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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
Electronic ballast powered by low-voltage sources. Electronic ballast on the KR1211EU1 microcircuit powered by the car's on-board network (11-15 V). Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Ballasts for fluorescent lamps One of the options for the practical implementation of electronic ballasts on KR1211EU1 powered by the vehicle's on-board network (11-15 V) is a device whose schematic diagram is shown in fig. 3.67. This device is useful both at home and when outdoors. Technical specifications:
Terminal blocks are placed on the board for connection to the mains supply and to the lamp. The printed circuit board of the converter can be placed in a case with overall dimensions of 72x50x28 mm. Description of work. The electronic ballast is made according to the scheme of a push-pull voltage converter based on a specialized generator KR1211EU1 (DA1). The generator generates two sequences of antiphase pulses with a protective gap to control a pair of powerful switches (VT1) that switch the windings of the power transformer T1. As a power switch, an assembly of field-effect transistors IRF7103 is used. The generation frequency is regulated by a variable resistor R3 in the range of 20-30 kHz. LED HL1 indicates power supply to the device. This circuit has overvoltage protection and output stage overcurrent protection. The supply voltage is connected to contacts X5 (+), X6 (-).
The lamp is connected to contacts XI, X2 and X4, XXNUMX. Winding knots. The inductor L1 with an inductance of 3,3 mH is made on a W-shaped magnetic core made of M2000NM ferrite. Core size - Ш5х5 with a gap δ = 0,4 mm. Wire with a diameter of 0,2 mm, the winding contains 230-240 turns. The pulse transformer T1 is made on the B22 armored core of 2000NM ferrite; windings 1-2 and 2-3 contain 18 turns of PEL wire with a diameter of 0,5 mm; winding 4-5 contains 150-160 turns of PEL wire with a diameter of 0,2 mm. Structurally, the ballast is made on a printed circuit board made of foil fiberglass with dimensions of 67x45 mm. The printed circuit board is shown in fig. 3.68. It should be noted that instead of KR1211EU1, it is quite possible to use specialized microcircuits IR2153, IR2156, IR2520, UBA2021, which are designed to implement high-voltage ballasts, given that the minimum supply voltage for these microcircuits is about 9-10 V. Another electronic ballast design using KR1211EU1 shown in fig. 3.69. A fluorescent lamp with a power of 18-20 W is used as a light source. The supply voltage (8 V) is supplied to the DA3 controller from the DA2 integral stabilizer. Immediately after turning on the device, the capacitor C4 is discharged, the voltage at the input IN of the controller corresponds to a low logic level. In this mode, the frequency division factor of the clock generator of the microcircuit has the smaller of the two possible values. Concept work. With the values of the elements R7 and C3 (frequency-setting generator circuit) indicated in the diagram, anti-phase pulse sequences with a frequency of 2 kHz are supplied to the gates of transistors VT3 and VT44. The impulse voltage of the same frequency on the secondary winding of the output transformer T1 has a range of 300 V. The load of the secondary winding of the transformer T1 is a series oscillatory circuit L2C10C11 with a resonant frequency of 32,2 kHz. The gas-discharge gap of the EL1 lamp, which is not yet lit, has a resistance close to infinite and does not affect the operation of the device.
Since the frequency of the pulses generated by the controller is far from resonant, the voltage on the lamp does not exceed 200 V. This is not enough for ignition, but a heating current of 0,5 A flows through its filaments.
