The circuit protection output of power amplifiers and loudspeakers, dispose certain interesting elements, as the isolation of loudspeakers from the exit of amplifier, when is presented continuous voltage in his exit or when the temperature of heatsink, goes up excessively, providing simultaneously and time delay in the connection loudspeakers in the amplifier, so that we avoid pass in them, the known annoying noises from the charge - discharge of capacitors of supply. It’s constituted by a binary comparator [ IC1 ], the transistors Q1-2 and indicative LED D5-6. The supply of circuit can become from positive voltage [ Point A ] of mainly power supply, which is stabilised by the D3 and the R17, in + 15V. Point B is connected in one of the secondary AC coil of main transformer. If close the main switch of main line AC, then a AC voltage (in secondary coil of transformer), is presented in the point B, it’s rectified from the D2 and it supply with negative voltage, via the R9, the Q3, this cut off and it begin charge the C4 via the R10-11. As long as time therefore last the charge of capacitor, the input [+] of comparator IC2B is found in low level concerning input [-]. The exit of IC2B, has low level, therefore the Q4 remain in cutt off and the RL1 remain deactivate, the D6 he is ON. Just the C4 charged, change the situation of IC1B, is activated the RL1, the loudspeaker are connected in the amplifier output, the D6 it OFF. When it’s interrupted the supply, all the process is inverted, and the loudspeaker disconnect, without pass annoying noises. If as long as it work the circuit, is presented problem of continuous voltages in the exit of amplifier then turn off of RL1 and protected the loudspeakers. This become with help the Q1-2. The acoustic signal from the exit of power amplifier, is led to point D, the alternate voltages is led to the ground via the C1-2, that create a not polar capacitor. Continuous voltages, bigger than + 1.7V or smaller than –4.8V, activate immediately the Q1 or the Q2, respectively. With the activation of transistors goes down the level of input [+] of IC1B, so that turn off and the RL1. A other section of protection that for us provide the circuit is the thermic protection. This become with the help of sensor temperature the RTH, which is a resistor of type PTC (positive factor of temperature), and is found placed above in heatsink, where are found also the power transistors. Her price increase, with the increase of temperature, until the potential in input [-] of IC1A, goes up above from the level of input [+], which is determined with the voltage divider R2-3. As soon as the level in input [-] exceed the level of input [+], the exit of IC1A, it return in low level, compelling and the IC2B to change situation, turn off the RL1 and to turn on the D5, that show the thermic protection. In the circuit the limit above which it exist clue of thermic protection they are 70?C. If it’s presented instability, in this stage, in operation of RL1, should be changed the R4, with other of smaller price. The circuit was published in magazine “Elektor” 12/95.
Circuit diagram
Part List
- R1-2=27 Kohms
- R3=1.4 Kohms
- R4=1 Mohms
- R5-14-17=3.3 Kohms
- R6-7=100 Kohms
- R8=47 Kohms
- R9-11=120 Kohms
- R10=470 Kohms
- R12=15 Kohms
- R13=1.5 Kohms
- R15=4.7 Kohms
- R16=33 Kohms
- R18=1.5 Kohms 5W
- RTH=KTY81-122
- C1-2=100uF 63V
- C3=470nF 100V MKT
- C4-5=47uF 25V
- IC1=LM393
- Q1-2-3=BC337
- Q4=BC639
- D1=1N4148
- D2=1N4004
- D3=15V 1.5W Zener
- D4=1N4002
- D5-6=LED
- RL1=24V Relay
