Introduction Regulators Other Options Supply Project

Download this project in PDF

Bench Power Supply


After some fooling around with batteries every hobbyist should think seriously investing into a decent power supply . You could buy one of course but if you would be a lot more rewarding to build your own and save yourself a bundle . Aside from saving a lot of money , you will also get to know what is inside your supply and be able to service it if anything goes wrong down the road .

Within the following descriptions of the power supply , you will be able to chose many options available to upgrade your project into a really versatile high precision instrument , such as a variable voltage output , presettable digitally selectable voltages , a current limiting function , digital LCD or LED read outs , or just a basic accurate power supply .

The Basic Circuit

The circuit shown below is the basic variable supply using the LM317 adjustable regulator with RV1 being the main adjustable element to produce a regulated voltage output from 1.2 volts to a maximum of 37 volts at 1.5 amp current when used with a heatsink .

If only the basic supply will be build , then, use only RV1 and connect it directly to ground marked "B " . But you migh still be interested into using the LCD readouts and current limiting function as described further , so read on .
With the additional components RV2 to RV5 and Q1 to Q5 we expand the one variable function privided by RV1 into four additional switchable preset voltage range for for fast highly accurate selectable voltage outputs .

You are going to ask "why don't we do away with Q2 - Q5 and connect the trimmers directly to the switch" Well we want precision and we cannot guaranty the switch contacts will not eventually build up resistance and affect the trimmer setting . By using transistors we can be assured that the setting will not drift even if the transistor base voltage from the switch may drift a few milivolts . You can omit the transistors if you wish but they are essential for the proper function of the push button circuit .

A list of parts is availble for your requirement to be selected depending on your application of the project . All transistors used are the 2N2222 or 2N3904 or equivalent NPN type .


The Transformer :
The LM317 will regulate a voltage to a maximum of 37 volts under load and needs at least 1.5 volts above the required regulated voltage , based on this you must use a transformer which will output the required voltage and at a current output to meet your requirements . A center tapped or two wires output transformer may be used.
The rectifier bridge may be substituted with four single rectifier diodes . If you are in doubt about this please go to Introduction for more informations on this subject .
As shown on the Layout RS1 and RS2 are precision resistors used as shunts for the Current meter if used , if not , they can be omitted and the voltage output taken directly from the " OUT " position pad .
Since the LM317 has built in overcurrent shut down function the Fuse may be optional but it is a wise investment .
The Heatsink used is a vertical type , the larger you can fit , you can buy or make your own using 1/8" thick copper or aluminum plate about 1.5" square bolted to the regulator , no other support should be required .

The Circuit Layout (Enlarged)

Parts list and actual size of PCB suggested layout

Presettable Switching

Two methods are shown for presettable switching . One uses a rotary one pole 5 positions switch as shown below and the other using a single push button triggering a CMOS 4017 decade counter wich in turn activates in rotation the transistor to enable the selected voltage output .

For both mode of switching a fixed supply voltage is required for the base of the transitors , the 4017 and the optional LEDs indicators which are used to indicate which voltage is selected .
For that purpose a reguator is used as shown and the supply source is taken directly from the output of the rectifier bridge . The regulator can be a small TO-92 9 or 12 volts /100 mA or a zener type shown as an option .
Using the single push button method , the LEDs will be necessary to indicate which voltage range is in use and should be mounted on the panel face . The LEDs can be connected to the main board with multicoloured ribbon wires for easy tracing and indentification . Of course if digital read out are used the LEDs may be omitted .
When using the rotary switch the LEDs can be omitted and the panel can be lettered indicating the switch position and the voltage output .
For easy identification , again , coloured ribbon wires is suggested for connections from the 4017 or the panel mounted rotary switch to the bases of each transistors .


Once the circuit is assembled , select the main RV1 position and set RV1 to its minimum , connect a voltmeter between ground and the positive of the rectifier output and apply power . Note the voltage output , it must be higher by at least 1.5 volts than the expected maximum regulated voltage output .

Move your meter connections to the regulated voltage output position , the reading should be 1.2 to 1.5 volts , rotate RV1 slowly toward its maximum setting and ensure the voltage increases to its maximum expected voltage , If so all is well . If not go over all your connections , capacitors and diode polarity connections as well as the regulator pins connections , see the LM317 connections on the diagram .

Once you have confirmed that the main control RV1 is producing a variable voltage output , it is time to calibrate the other selections . Using your ohmmeter across each trimmer from RV2 to RV5 , adjust the trimmer to indicate the sample resistance value for each corresponding voltage shown on the part list .
Connect your voltmeter to the output , power up and select each position in turn and read the voltage output , it should be very close to the corresponding sample voltage output listed . Adjust each trimmer for the precise listed sample , or you can adjust to any other voltage you chose . While you're at it , confirm that the LED is on and indicate the proper position , if not , do a tracing and adjust connection as required .

ERRATA=Please make correction to the TRIGGER ( S1)circuit on the schematic and layout .
The .4uF capacitor and the 1K resistor should be connected between the transistor( 2N2222) base and ground .

Continue on page two

Introduction Regulators Other Options Supply Project

© Laurier Gendron, Burnaby, B.C., Canada. 1998