DIY Variable Lab Bench Power Supply | 110V/220V to 30V 10A DC Power Supply

 DIY Variable Lab Bench Power Supply | 110V/220V to 30V 10A DC Power Supply

Power supply is an utmost essential tool for an electronic lab. It comes in handy for powering up various applications and circuits. However a fixed voltage, fixed current power supply is sufficient for basic needs but a variable one is good to have because different circuits and components operate at different voltages and consume different current. When it comes to an electronic hobbyist’s lab, a good power supply is must to have. Also if the power supply boosts additional features like on board voltage and current display, it comes in handy as one can know the exact voltage at the output terminals and also the current drawn by the load. But in the electronic market, those power supplies are not economic and are meant for industrial purpose. Here in this article I present an economical and cost effective yet efficient variable bench power supply that is capable of providing 1.2 to 25 Volt variable supply up to 5 Ampere through one channel while 5 Volt, 1 Ampere and 12 Volt, 1 Ampere supply through other two channels thus having one variable and two fixed supply channels. It also displays the voltage on the output terminals and also the current and instantaneous power drawn by the load on an on board LCD display. Not only this one can connect this power supply to his personal computer via the serial port and can see the voltage, current and power drawn by the load graphically. The project is portable and simple to build that even a newcomer can build this power supply with ease and add it into his lab. The project uses components that are easily and cheaply available. The project is composed of two modules, one is main power module that consists of linear voltage regulators with rectification and filtering circuitry for supply generation and regulation while the other is composed of a micro controller which is used to sense and display the current and voltage across the variable supply channel. The second section only provides an additional functionality of displaying current and voltage, however one can build the power supply even skipping the second section leaving it functional but without the display feature. (Actually this version 2, I have already built one which was later modified with new features).

The block diagram is divided into two parts, one for each section:

The main power section is the one which is used to derive a regulated DC voltage from the mains 220 V AC input. The diagram is divided into four sections. First one is the step down section which converts the 220 V AC input into 20 V AC. This uses a step down transformer. Then the second section is the rectification section which rectifies the AC component of the signal of its input. It uses a bridge rectifier. Then the signal is passed onto the third section which is the filtering section which consists of capacitors which act as a first order low pass filter and further smooths the signal. After this stage, we get a smoothened DC voltage which is ready to be interfaced to the next section which is the regulator section. Now as we have three channels for our supply output that is one variable supply channel and two others are fixed ones, so the regulator section is further divided into three sections one for each channel. The two regulator sections comprise of a fixed linear voltage regulator while the third one consists of a variable linear voltage regulator. Thus at the end, we get our three channels of power supply which goes to the output terminals.

The block diagram for the second (version 1) section is given below. This is the interface and display section which senses the current and voltage on the variable supply channel and gives off the readings to the LCD display and computer.It consists of a voltage divider section and a series shunt arrangement at the variable supply channel for sensing the Voltage across the terminals and current through load respectively. A non-inverting amplifier is used after the shunt arrangement to amplify the voltage drop across the shunt resistor. The outputs of the voltage divider network and the non-inverting amplifier goes to analog-to-digital converter inputs of a microcontroller in the next stage which converts these real time analog data into digital ones so that it can be processed in the microcontroller, interpreted and sent off to the serial interface and display section. Two buttons are provided to act as inputs and a buzzer is there to give audio messages.

Version 2 description (in addition to version 1):

Version 2 of this power section is similar to the version 1, rather only some minor improvements were made like we have increased two more 4700uf,50V filtering capacitors, and added 100nf ceramic caps after LM7805 and LM7812 for functional stability purpose. Now earlier, we were directly feeding >25v DC to the inputs of LM7805 and LM7812 regulators which was not a good idea because the power (as heat) dissipated from the regulator is directly proportional to the voltage difference across it and current taken from the regulator, so to minimize the power dissipation, we have to minimize the voltage difference across the regulators. So we have included a series array of 1N4002 (or any general purpose diode) to make an appropriate voltage drop before it goes to the two fixed voltage regulators.So after diode D18, we will get a nominal voltage of 16v which feed the LM7812 and a nominal voltage of 10v after D26 which feeds the LM7805. Although this is not the best way to do it, you can also use a dual secondary core transformer with lower voltage at the second secondary to feed the LM7805 or LM7812.

Version 1 description:

It comprises of a step down transformer (TR1), four 6A diodes(D1-D4), regulator LM338(IC1), regulator LM7812(IC2), regulator LM7805(IC3), and a few discrete components.

A fuse of 0.6 Ampere rating is connected in series with the primary of the transformer to prevent short circuit. The step down transformer can be any with primary input of 220V AC and secondary output of 20V AC with a current rating of >7Ampere. The four diodes used are any general purpose power diode with current rating of more than 6 Ampere, example GI820, MBR750, P600, etc. A 100nF, 50V capacitor is connected in parallel to each of the rectifier diodes (D1-D4).These capacitors mainly are meant for smoothing purpose. Instead of these four diodes, one can use a single module bridge rectifier with power rating of more than 6 Ampere.

