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0-40V Adjustable Power Supply Circuit – Construction Tutorial

0-40V Adjustable Power Supply Circuit – Construction Tutorial

This multipurpose general purpose supply generates as much as 2.5 amps from zero to 20 volts or up to 1.25 amps from 0-40 volts. Current limiting is variable within the entire range for either output options.

By Trupti Patil

0-40V Adjustable Power Supply Front View

Power Supply Main Specifications:


Technical Specs for the 0-40V Power Supply

AN IDEAL POWER SUPPLY must provide a voltage that is variable within a broad range, and that stays in the set voltage irrespective of line voltage or load disparities.

The supply must also be safe from a short circuit throughout its output and be able to restricting the load current to ensure that devices are not damaged by failing circumstances.

This particular project explains a power supply designed to deliver 2.5 amperes at up to 18 volts (up to 20 volts at lower currents). At the same time a few basic modifications will make the supply offer as much as 40 volts at 1.25 amperes.

The supply voltage is adjustable between zero and‘ the highest available, and current limiting can also be adjusted across the stipulated full range. The mode of operation of the power supply is indicated by means of two LEDs.

The one near the voltage control knob shows if the unit is in normal voltage-regulation setting and the one near the current limit knob shows if the unit is in current limit mode. Furthermore a large meter shows the current or voltage output as selected by a switch.


While in our preliminary design stages we researched different types of regulator and the positive aspects and drawbacks of each to be able to pick the one which gives the top cost-effective functionality. The specific strategies and their features could be summarized as follows.

The shunt regulator:

This layout would work primarily for low power supplies around 10 to_15 watts. It offers excellent regulation and is internally short-circuit resistant however dissipates the full volume of power it is equipped to handle under no-load conditions.

The series regulator.

This regulator fits medium-power supplies approximately about 50 watts.

It may and is intended for higher power supplies, although heat dissipation could be a issue particularly at very high current with low output voltages.

Regulation great, generally there is minor output noise and the cost is comparatively minimal.

SRC regulator:

Ideal for medium to high power purposes, this regulator provides low power dissipation, though the output ripple and response time are not nearly as good as the ones from a series regulator.

SCR pre-regulator and series regulator.

The very best features of the SCR and series regulators are put together with this kind of power supply circuit employed for medium to high-power applications. An SCR pre-regulator is employed to secure an roughly regulated supply around five volts greater than recommended, accompanied by a suitable series regulator.

This lessens power loss in the series regulator. However, it is a lot more costly to construct.

Switching regulator.

Also applied for medium to high-power applications, this technique provides affordable regulation and low power dissipation in the regulator nevertheless is pricey to construct and possesses a high frequency ripple on the output.

Switched-mode power supply.

The most successful technique of all, this regulator rectifies the mains to operate an inverter at 20 kHz or even more. To lower or boost the voltage a low cost ferrite transformer is commonly employed, the output from which is rectified and filtered to get the preferred DC output.

Line regulation is very good but it surely has the downside that it is not able to conveniently be applied as a variable source since it is just adaptable over a relatively smaller range.


0-40V Adjustable Power Supply Circuit Diagram


0-40V Power Supply Transformer Diode Wiring Details

Our initial design principle had been for a power supply of around 20 volts at 5 to 10 amps output.
Having said that, in the light of the varieties of regulator readily available, as well as the costs, it was opted to limit the current to about 2.5 amps.
This approach helped us to employ a series regulator, the most cost-effective model. Good regulation was necessary, along with adjustable current limiting feature, plus it was additionally chosen that the power supply could well be workable right down to practically zero volts.

To get the final qualification a negative supply rail or a comparator that may run using its inputs at zero volts is essential. As opposed to using a negative supply rail we made a decision to work with a CA3l30 IC operational amplifier as the comparator.

The CA3l 30 needs a single supply (maximum of 15 volts) and, in the beginning we made use of a resistor and l 2 volt zener to get a 12 volt supply. The reference voltage had been then created from this zener supply by one more resistor and a 5 volt zener.

It was believed this would have presented adequate regulation for the reference voltage however practically the output from the rectifier was identified to alter from 21to 29 volts plus some of the ripple and voltage switching which took place over the 12 volt zener, as a result, ended up being mirrored into the 5 volt zener reference.

Due to this reason the 12 volt zener has been substituted by an lC regulator that remedied the issue.

With all series regulators the series-output transistor from the characteristics of the layout, ought to dissipate plenty of power particularly in low output voltage and high current. For this factor a respectable heatsink is an important portion of the structure.

