Adjustable power supply from +/- 1.25V to +/- 22V 1A
About power supply
The power supply is the basis of every electronic device. As the name suggests, power supplies are suppliers of energy to any circuit. Every electronic circuit needs an adequate power supply at the input to obtain an ideal result at the output. We need to choose the power supply of any device or circuit according to the power requirements of the device. In this experiment, we are making an adjustable power supply, which will provide voltage in the range of +/- 1.25V to +/- 22V with 1A as maximum current.
Overview
In this experiment, we are making an adjustable regulated symmetric positive and negative power supply. To reduce any fluctuations and ripples in the output, the power supply must be regulated to be able to provide a constant voltage at the output. This power supply provides regulated and adjustable voltage at the output.
The power supply we are making receives 220 Vac as input and generates a variable DC voltage in the range of +/- 1.25 V to +/- 22 V. This power supply can provide a maximum current of 1A at the output.
Required components
| Component Name | Specification | Amount |
|---|---|---|
| Transformer Tr1 | Reduce 24V-0-24V/2A | 1 |
| Diode D1-D6 | 1N4007 | 6 |
| Variable Resistance
RV1, RV2 |
10k | two |
| Voltage regulator | LM317 | 1 |
| Voltage regulator | LM337 | 1 |
| Capacitor C1, C2 | 100uF 63V | two |
| Capacitor c3, c4 | 10uF 63V | two |
| Capacitor C5,C6 | 1uF 50V | two |
| Ceramic capacitor C7, C8 | 0.1uF | two |
| Resistance R1,R2 | 240ohm | two |
| Fuse | 1A | two |
Power supply basics
Every DC power supply needs to follow a few steps to get the proper DC voltage at the output. The diagram below shows these basic steps by which we obtain an AC regulated DC power supply.
Working
• Reduce the mains supply by input transformer
The mains voltage (electricity coming to our house from the government) is approximately 220V, but according to the circuit requirements, only 22V voltage is required at the output terminal. To reduce this 220 V to 22 V, a center strip step-down transformer is used. The use of center tapped transformer is to generate positive and negative voltage at the output. The center ribbon will provide ground to the circuit and the remaining two ribbons will provide positive and negative voltage. The step-down transformer we are using is rated 24V-0-24V/2A. This transformer reduces the main line voltage to 24V as shown in the image below. The circuit suffers some drop in the form of resistive loss and by IC LM317. Therefore, a voltage transformer higher than the voltage required for the application is placed in the circuit and can provide 2A current at the output, which is suitable for our 1A application.
• Rectification
Rectification is the process of converting AC to DC. There are two ways to convert an AC signal to DC. One is through half-wave rectifier and another is using full-wave rectifier. In this circuit, we are using a full wave bridge rectifier to convert +/- 24 Vac to +/- 24 V dc. As full wave rectifier is more efficient than half wave as it can provide full use of negative and positive pulse of AC signal. In the full-wave bridge rectifier configuration, four diodes are connected in such a way that it generates a DC signal at the output, as shown in the image below. The 1N4007 diode is used in full wave rectification as it can allow 1A current and 24V supply.
• Smoothing
As the name suggests, it is the process of smoothing or filtering the DC signal using a capacitor. A high value capacitor C1 and C2 is connected on the input side after the rectifier bridge to provide pure DC at the output. As the DC which is rectified by the rectifier circuit has many AC spikes and ripples, to reduce these spikes we use a capacitor. This capacitor acts as a filtering capacitor that shunts all the AC through it to ground. At the output, the remaining DC is now smoother and ripple-free.
• Output capacitor
At the output, capacitors C5, C6, C7 and C8 are also connected to the circuit. Capacitors C5 and C6 assist in quick response to load transients. Whenever the output load current changes, there is an initial shortage of current, which can be met by this output capacitor.
Capacitors C7 and C8 are ceramic capacitors, the impedance or ESR of ceramic is low compared to an electrolytic capacitor. Therefore C7 and C8 are used in parallel with the electrolytic capacitor just to decrease the equivalent output impedance.
The output current variation can be calculated by –:
Output current,Iout = C (dV/dt)
dV = Maximum allowable voltage deviation
dt = transient response timeConsider dv = 100mV
dt = 100us In this circuit C = 1uF Iout = 1u (0.1/100u) Iout = 1mAWith this, we can say that the output capacitor will respond to a current change of 1mA for a transient response time of 100us. On the tuning pin, capacitors C3 and C4 are connected. These capacitors prevent the ripple from being amplified as the output voltage increases.
