So if you are adverse to spending money like me and don’t want to fork out for a flexible DC bench power-supply then I may have a decent alternative for you.
A lot of people will have an old desktop computer lying about at work or in their homes that usually get thrown out or horded in the attic.
Well you can put it to good use if you take the power supply out of the back, it is a nice stable fan cooled voltage regulator with potentials of 12v, 5v for CMOS logic applications and 3.3v for those TTL hipsters.
If you don’t have an old desktop you can get a tidy 450W module for around £10 ≈ $15.
You 12v supply is used for various applications including vehicle electronics, a lot of industrial electronics and a lot of COTS actuators such as motors and pumps. (The power output will be more than enough to drive any home automation projects).
Your 5v Supply can be used to charge your phone if you’re willing to cut up an old micro USB cable (make sure you use the correct wires though) and because computer supplies usually have lots of ports you can run multiple devices such as an Arduino or Raspberry Pi at the same time.
If you are feeling adventurous then you could implement you own PWM switch mode supply controlling the output voltage with a potentiometer.
The circuit duagram above shows a simple PWM circuit. The duty cycle of the outputted square wave is controlled by the potentiometer. Because of the DC offset of the 555 timer you will need to compensate by adding resistors to the potential divider circuit to keep the potentiometer output voltage within the limits of the triangle wave output voltage of the 555 timer.
The duty cycle of the PWM wave is the percentage time spend at the 'high' voltage over the overall period of the wave and as such if you control the duty cycle of the wave you can map that to what percentage of your input voltage will be the average output. (12v input, at 50% duty cycle will give you 6v on average).
I have used a P-channel MOSFET as a power buffer for the PWM signal coming out of the operational amplifier. It is P-channel because it will be using the power supply as a reference which means that current will be flowing when the op-amp output is low (keep this in mind).
Then after the PWM output you will need to connect a smoothing circuit, I am using a second order LC circuit to cut the AC components of the PWM signal to give a smoothed average DC voltage from the 12v supply. You could use a first order LR circuit which would have no resonance frequency but less filtering effect.
The size of the components will depend on what frequency triangle wave you are using and where your cut-off frequency is (I suggest a high frequency for smaller components).
See my circuit help index for help with some of these circuits.