Industrial heatsinks are incredibly expensive and frequently challenging to attach. We as a result created our very own heatsink that was not only more affordable yet functioned a lot better than the commercial variation we had been thinking about - being simpler to attach.

Nevertheless at full load the heatsink continue to operate warm as will the transformer. and within high-current low-voltage circumstances the transistor could even become far too sizzling to touch.
This is fairly normal since the transistor within these situations remains to be functioning within its selected temperature range.

Together with any extremely regulated supply, steadiness could be a difficulty. For this motive the voltage-regulation mode of operations, capacitors C5 and C7 are included to minimize the loop gain in high frequencies and therefore avoid the supply from oscillating.

The value of C5 has been picked for ideally skimp on between stability and reaction period. When the value of C5 is too low the rate of reaction is increased.

However there exists a greater possibility of lack of stability. lf excessive reaction time is unduly increased. In the current-limit mode the identical functionality is completed by C4 and the exact same opinions implement as for the voltage scenario.

As the power supply has the ability to of relatively high current output there may be undoubtedly some voltage drop over the wiring to the output terminals.This is compensated by sensing the voltage on the output terminals through a independent set of leads.

Although the supply was principally made for 20 volts at 2.5 amps it ended up being recommended that the exact same supply may be accustomed to supply 40 volts at 1.25 amps and that this may be of more appropriate to many end users.

This could be accomplished by modifying the settings of the rectifier and by altering a few components. Some idea was handed to creating the supply switchable however the additional complexities and price were in a way that it was disregarded to be advantageous.

Therefore you need to basically choose configuration that matches your demand and build the supply as necessary.

The maximum regulated voltage accessible is restricted possibly by the input voltage to the regulator being too reduced (with more than 18 volts and 2.5 amps) or perhaps from the ratio of R14/R15 and by the value of the reference voltage. (Output = R14+R15/R15 )V ref

Because of the tolerance of ZD1 the complete 20 volts (or 40 volts) is probably not accessible. If it is identified like a situation R14 must be increased to the subsequent favored value.

Single turn potentiometers have been given for the voltage and current controls due to the fact that they are affordable. Nevertheless if accurate setability of voltage or current control is needed ten-turn potentiometers ought to be applied as a substitute.


The 240 volt mains is stepped-down to 40 Vac through the transformer and, based on which supply has been developed, rectified to either 25 or 5 Vdc.

This voltage is actually moderate since the actual voltage will be different between 29 volts (58 volts) on no-load to 21 volts (42 volts) on full load.

The identical filter capacitors are employed in both situation. These are attached in parallel for your 25 volt variant (5000uF) and in series intended for the 50 volt model (1250uF). ln the 50 volt model the centre tap of the transformer will be coupled to the centre tap of the capacitors hence guaranteeing accurate voltage. sharing amongst the capacitors. This set up additionally offers a 25 volt supply to the regulator lC.

The voltage regulator is essentially a series type in which the impedance of the series transistor is governed in such a method that this voltage throughout the load is kept constant at the predetermined value.

The transistor Q4 dissipates a great deal of power particularly at low output voltages and high current and it is hence installed on the heatsink within the backside of the product.

Transistor Q3 brings current gain to Q4,the collaboration performing like a high-power, high-gain, PNP transistor. The 25 volts is decreased to 12 volts through the integrated-circuit regulator ICI. This voltage is commonly employed as the supply voltage for the CA3130 lCs and it is additionally lowered to 5.1 volts by zener diode ZDI to use as the reference voltage.

The voltage regulation is conducted by lC3 that examines the voltage as determined by RV3 (O to 5.1" volts) with the output voltage as divided by R14 and R15. The divider provides a division of 4.2 (O to 21 volts) or eight (0 to 40 volts).

On the other hand in the high end the obtainable voltage is restricted to the point that the regulator manages to lose control at high current as the voltage through the filter capacitor reaches the output voltage plus some 100 Hz ripple may also be found. The output of IC3 regulates transistor Q2 that subsequently controls the output transistor in a way that the output voltage continues to be consistent irrespective of line and load disparities. The 5.1 volt reference is offered to the emitter of Q2 through Q1.

This transistor is actually a buffer stage to counteract the 5.1 volt line from becoming loaded. Current control is conducted by IC2 that analyzes the voltage determined by -RV1 (O to 0.55 volts) using the voltage created around R7 by the load current.

If say 0.25 volts is defined on RV1 and the current taken from the supply is small, the output of IC2 is going to be near 12 volts. This leads to LED 2 being lit up since the emitter of Q1 is at 5.7 volts.

