Download Variable Power Supply - USB Controlled

Variable Power Supply controlled via USB Interface
Jay Waghmare (05D07011) <[email protected]>
Vighnesh Rege (05D07024) <[email protected]>
Apoorva Oak (05D07027) <[email protected]>
Group No: D10
Supervisors: Abhay Karandikar, Dipankar
Abstract:
The objective of our project is to design a variable power supply, which can be controlled
using a computer, connected to the power supply module via a USB 2.0 interface. The
motivation behind the project is the need to integrate the various lab instruments with a
central computer so as to do away with the cumbersome mechanical knobs presently in use.
We have successfully implemented a standard linear power supply which provides a variable
voltage of 0-25 V at a current of 0.9 A. A USB to UART bridge interfaces the supply with a
computer for easy digital control.
Our circuit can be replicated with minor changes to obtain a computer-controlled multimeter,
function generator and other similar lab instruments. This has the potential to ease lab work
to a great extent as all the instruments can be controlled from the computer.
Design Approach:
The project consists of the following parts:
On board circuitry: Broadly speaking, this includes the following blocks:
 Power Supply unit: Step-down transformer, Rectifier, Power transistor [2N3055]
 Control Circuit : DAC0808, 741 as comparator, Sziklai pair of transistors.
 Current Limiting Protection: Optocoupler, RS Latch
 Internal power for the chips via LM7800 series regulators.
Interfacing:
The interfacing of the onboard circuitry to the computer is done via a USB 2.0 interface.
We have used the CP2102 USB to UART bridge along with an 89C51 microcontroller.
High level software:
A user friendly interface has been made using Visual Basic to set the required values of
voltage and for obtaining the readings and maximum limits depending on the devices
connected to the circuitry. The software part also includes the interfacing between the
microprocessor and the USB chip.
Design of circuit:
1. Power Supply unit: A standard power supply unit with a transformer and rectifier
has been implemented.
Figure 1: Transformer Rectifier Circuit
Figure 2:
Choice of components:

Power Transistor: Since we required a voltage range of up to 25V, power transistor
2N3055 was chosen.

NPN, PNP transistors: The BC548/558 series transistors have a good power rating.
Hence, they are used.

Resistors: R3 and R4 were chosen to be 10k and 3.3k respectively so as to limit the
current flowing through the transistors.

Capacitors: We have used high rating capacitors (2200µF, 63V) to smoothen out the
rectified voltage waveform.
2. Generation of DC voltages for Chip Operation:
The only voltage source available to the module is the AC line voltage of approximately
230 Volts. All DC voltages required for the functioning of the various chips should be
generated from this supply. The microprocessor requires +5V and -15V, while the OPAMP used needs +/- 15V. Voltage Regulators 7815, 7915, 7805 have been used to get
constant voltages +15V, -15V and +5V respectively, from the unregulated rectified
voltage. This voltage is used for driving the various chips in the circuit.
3. CP 2102 circuit:
The CP2102 chip allows USB is a highly-integrated USB-to-UART Bridge Controller. It
provides an easy implementation of USB interfacing with a computer without any
external components. It includes a USB 2.0 full-speed function controller, USB
transceiver, oscillator, EEPROM, and asynchronous serial data bus (UART).
The schematic diagram is given below.
Figure 3: CP2102 Microprocessor circuit
4. Current Limiting:
This includes a 74279 SR Latch and a PC 817 optocoupler. The PC817 senses the voltage
drop across a 1 ohm resistor added in series with the load. If the output current exceeds
0.9A , the optocoupler SETs the latch, the output of which is connected to the input of the
Figure 4: Flow Diagram
Test procedure and results:
The testing was done using loads ranging from 30 ohms to 20 kilo ohms. The power supply
gave constant output as per the input given from the computer. The power supply can provide
upto 0.9 A of current after which we limit it off.
The current protection worked satisfactorily for all the test cases, cutting off the power when
the current exceeded 0.9 A.
Conclusions and future improvements:
A power supply capable of a voltage range from 0 to 25V[upto 0.9 A current] has been
successfully implemented. We are able to control it from a computer via a USB interface.
The future improvements could be adding more functionalities to the module such as function
generator, multimeter etc.
References:
1) Muhammad Ali Mazidi, Janice Gillispie Mazidi, The 8051 Microcontroller and
Embedded Systems, Prentice Hall, 1999.
2) Brandon (2003), all kinds of datasheets. Available: http://www.alldatasheet.com/