IntroductionA bench power supply is an essential tool for any electrical engineer to have in his electronics lab. A power supply is the kind of test equipment that you can and should build yourself. Building a power supply is actually quite simple and it is an instructive and fun project to get started. There are different types of power supplies out there all the way from a simple battery to complex switched-mode power supplies. What we will be building is a so called 'linear regulated power supply'. They are easy to build yet very flexible and can deliver adjustable voltages over a variable range. We are aiming for a variable voltage range up to 20V and a maximum current of 1A. We will also include the possibility for a realtime voltage- and current readout.
Theory of operationIn a linear regulated power supply, a linear regulator is used to obtain a adjustable stable output voltage. Starting from the mains AC voltage (230V, 110V, ...) a transformer is used to obtain a lower-amplitude AC voltage (for example 48V, 24V, 9V, ...). To obtain a DC voltage, a bridge rectifier can be used, in combination with a capacitor to minimize the voltage ripple. This DC voltage should not be used to power external circuitry just yet. The voltage obtained at the output of the bridge rectifier will not be adjustable and the voltage ripple will highly depend on load current. For this purpose, a linear regulator is used. It delivers a stable output voltage with small voltage ripple for any input voltages above a certain value. Furthermore for certain types of linear regulators, the voltage it delivers can be made adjustable.
There are bridge rectifier ICs out there, but we are going to build one ourselves using four diodes. The basic schematic is shown below.
|Bridge rectifier consisting of four diodes.|
|Output of the bridge rectifier.|
DesignThe design consists of two main parts. There is the diode bridge that needs to be designed and the regulator circuit. The diode bridge is very easy, all you need is four diodes. Almost any diode will do, but we will be using a Schottky diode for its low forward voltage drop. During any time period, two diodes will be active simultaneously and will cause two forward voltage drops, so it's important to keep these low. The diodes should also be able to carry the needed current, so in our case this is 1A. The diodes we will use in this project are 1N5822 Schottky rectifiers. We will use a 10000μF capacitor at the output of the diode bridge.
A suitable and often used regulator is the LM317. The LM317 is special in that it delivers a constant voltage reference of 1.25V. We can use this voltage reference to obtain a variable output voltage.
|LM317 and the utility of its constant voltage reference (1.25V)|
|Complete schematic of the regulator circuit.|
ConstructionIn this picture, some of the parts needed for the construction are shown.
On the bottom right is the transformer which I salvaged from an old hifi radio. In the middle is a voltmeter panel which can measure and show voltage or current. Some other parts are a potentiometer, heatshrink tubing, a relais which I will use for my load switch, wires, some connectors a case and a fuse. On the bottom left is a heatsink that will be used for cooling the LM317 voltage regulator. This heatsink has a thermal resistance of 1.85°C/W. In the worst case scenario (output voltage = 1.25V and output current = 1A), the LM317 will dissipate around 20W meaning a temperature increase of around 40°C which is not at all problematic. The voltage regulator is connected to the heatsink with thermal paste for lowest thermal resistance between the regulator and the heatsink and it's leads are brought inside the case.
This is a picture of the circuit board itself. I decided to use just a solder breadboard since this design was relatively easy so it didn't need a custom PCB. At the bottom right is the input coming from the transformer followed by a fuse and a capacitor. The diode-bridge and smoothing capacitor are seen above them. At the left is the circuitry for the LM317 and the black terminal block is to insert wires coming from the voltage regulator. The terminal block on the bottom left is for the 5kOhm potentiometer connected to the front panel of the case.
The pictures below show the case with front/back panel and transformer installed in the case.
These pictures have the board installed in the case.
Finally this is a picture of the complete unit. The output voltage is set to 15V and a 1kOhm load resistor is connected. The current reading shows 0.015A as expected.