Adjustable mini power supply from 0 to 15V 1A (Part 3/13)

This project aims to build an adjustable power supply circuit from 0 to 15V 1A. The circuit will act as a mini portable power supply for most electronic devices. The circuit can be used as a power adapter for smartphones, wearables and computer devices. In this project, an adjustable regulated symmetric positive power supply is designed. To reduce any fluctuations and ripples in the output, the supply needs to be regulated so that it can provide a constant voltage at the output. Again, as with previous designs, the voltage is adjusted using a variable resistor. This power supply provides regulated and adjustable voltage at the output.

The circuit will have an input of 220 V-230 V AC and generates a variable DC voltage in the range of 0 V to 15 V at the output. This power supply can provide a maximum current of 1A at the output.

Designing a power supply circuit is a step-by-step process that involves reducing AC voltage, converting AC voltage to DC voltage, smoothing DC voltage, compensating for transient currents, voltage regulation, voltage variation, and protection against short circuit.

Required components –

Fig. 1: List of components required for 0 to 15V 1A adjustable mini power supply

Circuit Connections –

The circuit is assembled in stages and each stage serves a specific purpose. To step down the 230 V AC, an 18 V – 0 -18 V transformer is used. One end of the transformer's secondary coil and its center strip are connected to a full-bridge rectifier. The full bridge rectifier is constructed by connecting four 1N4007 diodes together designated as D1, D2, D3 and D4 in the schematics. The cathode of D1 and the anode of D3 are connected to one of the secondary coils and the cathode of D2 and the anode of D3 are connected to the central ribbon. The cathodes of D2 and D4 are connected, of which one terminal is taken from the output of the rectifier and the anodes of D1 and D3 are connected, of which another terminal is taken from the output of the full-wave rectifier.

A 470 uF capacitor (shown as C1 in the schematic) is connected across the output terminals of a full-wave rectifier for smoothing purposes. For voltage regulation, the LM317 is connected in parallel to the smoothing capacitor. A variable resistance is connected in series to the voltage regulator IC for voltage adjustment and a 220 uF capacitor (shown as C2 in the schematics) is connected in parallel at the output to compensate transient currents. There is a diode connected between the input and output voltage terminals of the voltage regulator IC for short circuit protection.

Get the schematic diagram drawn or printed on paper and make each connection carefully. Only after checking each connection made correctly, connect the power circuit to an AC source.

1. AC to AC Conversion

2. AC to DC Conversion – Full Wave Rectification

3. Smoothing

4. Transient Current Compensation

5. Voltage regulation

6. Voltage adjustment

7. Short circuit protection

AC to AC Conversion

The voltage of the main sources (electricity fed by the intermediate transformer after reducing the line voltage of the generating station) is approximately 220-230 Vac, which needs to be further reduced to the 15 V level. To reduce 220 Vac to 15 Vac , a step-down transformer with center strip is used. The use of the center tap transformer allows generating positive and negative voltages at the input, however, only the positive voltage will be extracted from the transformer. The circuit experiences some drop in output voltage due to resistive loss. Therefore, a transformer with a high voltage rating greater than the required 15V needs to be used. The transformer must provide 1A current at the output. The most suitable step-down transformer that meets the mentioned voltage and current requirements is 18V-0-18V/2A. This transformer reduces the main line voltage to +/- 18 Vac, as shown in the image below.

AC to DC Conversion – Full Wave Rectification

The reduced AC voltage needs to be converted to DC voltage through rectification. Rectification is the process of converting AC voltage to DC voltage. There are two ways to convert an AC signal to DC. One is half-wave rectification and the other is full-wave rectification. In this circuit, a full wave bridge rectifier is used to convert 36V AC to 36V DC. Full-wave rectification is more efficient than half-wave rectification as it provides full use of both the negative and positive sides of the AC signal. In the full-wave bridge rectifier configuration, four diodes are connected in such a way that current flows through them in only one direction, resulting in a DC signal at the output. During full-wave rectification, two diodes are forward biased and two other diodes are reverse biased.

Fig. 4: Full Wave Rectifier Circuit Diagram

During the positive half cycle of the supply, diodes D2 and D3 conduct in series, while diodes D1 and D4 are reverse biased and current flows through the output terminal passing through D2, output terminal and D3. During the negative half cycle of the supply, diodes D1 and D4 conduct in series, but diodes D3 and D2 are reverse biased and current flows through D1, output terminal and D4. The direction of current in both directions through the output terminal in both conditions remains the same.