• Voltage regulation by LM317
To supply IC LM317 regulated from 1.25V to 22V is used and for -1.25 to -22V at output IC LM337 is used. Both ICs are capable of delivering 1.5A current, therefore suitable for our 1A requirements. In this circuit, LM317 and LM337 will provide an adjustable voltage corresponding to their input voltage. Both have the good load regulation characteristic. They will provide regulated and stabilized voltage at the output regardless of the variation in input voltage and load current.
About LM317 and LM337
317 is a positive voltage regulator that provides output in the range of 1.25V to 37V with input voltage up to 40V. On the contrary, 337 is a negative voltage regulator that will provide -1.25V to -37V with voltage input voltage of up to -40V. At the output, both can supply a maximum current of 1.5A according to the data sheet under ideal conditions.
To set the desired voltage at the output, resistive voltage divider circuit is used between the output pin and ground. The voltage divider circuit has a programming resistor (fixed resistor) and another is a variable resistor. By taking a perfect proportion between the feedback resistor (fixed resistor) and the variable resistor, we can obtain the desired value of the output voltage corresponding to the input voltage.
In this experiment, resistances R1 and R2 are used as programming resistance for 317 and 337 respectively.
Variable resistors RV1 and RV2 are used to vary the output voltage by 317 and 337 respectively.
• Protection diode
A D5 diode must be connected to 317, as shown in the image below. To prevent the external capacitor from discharging through the IC during an input short circuit. When the input is shorted, the cathode of the diode is at ground potential. The anode terminal of the diode is at high voltage as C5 is fully charged. So in this case, the diode is forward biased and all the capacitor discharge current passes through a diode to ground. This will save the LM317 from reverse current.
In a similar way, a diode D6 is connected to IC 337 to prevent the IC from discharging capacitor C6 through the IC when the input is shorted.
• Output voltage
The output voltage can be varied using tuning pin 317 and 337. Variable resistor RV1 and RV2 provide the output voltage from 1.25V to 22V and -1.25 to -22V respectively.
Practical Observation
No-load output voltage
• On LM317
By varying RV1 we can vary the output voltage in the range
Vout = 1.25V to 22V When the load is connected to the output setting the voltage at 20V• Load RL1 = 50 ohms
Vout (noted) = 16V (voltage drop from 20V to 16V)Then current output
I go out = 300mA
By varying RV2 we can vary the output voltage in the range
Vout = -1.25V to -22V
When the load is connected to the output by setting the voltage at -20V
• Load RL1 = 50 ohms
Vout (observed) = 17.5V (voltage drop from -20V to -17.5V)
Then current output
I go out = 320mA
With this, it can be analyzed that when the current demand increases at the output, the output voltage will begin to decrease. As current demand increases, IC 317 and 337 will begin to heat up and the IC will experience more sags, which will reduce the output voltage. Hence, a suitable heat sink is required when the current drawn at the output is increased to dissipate excessive heat from the circuit. The LM317 internally can tolerate 2W of power dissipation above this power, requiring a heatsink.
Application
Points to Remember
• The current rating of a step-down transformer, bridge diode and output diode must be greater than or equal to the current required at the output. Otherwise, it will not be able to provide the required current at the output.
• The rated voltage of a step-down transformer must be greater than the maximum required output voltage. This is due to the fact that the 317 and 337 experience a voltage drop of around 2-3 V. Therefore, the input voltage should be 2 V to 3 V higher than the maximum output voltage.
• Use a high value capacitor at the input, as a high value capacitor can deal with mains noise. Also use a capacitor at the output, this capacitor helps to deal with fast transient changes and noise at the output. The value of the output capacitor depends on the deviation in voltage, output current and transient response time of the circuit.
• Use protection diode if a high capacitor value is used at the output of the IC. To prevent the external capacitor from discharging through the IC during an input short circuit.
• The capacitor used in the circuit must have a higher rated voltage than the input voltage. Otherwise, the capacitor will start to leak current due to excess voltage on its plates and will explode.
• To drive high output load, a heat sink must be mounted in the regulator holes. This will prevent the IC from being exploded.
• As our circuit can consume a current of 1A at the output. A 1A fuse must be connected to the rectifier output. This fuse will prevent circuit current exceeding 1A. For currents above 1A, the fuse will blow and this will cut off the electrical supply to the circuit.
Project source code