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POWER SUPPLY 0.8V - 5.0V & WAVEFORM GENERATOR |
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PROTOTYPE FEATURES:
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The project presented incorporates a power supply voltage and a waveform generator in a simple circuit. It based on microcontroller Atmel ATmega32 (or ATmega16) and the code can be easily modified in order to use newer microcontrollers that have 4 Ports, External Interrupts, USART and ADC. The settings are done by a 4x4 keypad and two push buttons and indicators displayed in an LCD 2x16 series 1602 screen. The settings of the power supply can also be done by PC with the LabVIEW through serial port RS232 or USB port with a suitable converter.
The keyboard is connected to PORTB0:7 and the push buttons to PORTD2:3. Through the keyboard is the choice of operating power supply or generator and selection of the output voltage of the power supply and frequency of waveform of the generator. The frequency can also be changed in steps by the two push buttons. The LCD screen is connected to PORTA1:7 and the output of the power supply is connected to PORTA0 in order the ADC of microcontroller to read the voltage output and appearance it on the screen.
The DAC circuit is connected to PORTC0:7 and is based on a R2R resistor ladder network and an operational amplifier. The operational amplifier does not need a negative supply voltage and with the power transistors are supplied with a voltage higher than 7V DC (V2). For this reason, placed 2 Zenner diodes in order to prevent the application of high voltage to the microcontroller in case of failure. The feedback to the inversion input of the operational amplifier is the output of the power transistor in order to eliminate the Vbe. The feedback through R29 is that it is possible to modify the software to produce higher output voltages. For the same reason has been placed R31. R30 has been placed for the Offset zeroing of operational amplifier.
For the circuit without modifications (as originally designed) the jumpers should be as shown in the schematic and variable resistors in regulation without voltage division. The J5 must be closed during operation of the power supply and open during operation of the generator. The output transistors must be Darlington to be stable for currents as 2.5A. The accuracy of the output voltage of the power supply is in the range of 0.05V for voltages greater than 2V and in the range of 0.2V for smaller, while it is not possible to produce smaller voltages of 0.8V.
The width of the output waveform is 2Vpp for sinusoidal waveform, 2.5Vpp for triangular and snaggletooth and 3Vpp for square, and there is a DC component which can be eliminated if the waveform received from the J2 (free end of C1) or from the J5 when it is open for the operation of generator (free end of C5). In the first case the maximum output current is 25mA (depends on the operational amplifier to be installed), while in the latter case can reach 2.5A and depends on the output transistors. The DC component is enough to bring the DAC in the area of operation with optimum accuracy.
The maximum output frequency of the generator, for a clock frequency of 4MHz tomicrocontroller, is about 800Hz for sinusoidal and triangular with 128B resolution, 1.6kHz for snaggletooth with 64B resolution and 1MHz for square which has a resolution 2B. Output frequencies can be significantly increased by increasing the clock frequency of microcontroller. Evenmore can be increased if the resolution reduced, which if combined with a corresponding reduction in output amplitude can not be at all affected the quality of the waveforms. Increasing the output frequency must be accompanied by a reduction in values of the capacitors C3, C4, C6 and C7.
The values of table for the realization of the sinusoidal waveform can be calculated using a spreadsheet (eg Excel) with the formula = (((SIN (x * 2 * PI () / Res) +1) * Vpp / 2) + DC-Vp ) * 255/5, where Res represents the resolution and x takes values between 0 and Res. Operate the instrument is easy. On the home screen are the necessary information for functions. Pressing the A starts operation of the power supply or pressing the B generator. For the power supply the desired output voltage consists of one integer and one decimal number by pressing * for decimal point and then # for execution. If any error, clears the last entry with D. The software not allow the introduction
of a greater number of 5.0 or incorrect entry but only in the correct order: [number * number #]. The screen displays at the top line the desired voltage labeled Set: and at the bottom line the measured labeled Output:. The generator is not available to indicate the output frequency (in this version) but only a division factor of maximum frequency. The entry of the division factor is the keyboard from 1 to 4 digits and then # for execution.The output waveform starts and stops by pressing C. During production of waveform does not work any other key and there is no other function than only stepwise decrease or increase the frequency from the push buttons.
The type of waveform can be changed when there is production of waveform by pressing the * key. The screen at the top line shows the division factor labeled f div: and at the bottom line the waveform type and the available keys to their function. With LabVIEW is possible (in original version) only the control of power supply. After setting the COM port, the output voltage can be regulated from the rotary knob or from the numeric indication and confirmation with the OK button. In Volts field is displayed the output voltage and there is a lamp, with the characterization Deviation, that lights when there is deviation from the desired to output value. The tolerance can be adjusted by the corresponding field in steps of 100mV. The PCB of the DAC is available for downloading. For the circuit of microcontroller can use the development board presented to the corresponding page. Caution is needed in the reception of PORTC it is the only slot on the board which in position 1 is connected PORTC7 and for this reason should be constructed cable with reverse turn 1-8.
© 2012 Dimitrios Porlidas
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