Fig. 5: Circuit Diagram showing the positive cycle of the Full Wave Rectifier

Fig. 6: Circuit diagram showing negative cycle of full wave rectifier

1N4007 diodes are chosen to build the full wave rectifier because they have maximum (average) forward current of 1A and in reverse bias condition can sustain peak reverse voltage up to 1000V. This is why 1N4007 diodes are used in this design for full wave rectification.

Smoothing

As the name suggests, it is the process of smoothing or filtering the DC signal using a capacitor. The output of the full wave rectifier is not a constant DC voltage. The rectifier output has twice the frequency of the main sources, but contains ripples. Therefore, it needs to be smoothed out by connecting a capacitor in parallel to the output of the full-wave rectifier. The capacitor charges and discharges during a cycle, providing a constant DC voltage as output. Thus, a capacitor (shown as C1 in the schematic) of high value is connected to the output of the rectifier circuit. As the DC that has to be rectified by the rectifier circuit has many AC spikes and unwanted ripples, to reduce these spikes a capacitor is used. This capacitor acts as a filtering capacitor that shunts all the AC through it to ground. At the output, the remaining average DC voltage is smoother and ripple-free.

Fig. 7: Smoothing Capacitor Circuit Diagram

At the output terminals of the power circuit, a capacitor (shown as C2 in the schematic) is connected in parallel. This capacitor helps in quick response to load transients. Whenever the current of the output loads changes, there is an initial shortage of current, which can be met by this output capacitor.

The output current variation can be calculated by

Output current, Iout = C (dV/dt) where

dV = Maximum allowable voltage deviation

dt = transient response time

Considering dv = 100mV

dt = 100us

In this circuit, a 220 uF capacitor is used, so

C = 220uF

Iout = 220u (0.1/100u)

Iout = 220mA

In this way it can be concluded that the output capacitor will respond to a current change of 220mA for a transient response time of 100 us.

Fig. 8: Circuit diagram of transient current compensator

Voltage regulation

The power supply circuit must provide regulated and constant voltage, without any fluctuation or variation. For voltage regulation, a linear regulator is required in the circuit. The purpose of using this regulator is to maintain a constant voltage at a desired level at the output. To provide regulated 0V to 15V IC LM317 is used. The voltage regulator IC is capable of supplying a current of 1.5A, so it is suitable for the current requirement of 1A. In this circuit, the LM317 will provide an adjustable voltage corresponding to its input voltage. The IC is capable of regulating the load by itself. It provides a regulated and stabilized voltage at the output, regardless of fluctuation in input voltage and load current.

LM317 is a positive voltage regulator that provides output in the range of 1.25V to 37V with input voltage up to 40V. At the output, it can supply a maximum current of 1.5A according to the datasheet under ideal conditions .

To define the desired voltage at the output, a resistive voltage divider circuit is used between the output pin and ground (central strip of the transformer). The voltage divider circuit has a programming resistor (fixed resistor) and another variable resistor. By taking a perfect relationship between the feedback resistor (fixed resistor) and a variable resistor, the desired value of the output voltage corresponding to the input voltage can be obtained. In this circuit, resistance R1 is used as programming resistance for LM317. Variable resistors RV1 are used to vary the output voltage in voltage regulator IC.

Fig. 9: Circuit diagram of IC based voltage regulator LM317

The LM317 has the following tolerable power dissipation internally –

Pout = (Maximum IC operating temperature)/ (Thermal Resistance, Junction-Environment + Thermal Resistance, Junction-Enclosure)

Pout = (150) / (65+5) (values ​​according to technical data sheet)

Pout = 2W

Therefore, the LM317 can internally sustain a power dissipation of up to 2W. Above 2W, the IC will not tolerate the amount of heat generated and will start to burn. This can also cause a serious fire hazard. Therefore, a heat sink is required to dissipate excessive heat from the IC.

Voltage adjustment

The output voltage can be varied using the tuning pin of the LM317 IC. The variable resistor RV1 is used to vary the output voltage from 0V to 15V. Since the minimum output of the LM317 is 1.25V, two 1N4007 diodes are connected in series with a 1K resistor (shown as R2 in the schematics) to make the minimum output close to 0V. Each diode suffers a drop of 0.7 V and the remaining drop is taken up by the 1k resistor. Thus, at the output we obtain a minimum voltage of 0.3V and a maximum voltage of 15.35V.

