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.
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.
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:R’ determines the voltage ratio.
The OP-AMP identifies any change in this ratio via Rf 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 for operational amplifiers from a single supply.