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STUDIA UNIVERSITATIS BABEŞ-BOLYAI, PHYSICA, SPECIAL ISSUE, 2003 MODULAR - MICROWAVE POWER SYSTEM, BASED ON MICROCONTROLLER UNIT V.Surducan, E. Surducan, C.Neamtu National Institute of Research and Development for Isotopic and Molecular Technology, Donath 75-103, Cluj-Napoca, Romania The “Modular-microwave power system” was designed for fine adjustment and control of any microwave power generator, based on continuous wave magnetron, with output power of max. 1000W. The destination for such power generators covers many scientific or industrial applications, from drying, microwave assisted digestion, polymerization, sample extractions etc. For scientific and other sensitive applications, it’s imperative to control precisely both the output power and the energy of the microwave generator. The scientific key for this complex power and energy adjustment is the use of a smart device, like a personal computer, or a standalone microcontroller unit, to control the microwave generator’s power supply. An user-friendly interface, having only four functional buttons and a large LCD display with 2 rows and 16 characters/row, assures the input parameters for the time pulse (energy) and power. A real time clock measuring the programming time and generates the markers for energy adjustment. Two different programming modes are implemented: a local mode and a programed mode via an optoisolated RS232/485 interface, using an external personal computer unit. Various resolutions for the microwave output power may be achieved, with a simple and stalwart procedure by switching the power supply coils of the high voltage transformer. Sample’s temperature can be logged and collected on the PC using some smart temperature sensors connected to one measurement bus. Design consideration Our goal was to design and manufacture an universal microwave power system, being able to generate inside the microwave coaxial chamber, a range between low microwave power (beyond 50W for small sample volumes) to high microwave power (900W for large sample volumes), in different effective power steps using the same magnetron (900W/2.45GHz) and the same microwave circuit, because the microwave sample preparation for small (less than 6ml) and very small sample volumes are requiring an expensive and specialized microwave digestors. This has the effect of decreasing the system cost and increase of versatility. It was necessary to modulate the output energy in a precisely time quantum, because 50W V.SURDUCAN E.SURDUCAN C.NEAMTU of absorbed microwave power radiation is still too much for small sample volumes. Thus, the probe may be iradiatted with any possible energy, by using a Pulse Width Modulation tehnique [fig.2], with four different amplitude energy pulses. If the pulse width is small enough and the pulse rate is extremely stable, then a precise Microwave power versus output energy 900 microwave power [W] 800 700 600 500 full power 10% energy 50% energy 90% energy 400 300 200 100 0 1 2 3 4 power modes fig. 1 The microwave generator output power versus 10% to 90% energy mode incident fig. 2 Puls Width Modulation output tehnique for 10% output energy. The signal period is kept constant while the pulse width is increased untill it equals the period time for 100% energy mode. MODULAR - MICROWAVE POWER SYSTEM, BASED ON MICROCONTROLLER UNIT power is generated in the probe chamber as the fig. 1 shows. However the user must know how much from this microwave radiation was absorbed by the probe, and for this purpose we have designed and patented a microwave thermal transducer [1]. Another problem in such systems may be the noise generation into the mains, the tehnique used to supress this noise is zero crossing firing for the power semiconductor devices involved in driving the mains load. The mains zero crossing is synchronised with the rising edge of the power pulse width. [fig.2]. Power design and control overview The generator [fig.3] is designed using a 900W CW magnetron [1]. The microwave power is extracted using a TE microwave guide and a coaxial adapter to the reaction chamber [2]. The chamber temperature is measured using a digital temperature sensor. The power supply [3] is entirely electronically controlled, both the anode voltage and the filament voltage can be directly controlled by the required program sequence. The system controller [4] is built with a PIC16F628 microcontroller. It uses a smart LCD driver [4b] with HD44780 which is able to display all the important parameters: the output power level (in relative power steps), the output energy (in procent indications), the treatment time (in minutes/seconds) and the chamber temperature (celsius degree). Al settings can be set from a small four buttons functional keyboard [4a]. A functional keyboard is sharing the button functions and displaying those on the LCD. When a button is pressed the microcontroller is running the specific routines and change the button function for the next push. Some buttons may have the same function in different screens displayed, other buttons may have different functions as the software requires. From one screen, the PC communication menu may be set. In this mode the whole generator control is under the PC command via the RS232 interface [5]. To avoid the computer breackdown, because of possible leackage currents from the high voltage modules, the RS232 interface is fully optoisolated. fig. 3 Modular microwave power system, block diagram V.SURDUCAN E.SURDUCAN C.NEAMTU Conclusions The modular-microwave power system can be used in any microwave application which request variable output power and energy. With a small firmware reconfiguration the system can be used in totally different aplications. This is possible because of some logic/analogic reserved input/ output ports in the microcontroller unit and interchange features of the microwave circuit. One usage example is our versatile and cheap microwave digestion system [fig.4]. fig. 4 Microwave digestion unit using the modular microwave power system Bibliography: 1. E.S ur d u ca n, V .S urd u ca n, Thermographic Transducer for Microwave Power Radiation, Patent RO116506 / jan.2001