Types of Power Supplies

Regulated power supplies usually refers to a power supply capable of supplying a variety of output voltages useful for bench testing electronic circuits, possibly with continuous variation of the output voltage, or just some preset voltages. Almost all electronic devices used in electronic circuits need a dc source of power to operate. A regulated power supply essentially con­sists of an ordinary power supply and a volt­age regulating device. The output from an ordinary power supply is fed to the voltage regulating device that provides the final output. The output voltage remains constant irrespective of variations in the ac input voltage or variations in output (or load) current but its amplitude is varied according to the load requirement.

Some of these types of power supplies are discussed below.

SMPS

The industry drive to more diminutive, lighter and more productive electronics systems has prompted the advancement of the SMPS, nothing but Switch Mode Power Supply. There are some topologies normally used to actualize SMPS. A switched-mode power supply is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently. In this by employing high switching frequencies, the sizes of the power transformer and associated filtering components in the SMPS are dramatically reduced in comparison to the linear. DC to DC converters and DC to AC converters belong to the category of SMPS.

In a linear regulator circuit the excess voltage from the unregulated dc input supply drops across a series element and hence there is power loss in proportion to this voltage drop, whereas in switched mode circuit the unregulated portion of the voltage is removed by modulating the switch duty ratio. The switching losses in modern switches (like: MOSFETs) are much less compared to the loss in the linear element.

The majority of electronic DC loads are supplied from standard power sources. Unfortunately, standard source voltages may not match the levels required by microprocessors, motors, LEDs, or other loads, especially when the source voltage is not regulated like battery sources and other DC as well AC sources.

SMPS Block Diagram:

The main idea behind a switch mode power supply (SMPS) can be easily understood from the concept of conceptual explanation of a DC-DC converter. If the system input is AC then the 1st stage is to convert to DC. This is called rectification. The SMPS with a DC input does not require the rectification stage. Many newer SMPS will use a special Power factor correction (PFC) circuit. By following the sinusoidal wave of the AC input, we can make the input current. And rectified signal is filtered by the input reservoir capacitor to produce the unregulated DC input supply. The unregulated DC supply is given to high frequency switch. For higher frequencies, components with more level capacitance and inductance are required. In this MOSFETs may be used as synchronous rectifiers, these have even lower conducting stage voltage drops. The high switching frequency, switches the input voltage across the primary of the power transformer. The drive pulses are normally fixed frequency and variable duty cycle. The output of the secondary transformer is rectified and filtered. Then it is sent to output of the power supply. Regulation of the output to provide a stabilized Dc supply is carried out by the control or feedback block.

Most SMPS. Systems operate on a fixed frequency pulse width modulation basis, where the duration of the on time of the drive to the power switch is varied on a cycle by cycle basis. The pulse width signal given to the switch is inversely proportional to the output of the output voltage. The oscillator is controlled by the voltage feedback from a closed loop regulator. This is usually achieved by using a small pulse transformer or an opto-isolator, hence adding to the component count. In an SMPS, the output current flow depends on the input power signal, the storage elements and circuit topologies used, and also on the pattern used to drive the switching elements. By using LC filters the output waveforms are filtered.

Advantages of SMPS:

• Greater efficiency because the switching transistor dissipates little power
• Lower heat generation due to higher efficiency
• Smaller in size
• Lighter weight
• Reduced harmonic feedback into the supply main

Applications of SMPS:

• Personal computers
• Machine tool industries
• Security systems

Along with SMPS another circuit for regulated supply and back up purpose is discussed below.

Linear Power Supplies

Work bench power supply with backup

A work bench power supply is a DC power supply unit which can provide different regulated DC voltages which is used for the purpose of testing or trouble shooting. A simple circuit of regulated power supply with battery backup has been designed which can be used as a work bench power supply. It gives 12 volts, 9 volts and 5 volts regulated DC to power prototypes while testing or trouble shooting. It also has a battery back up to continue the work if power fails. Low battery indication is also provided to confirm the battery status.

It Consists of Three Major Sections:

A rectifier and a filter unit which converts the AC signal to regulated DC signal using the combination of transformer, diodes and capacitors.

A Battery used as an alternative, which can be recharged during the main power supply and used as a source of power in case of absence of main supply.

A battery charge indicator which gives an indication of the battery charge and discharge.

A 14-0-14, 500 mA transformer, rectifier diodes D1, D2 and smoothing capacitor C1 form the power supply section. When the mains power is available, D3 forward biases and provides more than 14 volts DC to IC1 which then gives regulated 12 volts that can be tapped from its output. At the same time, IC2 gives regulated 9 volts and IC3 regulated 5 volts from their outputs.

A 12 volt 7.5 Ah rechargeable battery is used as backup. When mains power is available, it charges via D3 and R1. R1 limits the current for charging. To prevent overcharging, if the power supply is switched for long time and the battery is not using, Trickle charge mode is safe. The charging current will be around 100-150 mA. When the mains power fails, D3 reverse biases and D4 forward biases and battery takes the load. A UPS battery is an ideal choice.

