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.
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.
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Good day Swagatam, I would like to share with you the integration of the Bench Top Power Supply for your advice. As discussed, I used the diagram suggested by you and other diagram where the transistor were added to the LM317 and Lm337 for increasing current. I would really like to hear your advice to this respect but this plataform doesn’t allow me to send the picture.
Hi Jorge, yes unfortunately image upload facility is not available in this website by the user, but no problem, you can explain the problem through comments, I will try my best to understand it and solve it for you…
Hello, I really love this idea with the switch, but I was thinking to add the 2N3055 and MJ2955 at each LMXXX IC to increase the output current. Has you designed one with this option cause sometimes 1 amp is not enough but maybe 3 or 4 amps. I would really love to hear about this. I am a Hobbyist currently working in my new Workbench Power Supply.
Thanks
Hi, yes you can add an outboard transistor for increasing the current, you can read the following article for more info on the same:
LM317 with Outboard Current Boost Circuit
You can think about designing bench power using discrete parts without ICs, as discussed below:
Make a Workbench Multimeter With the IC 741
Pls swag,I need a very efficient circuit that can stepdown 48v to 12 voltage.I want to use it to power a microcontroller board.
Hi Joseph,
you can try the following concept:
https://www.homemade-circuits.com/5v-pwm-solar-battery-charger-circuit/
sir i hope am not disturbing you but am a little bit confused if i change the lm317 in the circiut to lm196 or lm396 what ic am i going to use to replace LM337
yes that may not be possible, because it seems there’s no negative complementary IC for LM196
sir can i use lm196 or lm338 for both ic to get high current up to 10amps or are there any other ic that can replace the ones in the diagram for higher current up to 10amp
Marvin, you can use LM196 or LM396 for getting upto 10 amps by adding adequate heatsinking to th IC.
sir what change do i have to make to get 6 or 7 amps from the circiut will i have to wire more lm312 ic in parallel at both sides
Abioye, adding more ICs in parallel can be complex, instead you can try the following alternative design
https://www.homemade-circuits.com/0-to-50v-0-to10amp-variable-dual-power/
Sir please am doing a project on a DC regulated power supply with voltage of 2v,6v and 9v…
I find this circuit diagram very useful but am having some doubt’s about excluding other resistors in the circuit and use the necessary ones I require.
Please its an urgent project
Flynn, you can remove the switching resistors which are not required…
I found it is important to use a rotary switch that connects the next pole before disconnecting the first one. Otherwise as you rotate the control there is a brief moment where it is open circuit. This will result in a high voltage spike on the output. Possibly damaging the connected device. With a “ make before break” type there will be a brief voltage drop as you rotate. Not normally an issue.
Thanks for pointing out the issue, I will update the info in the article soon.
sir, with a center tap 12-0-12 transformer will it be possible to get two positive 12v from each side?
if yes, kindly help me out with the circuit diagram
if no, can I use the negative (-12v) of the center tap transformer to supply circuit that requires +12
sir what I really what to do is to simply two separate circuit (that is 1. the egg Turner timer circuit and 2. the temperature control circuit) with the same center tap transformer
Patrick, for getting two separate +12V lines you simply need to bifurcate one +12V from a bridge rectifier output into to positives lines using two separate diode. that's…a center tap is not required for this.
the ground will be common for both these +12V lines
Sir,
In circuit diagram ic LM317 output pin
direct short with Adjust pin
thanks Ubaid, it's a drawing mistake, there should be a diode there instead of a short
Sir Swagatam Majumdar !!!
i have simulated this circuit but it is not giving proper output voltages.
i.e when rotatory switch is connected with 3k resistance ,it gives 21.43 voltages
when rotatory switch is connected with 3k resistance ,it gives 21.34 voltages etc what should i do.
Errors are expected in Simulation…have you made it on Hardware???
Asif, I have not made it practically, but the design looks very obvious and straightforward, so there's no harm in building it practically and tweaking the results with some trial and error….there's nothing in the design that might blow of or burn during the testing process…
Thanks sir.
i will give it a try very soon.
Thanks for your informative posts.
Sharing is Caring 🙂
you are welcome!!