 is the stabilization voltage of the zener diode VD1; Rn is the load resistance of the power amplifier.

Fig. 2

All parts of the device are placed on a single-sided printed circuit board made of foil fiberglass. The drawing of the board and the location of the elements are shown in fig. 2.

The device uses resistors MLT-0,125 or similar, capacitors - K50-35 or similar imported ones. Diode VD3 - any silicon series KD503, KD521, KD522. Transistor VT1-KT816 with any letter index. The fan must be designed for a constant voltage of 12 V and a current consumption of not more than 0,5 A. When connecting fans with a current consumption of more than 150 mA, the VT1 transistor must be installed on a small heat sink. The supply voltage can be increased to 24 V, but at the same time, a quenching resistor of the appropriate resistance must be connected in series with the fan or two fans connected in series must be used.

Establishing the device is reduced to setting the trimming resistor R5 to the required delay time for turning off the fan.

For a more correct operation of the device, it is recommended to connect a capacitor with a capacity of up to 0,01 uF in parallel with the Zener diode VD2.

Author: A. Zhurba, Taganrog, Rostov Region; Publication: radioradar.net

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