Zener diode ZD and the PNP transistor T1 form the low battery indicator.  This kind of arrangement is used in Inverters to indicate the low battery status. When the battery voltage is above 11 volts, Zener conducts and keeps the base of T1 high so that it remains off. When the battery voltage drops below 11 volts, the Zener turns off and T1 forward biases. (Zener diode conducts only when the voltage through it is above 1 volt or higher than its rated voltage. So here the 10 volt zener conducts only if the voltage is above 11 volts.) LED then lights to indicate the need for battery charging. VR1 adjusts the correct off point of the Zener .Charge the battery fully and measure its terminal voltage .If it is above 12 volts, adjust the wiper of the preset VR1 in the middle position, and slightly turn it till LED turns off. Do not turns the Preset to the extreme ends. Battery should always contain sufficient voltage above 12 volts (Fully charged battery will show around 13.8 volts) then only IC1 gets sufficient input voltage.

Self Switching Power Supply Free Circuit Diagram

In this circuit diagram, given a regulated power supply circuit that though a fixed-voltage regulator U1-LM7805 not only gives a variable but also auto switch off features. This is achieved by a potentiometer which is connected between the regulator IC common terminal and ground. For every 100-ohm increment in the in-circuit value of the resistance of potentiometer RV1, the output voltage increases by 1 volt. Thus, the output varies from 3.7V to 8.7V (taking into account 1.3-volt drop across diodes D7 and D8).

When no load is connected across its output terminals, then the supply is that it switches itself off. This is achieved with the help of transistors Q1 and Q2, diodes D7 and D8, and capacitor C2. When a load is connected at the output, potential drop across diodes D7 and D8 (approximately 1.3V) is sufficient for transistors Q2 and Q1 to conduct. As a result, the relay gets energized and remains in that state as long as the load remains connected. At the same time, capacitor C2 gets charged to around 7-8 volt potential through transistor Q2. But when the load ( a lamp here in series with S2 )  is disconnected, transistor Q2 is cut off. However, capacitor C2 is still charged and it starts discharging through base of transistor Q1. After some time (which is basically determined by value of C2), relay RL1 is de-energized, which switches off the mains input to primary of transformer TR1. To resume the power again, switch S1 Push button should be pressed momentarily. The delay in switching off the power supply varies directly with the capacitor value.

A transformer with a secondary voltage of 12V-0V, 250mA was used, it can nevertheless be changed as per user’s requirement (up to 30V maximum. and 1-ampere current rating). For drawing more than 300mA current, the regulator IC must be fitted with a small heat sink over a mica insulator. When the transformer’s secondary voltage increases beyond 12 volts (RMS), potentiometer RV1 must be re-dimensioned. Also, the relay voltage rating should be predetermined.

Variable Power Supply using LM338

DC power supply is often required to power electronic devices. While some require a regulated power supply, there are many applications where the output voltage needs to be varied. Variable power supply is the one where we can adjust the output voltage according to the requirements. Variable power supply can be used in many applications like applying variable voltage to DC motors, applying variable voltages to High voltage DC-DC converters to adjust the gain, etc. It is mostly used in testing electronic projects.

The main component in a variable power supply is any regulator whose output can be adjusted using any means like a variable resistor. Regulator ICs like LM317 provide a adjustable voltage from 1.25 to 30V. Another way is using LM33 IC.

Here a simple variable power supply circuit using LM33 is used which is a high current voltage regulator.

LM 338 is the high current voltage regulator that can supply an excess of 5 amperes current to the load. Output voltage from the regulator can be adjusted from 1.2 volts to 30 volts. It requires only two external resistors to set the output voltage. LM 338 belongs to the LM 138 family which is available in 3 terminal package. It can be used in applications such as adjustable power supply, constant current regulator, battery chargers etc. A high current variable supply is essential to test high power amplifier circuits, during trouble shooting or servicing. This allows the power supply to be used with high transient loads and speeds start up under full load condition. The over load protection remains functional even if the adjust pin is disconnected accidentally.

Circuit Description

The basic circuit consists of the following parts:

1. A Step down Transformer to cause a drop in ac voltage of 230V.
2. A rectifier module to rectify the AC signal.
3. A smoothing electrolyte capacitor to filter out the dc signal and remove the ac ripples.
4. LM338
5. A variable resistor

Working of the Circuit

The variable power supply using LM338 positive voltage regulator is shown below. The power is derived from a 0-30 volts 5 ampere step down transformer. The 10 amps rectifier module rectifies the low volt AC to DC which is made ripple free by the smoothing capacitor C1. Capacitor C2 and C3 improves the transient responses. Output voltage can be adjusted through the Pot VR1 to the desired voltage from 1.2 volts to 28 volts.D1 protects against C4 and D2 protect against C3 when switched off. Regulator requires heat sink.

Vout = 1.2V (1+ VR1 / R1) + I AdjVR1.