120 VAC Dimmer Lamp Circuit

It is a simple design full wave phase control. This circuit is supplied by AC line. In this circuit is there the four diodes provide a full wave rectified voltage to the anode of a SCR. This is the figure of this circuit;


This is a principle work of the circuit. Two small signal transistors are connected in a switch configuration so that when the voltage on the 2.2uF capacitor reaches about 8 volts, the transistors will switch on and discharge the capacitor through the SCR gate causing it to begin conducting. The time delay from the beginning of each half cycle to the point where the SCR switches on is controlled by the 50K resistor which adjusts the time required for the 2uF capacitor to charge to 8 volts. As the resistance is reduced, the time is reduced and the SCR will conduct earlier during each half cycle which applies a greater average voltage across the load. With the resistance set to minimum the SCR will trigger when the voltage rises to about 40 volts or 15 degrees into the cycle. To compensate for component tolerances, the 15K resistor can be adjusted slightly so that the output voltage is near zero when the 50K pot is set to maximum.
Note; Don’t touch this circuit when it is connected with Voltage AC and Current AC.

25W Hi-Fi Power Amplifier Circuit

25 W Hi-Fi power amplifier circuit is a one of simple design that’s build by transistor NPN and PNP. The output devices are MJL4281A (NPN) and MJL4302A (PNP), and feature high bandwidth, excellent SOA (safe operating area), high linearity and high gain. Driver transistors are MJE15034 (NPN) and MJE15035 (PNP). All devices are rated at 350V, with the power transistors having 230W power dissipation and the drivers are 50W. The amp may also be operated at lower supply voltages for less power, but I do not recommend less than ±18V, which will provide around 15W into 8 ohms. This supply voltage (approximately) may be obtained by using a 15-0-15V transformer. This figure is show the circuit and operation of the circuit.


The supply voltage should be a maximum of ±25V. This supply is easily obtained from a 20-0-20V transformer. All resistors should be 1/4W or 1/2W 1% metal film for lowest noise, with the exception of R9, R10 and R15 which should be 1/2W types, and R13, R14 must be 5W wire wound. The bootstrap capacitor (C5) needs to be rated at least 25V, but the other electrolyc can be any voltage you have available. The potentiometer (VR1) must be a multi turn type, as the current setting is critical. Each of these amps will require a 0.25°C/W heat sink (very large). Consider using a fan or even water cooling to keep temperatures as low as possible.