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Repairing Switching Power Supply
Posted on Friday, April 26, 2013   •   Category: Power Supplies


Modern power supplies are known as "switching regulator power supplies." In most switching supplies, the 110 volt AC input is first rectified by two diodes and filtered by a pair of capacitors. This creates two high- voltage sources; one positive and the other negative. A pair of transistors is then used to switch these high voltage supplies across the primary winding of a transformer. This switching action is very fast. A typical switching speed is around 40,000 cycles per second or 40KHz. An integrated circuit is commonly used to control the transistors. This IC not only controls the speed at which the transistors are switched, but also controls the amount of time that each transistor is energized. The output voltage of the power supply is determined by the "on" time of the transistors. If the transistors are keep on for a longer period of time, the output voltage of the supply will rise, while shorter times lower the output voltage. This is known as "pulse-width modulation."


1000W AC Motor Speed Controller
Posted on Sunday, April 21, 2013   •   Category: Motor Controllers


This triac based AC motor speed controller circuit is designed for controlling the speed of AC motors like drill machines, fans, vacuums, etc. The speed of the motor can be controlled by changing the setting of P1 potentiometer. The setting of P1 determines the phase of the trigger pulse that fires the triac. The circuit incorporates a self-stabilizing technique that maintains the speed of the motor even when it is loaded.


1W Long Range FM Transmitter
Posted on Tuesday, April 16, 2013   •   Category: FM Transmitters


Long range, very stable, harmonic free, FM transmitter circuit which can be used for FM frequencies between 88 and 108 MHz. With good antenna transmitter can cover 5km range. It has a very stable oscillator because it uses LM7809 voltage regulator which is a 9V stabilized power supply for T1 transistor. Frequency adjustment is achieved by using the 10K linear potentiometer. The output power of this long range RF transmitter is around 1W but can be higher if you use transistors like KT920A, BLX65, BLY81, 2N3553, 2SC1970 or 2SC1971.


40 Watt FM Transmitter Amplifier
Posted on Friday, April 12, 2013   •   Category: FM Transmitters


Building two stage 40 Watt FM Transmitter Amplifier. RF input power should be between 0.5 and 1 watt. Amplifier is powered by 28V power supply. The diagram shows a 2N3375 driving a 2N5643 but there are many other transistors that will work. I used these two transistors just because they were cheap at the time. If any of the variable capacitors are at full capacitance you can pad them out with a fixed ceramic capacitor of suitable value. Extra capacitance also might be needed on the base of the transistors (i had to add 3 100pF capacitors on the base of the 2N5643). The transistors are bolted to a piece of right angle aluminum which is fixed to the metal chassis to dissipate heat effectively.


XR2206 Function Generator
Posted on Thursday, April 4, 2013   •   Category: Frequency Wave Generators


Presented here is XR2206 function generator with multiple waveform selection and a frequency readout display. The diagram on the right shows the internal workings of the XR2206 in the form of a block diagram. Essentially the chip contains A VCO (voltage controller oscillator), wave shaper and buffer. The XR2206 frequency generator diagram frequency of the VCO is set with a capacitor and a resistor. The capacitor sets the frequency range whilst a variable resistor can be used to vary the frequency in the set range. The frequency is defined by = 1/(RC). For a starting point for the design of the frequency generator I used the test circuit from the XR2206 datasheet. I built this on bread board and experimented with the timing resistor and capacitor and managed to get the frequency up to 4MHz.


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