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Overview The objective of the circuit is to create a loudspeaker circuit to be used in final amplifiers while imposing a time delay mechanism with DC protection from the power supply. Terminology - 4093 – a quad 2-input NAND with Schmitt trigger inputs integrated circuit, generally characterized by small fluctuation in voltage supply, very high impedance, outputs that can sink and source, one output can drive up to 50 inputs, high speed gate propagation time, high frequency, and low power consumption
- 7812 – 3-terminal 1A positive voltage regulator with short circuit protection, thermal overload protection, output transistor safe operating area protection, output current up to 1A, and output voltages from 5V to 24V
Circuit Explanation The operation of the circuit involves an even distribution from AC line while having a 1 sec delay to the transformers of amplifier power supply through RL1 and Rx. The connection delay from final amplifiers is used to avoid the noise that comes from the charging & discharging of power supply capacitors which will provide the control of amplifier output due to continuous DC voltage.
If in the middle of the process, a continuous DC voltage exists, the loudspeaker should be unplugged which opens some transistor in the final stage. The error is visually indicated using LD3 together with a buzzer BZ sound. Finding the proportionality of the circuit gives difficulty where the presence of relay RL3 connected in parallel with the main relay RL2, connects to the amplifier loudspeaker which closes RL2 afterwards. Since RL2 is frequently used, the contacts are degraded by electric arcs during the opening and closing of the relay which results in the development of high resistance due to spectrum of frequencies which degrades the audio. This problem could also affect the other contacts which can be avoided by adding a circuit protection. The supply of the circuit may come from a separate transformer or from an existing coil of the main supply of power amplifier. The values of resistors are proportional to the value of voltage supplied to the amplifier. As the circuit comes into complete operation, the supply falls very fast when the line of network gets interrupted. This will result to relay cut off in the loudspeaker. During the continuous voltage, the loudspeaker is disconnected from the amplifier output until the cause of DC presence is detected. The error is signified as the buzzer sounds and turns on LD3. TR1 regulates the intensity of the buzzer sound but can also be suppressed if the visual indication is not needed. The intensity may change by changing the values of capacitors C7-8. A suitable heatsink should be used on IC2 in the event of exceeding the 15V value while the contacts of RL1 and resistances around Rx/Cx is taken into account since the network voltage is dangerous. This is the main reason for sufficient insulation between the components especially considering the quality of all relays used. Application The concept of the circuit where DC protection and time delay are used is mainly used in power amplifiers where the time delay is provided for 1 seconds before the relay closes while the protection occurs on the presence of continuous DC voltages. Source:users.otenet.gr/~athsam/protection_1.htm
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