Adjustable 3V, 5V, 6V, 9V,12V,15V Dual Power Supply Circuit

The purpose of this paper is to detail a variable dual lab power supply circuit which has an adjustable range from 3V, 5V, 6V, 9V, 12V, and 15V or even more at an output current rate of 1 amp.

Written By: Dhrubajyoti Biswas

The Dual Power Supply Concept

In regard to positive volt it is preferable to use IC LM317 [-3V,-5V,-6V,-9V,-12V,-15V at 1A] and use LM337 as the negative volt. The voltage can further be controlled by S2 [+Vout] and S3 [-Vout]. The size of the transformer is set to 2A and furthermore the IC enables holding the heat sink.

However, for this development we would like to develop a dual positive power supply, ground and negative so as to experiment it in different circuits.

In addition, we can also experiment OP-amp IC – LM741, which uses the power supply voltage of +9 volts and -9 Volts. Even when we use tone control circuits or preamplifier circuit, they will use voltage supply of +15 volts and -15 volts.

Nevertheless, the circuit that we design here will be useful because a) The circuit has the capacity to enable positive voltage and even negative voltage [at 3 volts, 5 volts, 6 volts, 9 volts, 12 volts, 15 volts respectively keeping the output of the current under 1.5 amps; b)

The circuit is best to use with rotating selector switch, which will give the freedom to select the level voltage. Moreover, you won’t require any voltsmeter to measure the voltage of the output; c) The circuit is simple and the IC used for it LM317 and LM337 are cheap and can be easily procured from the market.

Circuit Diagram

How the circuit works

In this dual variable power supply circuit IN4001 – D3 and D4 diode acts as the full-wave rectifier. The waveform is then filtered to ease the capacitor C1 (2, 200uF).

Then the input of LM317T (ICI) acts to regulate the IC in a positive mode. Furthermore, it also adjusts the voltage of 1.2-37 volts and enable provision of maximum current output of 1.5 amps.

Point to Note

- The output of the voltage can alter because of the value of change in Resistor R2 and further alters R3 to R8. This is accomplished by S2 selector switch and you can choose the resistance as per your need, in order to gain the voltage level from 3, 5, 6, 9, 12 and 15 volts.

- The C2 (22uF) measured with high impedance and further reduces to transient on the output of ICI-LM317T.

- The C3 (0.1uF) capacitor is used when IC1 is installed keeping distance from C1.

- The C5 (22uF) capacitor, before being amplified and as the output of the voltage goes up acts as ripple signal.

- The C9 capacitor is used to low down the ripple in the output.

- The D5 and D7 diode (IN4001) in the circuit is used to protect IC1 from the discharge of C7 and C5, in situation when the input is in short circuit.

- With regard to the negative mode, it follows similar principle like that of the positive mode. Here, D1, D2 are the rectifier diodes in a model where the rectifier is in full wave. The IC IC2-LM337T is regulated by negative DC.

Aforesaid is the process to develop an adjustable dual power supply. However, if you need the voltage to be variable in nature [for instance, 4.5V,7.5V,13V et al], simply add the VR1 in IC1-LM317 and IC2- LM337 pin.

If a rotary switch is used instead of a potentiometer, as shown in the diagram, make sure to use a rotary switch having a "make before break " feature which will ensure that while operating the rotary switch, the output does not swing to the maximum voltage level during the split second transitional disconnection of the switch contacts. The "make before break" feature is specially designed to prevent such situations from occurring.

Calculating the Resistor Values:

The values of the various fixed resistors could be calculated either through this calculator software or using the following formula:

VO = VREF (1 + R2 / R1) + (IADJ × R2)

Where R1 = 270 ohms as given in the diagram, R2 = the individual resistors connected with the rotary switch,  and VREF = 1.25

For most applications IADJ could be simply ignored since it's value will be too small.

Another LM317 Simple Dual Power Supply Circuit

The diagram above shows how a simple yet higher versatile, adjustable dual power supply circuit could be built through just a couple of LM317 ICs. The circuit will produce an adjustable dual supply of 12V, 5V, and 9V

It means, an effective variable dual supply output could be achieved by using readily available IC like LM317, which is very easily accessible in any electronic market.

