Power Supplies Tutorial

Power supplies tutorial.  
//This article was gathered to develop a power supply for a Paia 2720 VCO.
1.) Dual bi-polarity 9 VDC
2.) Dual bi-polarity 5 VDC
3.) Variable 0 to 5 VDC
4.) 18 VDC

Step 1: The Actual Circuit

As I only have a couple of modules of the 2720 I thought I would build a simple power supply to test them.  I didn't want to use transformers so the project became a good refresher of how power supplies work in general. 

Hope you find this useful or educational.  Most of the information I present here is web based, with a little experience thrown in for good measure. 

Step 2: 12 VDC to 9 VDC Conversion

A typical 12 VDC to 9 VDC conversion.

Parts List:
R1 = 560 ohm
C1 = 1000uF/40V, Electrolytic
C2 = 10uF/25V, Electrolytic
C3 = 330nF, Ceramic
Z1 = 9.1V, 1watt zener
Q1 = ECG184, NTE184

To get a more precise output voltage, replace zener diode Z1 with 10V and R1 with a 1Kilo ohm potentiometer. A Coolrib for Q1 is optional but highly recommended. You can replace Q1 for a more robust type to get more output amps depending on your requirements. Simple circuit to power your 9 volt cassette recorder and other stuff.

This circuit is nice as a conversion circuit for DC to DC applications but does not cover the negative side of the voltage rail which will be required by the Paia 2720.

So we begin to look at dual power supplies with bipolarity...

Step 3: AC to DC Conversion Block Diagram

A basic functional block diagram of a power supply with AC to DC conversion.

Step 4: Dual Polarity Power Supply 78-79xx Series

This power supply is very simple dual power supply. With using 3 terminals voltage regulator, it’s best and simplest choice for non-critical applications. 3 terminals term is for input, ground, and output. The 3 terminals voltage regulator here are 78xx & 79xx IC (integrated circuit) series that can provide up to 1A load current.

This voltage regulator IC series have on chip circuitry to prevent damage in over heating or excessive current conditions. If the conditions happened, the chip simply shuts down rather than blowing out. These regulators are inexpensive, easy to use, and they make it practical to design a system with many PCBs in which an unregulated power supply is brought in and regulation is done locally on each circuit board.

This Dual Linear Power Supply provides a dual polarity power supply. It can be build easily for delivering up to 1 ampere with the appropriate choice of center tapped transformer and 3-terminal voltage regulator pairs at +/- 5V, +/- 9V, +/- 12V, and so on.

The 3-terminal pair of regulators you want: 7805 & 7905, 7809 & 7909, 7812 & 7912, 7815 & 7915or 7818 & 7918. The + and – regulators do not have to be matched: you can for example, use a +5v and -9V pair. However, the positive voltage regulator must be a 78xx regulator, and the negative voltage regulator a 79xx one. The user must choose the pair he needs for his particular application.
Read more: http://www.electronics-project-design.com/DualPowerSupply.html

Step 5: Variable Dual DC Power Supply

Here is a simple dual dc power supply constructed by using regulator IC 7805/7905 which provides symmetrical voltage 5 to 25VDC.

An improvement would be to replace thew regulators with a pair of variable IC regulatorsLM317/LM337.

It is advisable to use the IC heatsink.

Variable dual DC power supply is capable of providing current of 1A per polarity. When using a transformer CT 2A power supply for 15V or more output currents can be well distributed. Use fuse size 500mA to prevent damage to components due to a short-circuit.

As we can see both these circuits use a floating ground from the center tap of the step down transformer. Since we are using a power supply which has a step down transformer we would like to avoid redundancy.

Step 6: Simple Add-on for a Single-rail Supply, �2.5V to �15V Output

This design was conceived as an add-on for the Variable DC Power Supply, a very successful circuit posted to this website. This simple unit provides a dual-rail variable output ranging from ±2.5V to ±15Vdc with precise tracking of the positive and negative output voltages, still retaining the current limiting and short-proof capabilities of the "master" circuit. As the purpose of such a dual-rail design is to supply experimental or under-repair circuits, the maximum current output delivered was deliberately kept to about 500 - 600mA per rail, thus avoiding the use of expensive power transistors and complex circuitry.


R1 = 4.7K-1/2W
R2 = 4.7K-1/2W
C1 = 100nF-63V
C2 = 220µF-25V
C3 = 220µF-25V
C4 = 100nF-63V
C5 = 100nF-63V
Q1 = BD437
Q2 = BD438
IC1 = LM358


* The circuit can be placed into the existing Variable DC Power Supply metal cabinet.
* Q1 and Q2 must be mounted on heatsinks. Usually, bolting them to the metal case (through insulating washers etc.) proved effective.
* The full ±15V output can be achieved only if the secondary winding of the supply Transformer used in the Variable DC Power Supply is rated at 48V minimum (center tapped).
* When using this circuit, please set the Current-limit control (P1) of the Variable DC Power Supply to any value comprised in the 50mA - 1A range but not higher.
* The second Op-amp (IC1B) contained in the LM358 chip was not used, but its input pins were tied to the negative supply and the output was left open.

Source : www.redcircuits.com

Step 7: After All

With the combination of these notes I was able to get a 18VDC wall wort to split into +/- 9 Vdc which also ran in parrallel with a variable step down LM circuit to get my 5 volts, which was enough to test out the basic functionality of the Paia 2720 modules. 

Next consideration will be current consumption,  but that is another lesson...



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    10 Discussions


    7 years ago on Step 2

    Your Right!!! The Zener should be 10volts,Why You Ask?? Vbe= ,7 volts so the Voutput would be Vo= Vz- Vbe or Voltage Output = Zener Voltage- Voltage base to emitter====== 10v- .7v= 9.3v I hope this helps !!


    7 years ago on Step 6

    These are really great modifications to the circuit Mr Ohm! Thanks for bringing them to light.

    1 reply

    Reply 7 years ago on Step 6

    Hi MR Orbitalgun This is what makes Electronics Fun,,, when we can already Improve on a otherwise outstanding circuit!! I must go now,,,I smell my Weller Heating up!!! PS*** Nice Instructable !!!


    7 years ago on Step 6

    Actually I designed a Circuit just like this a few years ago,,I used Tip41 and Tip42 Transistors,, Heat sinked them to a Chassis and was able to get 1 amp output from each rail, I also used a 3 amp transformer to provide 1amp headroom for this circuit.Also I used a 100K Resistor for R1 and R2 to prevent loading on the Op Amp. I also used a 741 Op Amp which is available Everywhere,, So if I pushed This circuit ,,,1 amp per rail would be a 2 amp output,,,but the Transformer was rated 1 amp higher(3 amps) to provide the necessary headroom and to prevent Mr.transformer from overheating!!! By Providing These Minor Adjustments the user can have a Cost effective, Near Lab Quality piece of Test Gear which Would be a Welcome Addition To Any Test Lab!!!!

    You could do this a variety of ways... If you had a second battery you could use them as a 12v source by putting them in series, or you could use a simple voltage doubler( google voltage doubler), You might even consider using a charge pump.

    I need to convert a single 6v battery to 12v with more then 2A current . Voltage doubler available on websites gives only 100mA or 200mA. current.


    7 years ago on Step 4

    This article was very useful to me. Here's the circuit in practice, with some additional pictures and part numbers:


    1 reply

    Reply 7 years ago on Introduction

    Thank you for looking over this, my first instructable. I 'm glad to hear it pointed you in the right direction. Also, I read your article and it was very clean and forth right. Thumbs up!