Triple Bench Power Supply (Part 8/13)

In previous designs adjustable power supplies and symmetrical power supplies at constant voltage were made. In this project an asymmetric power supply will be designed. The triple bench power supply acts as a power supply providing three different constant voltages. The different voltage levels are extracted using a single transformer. The triple bench power supply designed in this project provides 5V, 9V and 12V at the output with 1A as maximum current limit for each voltage. The output voltages are regulated sources independent of unwanted fluctuations in the input voltage.

The power supply circuit designed in this project uses voltage regulator IC 7805, 7809 and 7812 to produce regulated supplies of 5V, 9V and 12V. The conventional power circuit design steps such as reducing AC voltage, converting AC voltage to DC voltage and smoothing DC voltage to obtain direct input from AC grid are employed to assemble the triple bench circuit.

Fig. 1: List of components required for triple bench power supply

Block diagram –

Fig. 2: Triple Bench Power Supply Block Diagram

Firstly, to step down the 230V AC, a 18V-0-18V center strip transformer is used. The secondary coil of the transformer is 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 D2 are connected to one of the secondary coils and the cathode of D4 and the anode of D3 are connected to the central ribbon of the secondary coil. The cathodes of D2 and D3 are connected, of which one terminal is taken from the output of the rectifier and the anodes of D1 and D4 are connected, of which another terminal is taken from the output of the full-wave rectifier.

For 5V supply, a 1A fuse is connected in series to the output of the full-wave rectifier for protection against AC sources. A 100 uF capacitor (shown as C1 in the schematic) is connected across the output terminals of the full-wave rectifier for smoothing purposes. For voltage regulation, IC LM-7805 is connected in parallel with the smoothing capacitor. The output is taken from the voltage output terminal of the 7805 IC. A 10 uF capacitor (shown as C2 in the schematic) is connected to the source output to compensate transient currents.

For 9V supply, again a 1A fuse is connected in series to the full wave rectifier output for protection against AC sources. A 100 uF capacitor (shown as C3 in the schematic) is connected across the output terminals of the full-wave rectifier for smoothing purposes. For voltage regulation, IC LM-7809 is connected in parallel with the smoothing capacitor. The output is taken from the voltage output terminal of the 7809 IC. A 10 uF capacitor (shown as C4 in the schematic) is connected to the source output to compensate transient currents.

Similarly for 12V supply, again a 1A fuse is connected in series to the full wave rectifier output for protection against AC sources. A 100 uF capacitor (shown as C5 in the schematic) is connected across the output terminals of the full-wave rectifier for smoothing purposes. For voltage regulation, IC LM-7812 is connected in parallel with the smoothing capacitor. The output is taken from the voltage output terminal of the 7812 IC. A 10 uF capacitor (shown as C6 in the schematic) is connected to the source output to compensate transient currents.

The power circuit operates in well-defined stages, each stage serving a specific purpose. The circuit operates in the following steps –

1. AC to AC Conversion

2. AC to DC Conversion – Full Wave Rectification

3. Smoothing

4. Voltage regulation

AC to AC Conversion

The voltage of the main sources is approximately 220-230 Vac, which needs to be reduced to the 12 V level. To reduce 220 Vac to 12 Vac, a step-down transformer with a 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 12V 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.

Fig. 3: 18-0-18V Transformer Circuit Diagram

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 D4 conduct in series while diodes D1 and D3 are reverse biased and current flows through the output terminal passing through D2, output terminal and D4. During the negative half cycle of the supply, diodes D1 and D3 conduct in series, but diodes D1 and D2 are reverse biased and current flows through D3, output terminal and D1. 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

Figure 6: Circuit diagram showing the negative cycle of the 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

Smoothing is the process of 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. Therefore, 100 uF capacitors (shown as C1, C3 and C5 in the schematics) are 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. These capacitors act as a filtering capacitor that shunts all the AC through them to ground. At the output, the remaining average DC voltage is smoother and ripple-free.

Voltage regulation

The output voltages are taken from the three regulator ICs. To obtain three different voltages at the output, three regulators are used. 5V output is taken from IC 7805, 9V is taken from IC 7809 and 12V is taken from IC 7812 output. These ICs provide a constant and regulated output voltage. Consequently, any fluctuations and unwanted spikes in the input voltage do not affect the output voltage.