The design employs a couple of identical LM317 variable regulator circuits driven through separate bridge rectifiers and AC inputs from the transformers.

This allows us to join the + and - of the two supplies to create a dual supply of our own choice as per specific requirements.

Considering that it should be achievable to adjust the output voltage to 3 variable ranges, the voltage regulator applied is a kind whose output could be fixed using a handful of resistors, as shown in the circuit diagram. The output voltage is determined using the formula

Uout = 1.25(1+R2/R1) + IadjR2, in which 1.25 signifies the reference voltage of the IC, and ladj indicates the current moving through the 'ADJ(ust)' pin of the device towards ground.

The IC LM317 has internal compartaors, which constantly analyzes portion of the output voltage, fixed by resistive divider R1/R2, with the reference voltage. In the event that Uout is required to be higher; the comparator output is switched high which forces, the internal transistors to conduct harder.

This action decreases the collector-emitter resistance, causing a boost in the Uout. This set up guarantees a practically constant Uout. Practically , the value of Iadj falls between 50 µA and 100 µA. Due to this lower value, the factor Iadj R2, could typically be removed from the formula. Therefore, the refined formula

Uout = 1.25[1+(1270+1280)280] = 12.19 V.

Precision Dual Voltage Power Supply

This circuit has the benefit over the standard 2-resistor voltage divider where the voltage ration V:V does not rely on the current flowing from it. The ratio of resistances R:R1 determines the voltage ratio.

The OP-AMP identifies any change in this ratio via R_f and quickly executes the correction. The actual voltages utilized will be limited by the upper and lower operating voltages of the OP AMP. The circuit shown was developed to deliver +15 V, -15V dual supply, specifically for the operational amplifiers.

Balanced Power Supply using LM324

This stream-lined balanced power supply utilizes the 4 opamps from one LM324 IC. These are employed to stabilize the output voltage and also to control the output current. The current limiter circuit is defined at 60 mA and consists of lowest number of parts.

It has to be taken into account that under certain situations it might appear that the (input) power supply of only ±16 V is actually very low. However, the highest output voltage is determined by the specifications of the IC employed. It is far from safe to raise the input supply voltage; any kind of rise in the voltage could destroy the IC, depending on its maximum input voltage tolerating specifications.

A 5.6 V zener diode is employed for fixing the reference voltage. The zener value is not really crucial; if it is small, the output voltage is going to be a bit lower.

P1 works like the voltage adjustment pot simultaneously for controlling both the +15 and -15 supplies. Using P1, the reference voltage gets broken down and is given to the + input pinout of the (upper) opamp.

This particular opamp manages the positive output voltage by governing the base current of the series regulating transistor (BC140). The stabilization of the output voltage is influenced through a negative feedback loop through a voltage divider network composed of the 22 k and 10 k resistors.

The regulation of the negative voltage tends to be relatively more complex. The + input pinout of the lower opamp is coupled to the zero voltage '0', by means of a 6k8 resistor.

The reference voltage is employed through control pot P1 along with various other parts with the - input pinout. The negative output voltage is well balanced with respect to the positive reference voltage using the voltage divider `see -saw' network established through the 33 k and 10 k resistors (that are bridged together through a trimming circuitry).

Trimming preset controller P2 cancels out the impact of small tolerances in the circuit elements, and P2 can be tweaked to balance the positive and negative output voltages. Over Current safety is achieved by the a couple of leftover opamps in the IC.

In case the voltage difference across one of the 10 ohm resistors becomes greater than 0.6 V, the reference voltage is going to decrease to zero and, as a result, the output voltages also reduces along with it. Simultaneously the LED's illuminate to show that the protection feature of the circuit is working.

Another Simple 3V to +15V, -15V Dual Power Supply Circuit

The following figure shows another simple dual power supply circuit which can be customized to get any dual voltage between 3V and 15V. THe

By appropriately changing the values of the resistors R2, and R4 resistors it may be possible to alter the output from anywhere between 3V, 4.5V, 6V, 9V, 12V, 15V dual supplies.

For a fixed dual power supply you can use the following configuration. Here we can see that, for the positive supply the IC 7812 is used, and for the negative supply, the IC 7912 has been used:

Dual Power supply circuit using IC 7815 and IC 7915