The LM338 is an adjustable 3-terminal positive voltage regulator, capable of supplying in excess of 5A over a 1.2V to 32V output range. It is exceptionally easy to use and requires only 2 resistors to set the output voltage. A unique feature of the LM338 is time-dependent current limit. The current limit circuitry allows peak currents of up to 12A to be drawn from the regulator for short periods of time. This allows the LM338 to be used with heavy transient loads and speeds start-up under full-load conditions. Under sustained loading conditions, the current limits decreases to a safe value protecting the regulator. Also included on the chip are thermal overload protection and safe area protection for the power transistor. Overload protection remains functional even if the adjustment pin is accidentally disconnected. The device provides an excellent line regulation of 0.005%/V when 3V ≤ (VIN − VOUT) ≤ 35V and load regulation of 0.1% when 10 mA ≤ IOUT ≤ 5A.

The LM7805 and LM7812 are three terminal positive regulators are available in the TO-220 package and with fixed output voltages of 5V and 12V, making them useful in a wide range of applications. Each type employs internal current limiting, thermal shut down and safe operating area protection, making it essentially indestructible. If adequate heat sinking is provided, they can deliver over 1A output current. A maximum of 35 V DC can be supplied to the input of these regulators provided that the device is properly mounted on a heatsink.

The 20V AC secondary output of the transformer(TR1) is fed directly to the bridge rectifier which rectifies the AC component and converts it into DC voltage.

As because, we are using bridge rectification configuration, the ideal average magnitude of the output of the bridge rectifier is given as:

V(peak) = V(rms)

And after subtracting the voltage drop of the two diodes that work in series in working of bridge rectification, we get the actual approx output voltage as:

V(actual) = 28.2848 - (2

which is approximate to 27 volt.

This is filtered with the help of C1 capacitor which acts as a low pass filter there.

The LED(LED1) lights up when the circuit is switched ON.Then this signal is fed to the input of the three regulators.

Referring to the datasheet of LM338, a dropout of 1.5 to 3 volt appears in the output of the regulator from the input when the working temperature is increased from -75 degree celcius to +150 degree celcius. However at normal working range from 20 to 40 degrees celcius, a typical dropout of 2.6 V is obtained when the current drawn from the device is max 5 ampere.

 DIY Variable Lab Bench Power Supply | 110V/220V to 30V 10A DC Power Supply

Power supply is an utmost essential tool for an electronic lab. It comes in handy for powering up various applications and circuits. However a fixed voltage, fixed current power supply is sufficient for basic needs but a variable one is good to have because different circuits and components operate at different voltages and consume different current. When it comes to an electronic hobbyist’s lab, a good power supply is must to have. Also if the power supply boosts additional features like on board voltage and current display, it comes in handy as one can know the exact voltage at the output terminals and also the current drawn by the load. But in the electronic market, those power supplies are not economic and are meant for industrial purpose. Here in this article I present an economical and cost effective yet efficient variable bench power supply that is capable of providing 1.2 to 25 Volt variable supply up to 5 Ampere through one channel while 5 Volt, 1 Ampere and 12 Volt, 1 Ampere supply through other two channels thus having one variable and two fixed supply channels. It also displays the voltage on the output terminals and also the current and instantaneous power drawn by the load on an on board LCD display. Not only this one can connect this power supply to his personal computer via the serial port and can see the voltage, current and power drawn by the load graphically. The project is portable and simple to build that even a newcomer can build this power supply with ease and add it into his lab. The project uses components that are easily and cheaply available. The project is composed of two modules, one is main power module that consists of linear voltage regulators with rectification and filtering circuitry for supply generation and regulation while the other is composed of a micro controller which is used to sense and display the current and voltage across the variable supply channel. The second section only provides an additional functionality of displaying current and voltage, however one can build the power supply even skipping the second section leaving it functional but without the display feature. (Actually this version 2, I have already built one which was later modified with new features).

The block diagram is divided into two parts, one for each section:

The main power section is the one which is used to derive a regulated DC voltage from the mains 220 V AC input. The diagram is divided into four sections. First one is the step down section which converts the 220 V AC input into 20 V AC. This uses a step down transformer. Then the second section is the rectification section which rectifies the AC component of the signal of its input. It uses a bridge rectifier. Then the signal is passed onto the third section which is the filtering section which consists of capacitors which act as a first order low pass filter and further smooths the signal. After this stage, we get a smoothened DC voltage which is ready to be interfaced to the next section which is the regulator section. Now as we have three channels for our supply output that is one variable supply channel and two others are fixed ones, so the regulator section is further divided into three sections one for each channel. The two regulator sections comprise of a fixed linear voltage regulator while the third one consists of a variable linear voltage regulator. Thus at the end, we get our three channels of power supply which goes to the output terminals.