This LED consequently signifies that this supply is functioning within the voltage regulator mode. lf however the current driven is elevated in a way that the voltage around R7 is little over 0.25 volts (in our illustration) the output of IC2 may drop. Once the output of IC2 drops below around 4 volts Q2 begins switch off through LED 3 and D5. The result of this would be to minimize the output voltage in order that the voltage throughout R7 is unable to surge more.

While this takes place the voltage comparator IC3 attempts to counter for the problem and its output soars to 12 volts. IC2 then consumes more current to make up and this current brings about LED 3 to lumination, implying that the supply is working in the current-limit mode.

To make sure precise regulation the voltage sensing terminals are delivered to the output points Independently from those transporting the load current. The meter includes a one milliamp movement and reads the output voltage (immediately along the output terminals) or current (by ‘measuring the voltage around R7) as chosen from the front panel switch SV2

PCB Layout for the 40V power supply circuit

0-40V Adjustable Power Supply PCB Track Layout


0-40V Power Supply PCB Component Overlay


The suggested PCB layout for this 0-40V variable power supply circuit must be utilized since construction is in so doing tremendously simplified.

The components must be put together onto the board ensuring that the polarities of diodes, transistors, lCs and electrolytics are proper. The BDl40 (Q3) must be installed in a way that the side using the metal surface confronts in the direction of lCl . A small heatsink must be bolted onto the transistor as demonstrated in the picture.

If the metalwork as detailed is used pursuing assembly arrangement must be employed.


0-40V Meter Connection


a) Join the front panel into the front of the framework and bolt them with each other by fitting the meter.

b) Fix the output terminals, potentiometers and meter-switch onto the front panel.

c) The cathodes of the LEDs (which we applied) had been designated by a notch within the body that could not possible be noticed while the LEDs were fitted onto the front panel.

If this sounds the situation with yours, reduce the cathode terminals slightly smaller to recognize them after which install the LEDs into place.

d) Solder lengths of wire (around 180 mm long) to the 240 volt terminals of the transformer, insulate the terminals using tape after which attach the transformer into place inside the framework.

f) Mount the mains cord and the cord-clip. wire the power switch, insulate the terminals and after that attach the switch on the front panel.

g) Fix the heatsink and screw it onto the back of the framework by using a couple of bolts - after that install the power transistor utilizing insulation washers and silicon grease.

h) Install the assembled PCB on the framework utilizing 10 mm spacers.

i) Wire the transformer secondary, rectifier diodes and filter capacitors. The diode leads are rigid enough not really to want any extra support.

j) The wiring involving the board and the switches may possibly now come in by hook up points with matching letters in the front panel diagram and component overlay diagrams. The only establishing needed would be to calibrate the meter. Hook up an genuine voltmeter to the output control of the power supply so that the external meter deciphers 1 5 volts (or 30 volts on the alternative set up).

Parts List for the proposed 40V 2 amp power supply circuit

0-40V Power Supply Parts List


About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

13 thoughts on “0-40V Adjustable Power Supply Circuit – Construction Tutorial”

  1. you’re right.i forgot to calculate wattage.thank you for help and formula.keep helping. i built my first dual power supply from studying your dual power supply concept.i am planning to build little by little power supply,stabilized,precised,and much more efficient.you’re a BIG HEP!

  2. hi swag.
    i built a power supply and i used LED and 12k resistor as mains ac indicator.my LED got busted.how to calculate led’s resistor from any ac source like 220,110 vac.thnx

    • Hi Dennis, Your resistor value is correct, but wattage may be wrong….you can do it through simple Ohm’s law, as given below:

      R = V/I = 220V / 0.02(LED Current) = 11000 ohms or 11K

      wattage = R x I^2 = 11000 x (0.02)^2 = 4.4 watts, or simply 5 watt….but this will still dissipate a lot of heat, so better to go with a 10 watt resistor

  3. sir if i have two rectified dc power supply from same tranformer 12 0 12
    Q1 can i connect dc output of these transformer in parallel
    Q2 wht happen if dc outputs of two different trafo connected ​in parallel ?

    • you can connect their output in parallel, just make sure the center tap has two wires, in total 4 wires should be there, connect two wires with one bridge, the other wires with another bridge and then you can connect the output from the bridge in parallel.

      • no no sir i did spelling mistake in my previous comment.
        i means to say i have two transformer of same rating.both trafo have 12 0 12 tapping.
        basically i want to make uninterrupted dc power supply bcoz i have two ac power source if ones then their will another give supply.

  4. I am not able to interpret this circuit, how will the cells get charged? there's no negative supply outlet for the cells, neither is there any positive input??

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