This DC power supply controller is controlled by pulse width modulation PWM, generated by the circuit around timer IC2 7555 according to the formula:
Duty cycle = ((R1 + (VR1/2)/(R1+R2+VR1)) x 100%
Diode D1 is used to bypass resistor R2 and half the effective resistance of the “lower” section of VR1, allowing the duty cycle to be varied from 0.01% to 99.9%.

The output at IC2 pin 3 is fed directly to power MOSFET TR1 to control the load. For loads greater than 3A, a heatsink must be used with TR1.
The DC power controller circuit is powered at 12V, regulated down to 8V by IC1 78L08. Capacitors C1, C2 and inductor L1 prevent noise being fed back into the power line when the load is being switched. Diode D2 prevents the generation of back-e.m.f. when switching inductive loads. It should be changed to a high power version if the load is greater than 3A.

source:http://apowersupply.com/dc-power-controller-30.html

This 120V ac power source is built with a simple 120V : 24V center-tapped control transformer and four additional component. This circuit outputs a clean 200-V pk-pk square wave at 60 Hz and can supply up to 20W. The circuit is self-starting and free-running.

If Q1 is faster and has a higher gain than Q2 is will turn on first when you apply the input power and will hold Q2 off. Load current and transformer magnetizing current then flows in the upper half of the primary winding, and auto transformer action supplies the base drive until the transformer saturates. When that action occuts, Q1 loses its base drive. As it turns off, the transformer voltage reverse, turning Q2 on and repeating the cycle.

Source link:http://apowersupply.com/12-vdc-battery-operated-120-vac-power-source-40.html

In laboratory and experimental work, the source of electric energy is very mportant for the project. Most electronic products require power to function so power supplies are an integral part of any instrumentation system. Sometimes it is difficult to use a power supply when it comes to getting an accurate output value and often the more features on the power supply the more difficult it is to operate. When a power supply has all these different features they also become expensive to purchase.

The programmable power supply (PPS) that I am designing will have several advantages over a typical power supply since it will eliminate the problem of trying to get an accurate output value, it will have fewer features making it easier to use, and a smaller size. The size of this power supply will make it easier to carry and it will save space in the working environment.

Source: http://powersupplycircuit.blogspot.com/2009/01/programmable-power-supply.html