Short circuit protection

A diode D7 is connected between the voltage input and output terminals of the 317 IC, 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 C2 is fully charged. Therefore, in this case, the diode is forward biased and all the capacitor discharge current passes through the diode to ground. This saves IC LM317 from reverse current.

In the circuit, two diodes are already connected in series at the output which prevents the IC from countercurrent. Therefore, it is not necessary to connect a protection diode in this circuit; Still, connecting a protection diode across the voltage regulator just provides an extra layer of safety.

Fig. 10: Short circuit protection circuit diagram

The following precautions must be taken while assembling the circuit –

• The rated current of the step-down transformer, bridge diodes and voltage regulator ICs 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 the step-down transformer must be greater than the maximum required output voltage. This is due to the fact that the IC 317 experiences a voltage drop of around 2 to 3 V. Therefore, the input voltage must be 2V to 3V greater than the maximum output voltage and must be at the limit of the IC 317 input voltage. LM317.

• Capacitors used in the circuit must have a higher voltage rating than the input voltage. Otherwise, the capacitors will start leaking current due to excess voltage on their plates and will explode.

• A capacitor must be used at the output of the rectifier so that it can deal with unwanted noise from the mains. Likewise, the use of a capacitor at the output of the regulator is recommended to deal with rapid transient changes and noise at the output. The value of the output capacitor depends on the voltage deviation, current variations and the transient response time of the capacitor.

• A protection diode should always be used when using a capacitor after a voltage regulator IC, to prevent the IC from countercurrent during capacitor discharge

• For high output load activation, a heat sink must be mounted in the regulator holes. This will prevent the IC from exploding due to heat dissipation.

• As the regulator IC can only draw current up to 1A, a 1A fuse needs to be connected. This fuse will limit the current in the regulator to 1A. For currents above 1A, the fuse will blow and this will cut off the input power to the circuit. This will protect the circuit and regulator ICs from currents greater than 1A.

Once the circuit is assembled, it's time to test it. Connect the circuit to the main sources and change the variable resistance. Take voltage and current readings at the output terminal of the power circuit using a multimeter. Then connect fixed resistors as load and check voltage and current readings again.

At the output terminals the input voltage was 18V and when adjusting the variable resistance, the output voltage was between 0.34 to 15.35V when no load was connected. Expected values ​​were 0V to 15V. Therefore, the percentage error ends up being –

%Error= (Experimental value – expected value)*100/Expected value

% error = (15.35 – 15)* 100/15

% Error = 2.3%

When a load is connected to the output the maximum voltage is read as 15.35V. With a load resistance of 470Ω, the output voltage reads as 15.35V, showing no voltage drop. The output current is measured at 38.2 mA, so the power dissipation in the 470Ω resistance load is as follows –

Pout = (Vin – Vout)*Iout

Pout = (18-15.35) *(0.0328)

Pout = 86 mW

With a 47Ω resistance load, the output voltage reads at 15V showing a voltage drop of 0.35 V. The output current is measured at 310 mA, so the power dissipation in the 47Ω resistance load is Following -

Pout = (Vin – Vout)*Iout

Pout = (18-15) *(0.310)

Pout = 0.93W

With a 14.1Ω resistance load, the output voltage is read at 13V showing a voltage drop of 2.35V. The output current is measured at 870 mA, so the power dissipation in the 14Ω resistance load ,1Ω is as follows –

Pout = (Vin – Vout)*Iout

Pout = (18-13) *(0.870)

Pout = 4.35W

While testing the circuit, it was found that when the current demand increases at the output, the output voltage starts to decrease. As the current demand increases, the IC 317 starts to heat up and the IC experiences more sags, which reduces the output voltage. From the above practical experience, the power dissipation in the IC is greater than its internal tolerable limits. Therefore, it is recommended to use a heatsink to help cool the IC and increase its lifespan.

The power supply circuit designed in this project can be used as a portable power adapter for common electronic appliances and devices. It can also be used as a power station for electronic circuits and component testing.

Circuit Diagram-Adjustable-0-15V-1A-Mini-Power Supply