The block diagram for the second (version 1) section is given below. This is the interface and display section which senses the current and voltage on the variable supply channel and gives off the readings to the LCD display and computer.It consists of a voltage divider section and a series shunt arrangement at the variable supply channel for sensing the Voltage across the terminals and current through load respectively. A non-inverting amplifier is used after the shunt arrangement to amplify the voltage drop across the shunt resistor. The outputs of the voltage divider network and the non-inverting amplifier goes to analog-to-digital converter inputs of a microcontroller in the next stage which converts these real time analog data into digital ones so that it can be processed in the microcontroller, interpreted and sent off to the serial interface and display section. Two buttons are provided to act as inputs and a buzzer is there to give audio messages.

Version 2 description (in addition to version 1):

Version 2 of this power section is similar to the version 1, rather only some minor improvements were made like we have increased two more 4700uf,50V filtering capacitors, and added 100nf ceramic caps after LM7805 and LM7812 for functional stability purpose. Now earlier, we were directly feeding >25v DC to the inputs of LM7805 and LM7812 regulators which was not a good idea because the power (as heat) dissipated from the regulator is directly proportional to the voltage difference across it and current taken from the regulator, so to minimize the power dissipation, we have to minimize the voltage difference across the regulators. So we have included a series array of 1N4002 (or any general purpose diode) to make an appropriate voltage drop before it goes to the two fixed voltage regulators.So after diode D18, we will get a nominal voltage of 16v which feed the LM7812 and a nominal voltage of 10v after D26 which feeds the LM7805. Although this is not the best way to do it, you can also use a dual secondary core transformer with lower voltage at the second secondary to feed the LM7805 or LM7812.

Version 1 description:

It comprises of a step down transformer (TR1), four 6A diodes(D1-D4), regulator LM338(IC1), regulator LM7812(IC2), regulator LM7805(IC3), and a few discrete components.

A fuse of 0.6 Ampere rating is connected in series with the primary of the transformer to prevent short circuit. The step down transformer can be any with primary input of 220V AC and secondary output of 20V AC with a current rating of >7Ampere. The four diodes used are any general purpose power diode with current rating of more than 6 Ampere, example GI820, MBR750, P600, etc. A 100nF, 50V capacitor is connected in parallel to each of the rectifier diodes (D1-D4).These capacitors mainly are meant for smoothing purpose. Instead of these four diodes, one can use a single module bridge rectifier with power rating of more than 6 Ampere.

The LM338 is an adjustable 3-terminal positive voltage regulator, capable of supplying in excess of 5A over a 1.2V to 32V output range. It is exceptionally easy to use and requires only 2 resistors to set the output voltage. A unique feature of the LM338 is time-dependent current limit. The current limit circuitry allows peak currents of up to 12A to be drawn from the regulator for short periods of time. This allows the LM338 to be used with heavy transient loads and speeds start-up under full-load conditions. Under sustained loading conditions, the current limits decreases to a safe value protecting the regulator. Also included on the chip are thermal overload protection and safe area protection for the power transistor. Overload protection remains functional even if the adjustment pin is accidentally disconnected. The device provides an excellent line regulation of 0.005%/V when 3V ≤ (VIN − VOUT) ≤ 35V and load regulation of 0.1% when 10 mA ≤ IOUT ≤ 5A.

The LM7805 and LM7812 are three terminal positive regulators are available in the TO-220 package and with fixed output voltages of 5V and 12V, making them useful in a wide range of applications. Each type employs internal current limiting, thermal shut down and safe operating area protection, making it essentially indestructible. If adequate heat sinking is provided, they can deliver over 1A output current. A maximum of 35 V DC can be supplied to the input of these regulators provided that the device is properly mounted on a heatsink.

The 20V AC secondary output of the transformer(TR1) is fed directly to the bridge rectifier which rectifies the AC component and converts it into DC voltage.

As because, we are using bridge rectification configuration, the ideal average magnitude of the output of the bridge rectifier is given as:

V(peak) = V(rms)

And after subtracting the voltage drop of the two diodes that work in series in working of bridge rectification, we get the actual approx output voltage as:

V(actual) = 28.2848 - (2

which is approximate to 27 volt.

This is filtered with the help of C1 capacitor which acts as a low pass filter there.

The LED(LED1) lights up when the circuit is switched ON.Then this signal is fed to the input of the three regulators.

Referring to the datasheet of LM338, a dropout of 1.5 to 3 volt appears in the output of the regulator from the input when the working temperature is increased from -75 degree celcius to +150 degree celcius. However at normal working range from 20 to 40 degrees celcius, a typical dropout of 2.6 V is obtained when the current drawn from the device is max 5 ampere.