IC 7805 provides an output voltage in the range of 4.75V to 5.25V with the input voltage range of 7V to 20V. IC 7809 provides an output voltage in the range of 8.6V to 9.4 V with the input voltage range of 11.5 V to 24 V. IC LM7812 can have input voltages of 14.5 V to 27 V and provides a constant output voltage of 11.4 V to 12 .6 V. The ICs have a maximum current limit of 1A.

Fig. 8: Table listing the input and output voltage of the 7805, 7809 and 7812 voltage regulator ICs

These ICs have the following tolerable power dissipation internally –

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

Pout = (125) / (65+5) (values ​​according to technical data sheet)

Pout = 1.78W

Therefore, 7805, 7809 and 7812 can internally sustain up to 1.78W of power dissipation. Above 1.78W, these ICs will not tolerate the amount of heat generated and will start to burn. This can also cause a serious fire hazard. Therefore, heat sink is necessary to dissipate excessive heat from ICs.

Compensating Transient Currents

At the output terminals of the power circuit, 10 uF capacitors (shown as C2, C4 and C6 in the schematics) are connected in parallel. These capacitors help in quick response to load transients. Whenever the output load current changes, there is an initial shortage of current, which can be met by these output capacitors.

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 10 uF capacitor is used, so,

C = 10uF

Iout = 10u (0.1/100u)

Iout = 10mA

In this way it can be concluded that the output capacitor will respond to a current change of 10mA for a transient response time of 100 us.

Fig. 9: Circuit diagram of transient current compensator

Short circuit protection

Diodes D5, D6 and D7 are connected between the voltage input and output terminals of the ICs 7805, 7809 and 7812 respectively, so that they can prevent the external capacitors (C2, C4 and C6 respectively) from being discharged through the respective ICs 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 the respective capacitor 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 the respective feedback current regulator IC.

Fig. 10: Short circuit protection circuit diagram for triple bench power supply

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 7805, 7809 and 7812 ICs experience a voltage drop of around 2 to 3 V. Therefore, the input voltage must be 2 V to 3 V greater than the maximum output voltage and must be in the input voltage limit of regulator ICs.

• 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.

• Protection diode should always be used when using the 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 regulator ICs 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 can be tested with a multimeter. Measure the output voltage at the terminals of the 7805, 7809, and 7812 ICs. Then measure the voltage outputs when the loads are connected.

In regulator IC 7805, the input voltage is 12V and the output voltage is 5.06V. With a 5 Ω resistance load, the output voltage reads at 3.43 V showing a voltage drop of 1.63 V. The output current is measured at 590 mA. With a 10 Ω resistance load, the output voltage is read as 4.08 V, showing a voltage drop of 0.98 V. The output current is measured at 370 mA. So the power dissipation in a 10 Ω resistance load is as follows-

Pout = (Vin – Vout)*Iout

Pout = (18–4.08)*0.370

Pout = 5.2W

In regulator IC 7809, the input voltage is 12V and the output voltage is 9.15V. With a load resistance of 20 Ω, the output voltage reads at 8.18 V showing a voltage drop of 0.97 V. The output current is measured at 400 mA. With a 10 Ω resistance load, the output voltage reads at 7.38 V showing a voltage drop of 1.77 V. The output current is measured at 680 mA. so the power dissipation in a 20 Ω resistance load is as follows-

Pout = (Vin – Vout)*Iout

Pout = (18 –8.18)*0.4

Pout = 3.9W

In regulator IC 7812, the input voltage is 12V and the output voltage is 12.22V. With a load resistance of 20 Ω, the output voltage is read as 10.86 V showing a voltage drop of 1.36 V. The output current is measured at 490 mA. With a 10 Ω resistance load, the output voltage is read as 9.02 V, showing a voltage drop of 3.2 V. The output current is measured at 830 mA. So the power dissipation with a load resistance of 20 Ω is as follows –

Pout = (Vin – Vout)*Iout

Pout = (18 – 10.86)*0.490

Pout = 3.5W

During circuit testing, it was analyzed that when the current demand increases at the output, the output voltage begins to decrease. As current demand increases, the 7805, 7809, and 7812 ICs begin to heat up and the ICs experience more sags, which reduces the output voltages. From the above practical experience, the power dissipation in ICs is higher than their internal tolerable limits. Therefore, it is recommended to use heat sinks to help cool the ICs and increase the useful life of these voltage regulator ICs.

The circuit designed in this project can be used as a regulated power supply and installed as standard power adapters. It can be used to bias Integrated Chips that require differential voltages.

Circuit Diagram-Triple Bench-Power Supply