Linear Regulated Bench Power Supply

Introduction

A 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 operation

In 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.

At the input of the transformer, the mains AC voltage should be connected. The voltage obtained at the output of the transformer is sinusoidal, with the same frequency as the mains voltage but with lower amplitude. The output of the bridge rectifier is shown below.

Output of the bridge rectifier.

If we put a capacitor at the output of the bridge rectifier, the capacitor will charge and discharge to reduce the voltage ripple. As long as the voltage at the output is higher than the voltage required by the regulator for normal operation, the voltage ripple will not be a problem. The voltage ripple can be reduced by choosing a higher value capacitor.

Design

The 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)

Assuming that no current flows in/out the adjust pin (few μA according to datasheet) we can write the output voltage as: \[ V_{OUT} = V_{ADJ} + 1.25V = R_2 \frac{1.25V}{R_1} + 1.25V = 1.25V \Bigl(1+\frac{R_2}{R_1}\Bigr)\] This equation shows that when varying R2, we can let the output voltage rise up to any value (40V is the maximum for the LM317, as long as the input voltage is high enough). It also shows us that the output voltage will be lower bounded by 1.25V. This is a limitation in our design but most applications require voltages from 3.3V and up so this is not really a big problem. The LM317 datasheet also suggest to use a 0.1μF capacitor as close to the regulator input as possible. This filters unwanted noise in your input voltage. It is also advised to use a 1μF capacitor at the output to improve transient response of the output voltage. Further, we might want to decouple the adjust pin of the regulator. If noise enters this pin, the output voltage will fluctuate according to this noise. A decoupling capacitor will filter this noise, leading to a more stable output voltage. At last, it is good practice to use protection diodes on any regulator IC. Charged capacitors can discharge through low impedance points of the regulator and dammage it. For example, most 10μF capacitors have low enough internal series resistance to deliver 20A spikes when shorted and have a high chance of dammaging the regulator. The complete schematic of the regulator including decoupling capacitors and protection diodes is shown below.

Complete schematic of the regulator circuit.

The protection diodes are 1N4002 rectifiers, but any basic diode able to carry 1A or more will do the trick. For R2 we will use a 5kOhm potentiometer and for R1 a 300Ohm resistor. This allows the output voltage to go up to 20V.

Construction

 In 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.