After 1-2 s, the capacitor C4 will be charged through the resistor R5 to a voltage exceeding the threshold for the operation of the DA3 controller at the IN input. The frequency division ratio of the clock generator will increase, and the frequency of the output pulses of the controller will decrease to 34,2 kHz, approaching the resonant frequency of the oscillatory circuit. As a result, the amplitude of the voltage applied to the EL1 lamp will begin to increase, and after several periods of oscillation it will reach 500 V, which is necessary for the occurrence of a gas discharge. Since the lit lamp shunts the SI capacitor, the quality factor of the oscillatory circuit will decrease, and the voltage amplitude between the lamp electrodes will stabilize at 80 V. This is the operating mode with an effective current through the lamp of about 0,35 A. To prevent excessive discharge of the battery, a DA1 undervoltage detector with a threshold of 10 V is provided. When the voltage between terminals 1 and 2 of the detector is below the threshold, its internal npn transistor is opened, the collector of which is connected to terminal 3, and the emitter to terminal 2. As a result, it is open transistor VT1 lights up, signaling an unacceptable battery discharge, the HL1 LED, and a voltage (~ 3 V) is supplied to the FC input of the DA5 controller, which prohibits the generation of pulses. The EL1 lamp goes out, and the current consumed by the electronic ballast decreases to a few milliamps. If the undervoltage detector is triggered by disconnecting the electronic ballast from the power source (battery), the HL1 LED will continue to light for a few more seconds until the capacitors C6 and C9 are discharged. Attention! Electronic ballasts must be protected from emergency idling, which occurs when the contacts in the lamp armature are broken, when one of its filaments burns out, or when the emission is lost by the electrodes. The documentation of the KR1211EU1 microcircuit does not contain any recommendations on the implementation of such protection. You can apply your own technical solution by connecting a voltage divider from a varistor RU1 and a resistor R14 in parallel with the lamp. If the voltage amplitude on a faulty or missing lamp EL1 exceeds the classification voltage of the varistor RU1, its resistance is relatively small. Zener diode VD4 limits the positive pulses coming from the divider RU1R14 to 6,8 V, and they charge capacitor C6 through resistor R3 and diode VD2. Negative pulses, limited by the same zener diode to an amplitude of less than 1 V, do not participate in the operation of the device. The time constant of the R6C2 circuit is chosen such that during the normal warm-up and ignition of the lamp (-2 s), the voltage on the capacitor does not reach the controller's response threshold at the FC input. In operating mode, the voltage on the lamp does not exceed 80 V, which is less than the classification voltage of the varistor, its resistance is very high and the capacitor C2 does not charge. But if the lamp for any reason does not light up for too long or goes out during operation, the voltage on the capacitor C2 will rise to the threshold level in about 5 seconds, and the controller will be blocked. Diodes VD1 and VD2 eliminate the mutual influence of the two protection nodes. A voltage proportional to the discharge current in the lamp is applied to the FV input of the DA3 controller. It is obtained using a current sensor - resistors R12, R13 connected in parallel and a rectifier on a VD5 diode. With the ratings indicated on the diagram, the current protection threshold is 0,7 A, which is twice the normal current of a burning lamp (0,35 A) and more than its glow current in the heating mode (0,5 A). When the current drops to the nominal value, the operation of the controller resumes automatically. Capacitor C7 suppresses impulse noise, preventing false protection trips, including during single flashes of the lamp. The circuit designer deliberately refused to dampen the transformer windings with RC circuits, which is usually done to reduce the level of interference generated by electronic ballasts. Autonomous power supply and shielding of the device with metal fittings of the lamp effectively suppress low-power parasitic electromagnetic radiation, making them almost imperceptible. PCB and mounting. All electronic ballast elements are mounted on a single-sided printed circuit board, the drawing of which is shown in fig. 3.70. Diode VD3 and resistor R6 are installed perpendicular to the board, their "upper" outputs are connected. The field-effect transistors are equipped with finned or pin heatsinks with a cooling surface of approximately 50 cm2. Radiators are raised above the board by 8-10 mm with the help of mounting bushings. In this case, the heat-removing surface of the transistor VT2 is located parallel to the board, and VT3 is perpendicular to it. It is desirable to select these transistors identical in terms of threshold. Replacing items. The KT3107B transistor can be replaced by any low-power pnp silicon structure. Varistor RU1 can be domestic CH1-2 180 or imported TVR 10 181. About chokes. The inductor L1 with an inductance of 100 μH is taken from a faulty computer power supply. It is wound on a "dumbbell" magnetic core and pressed with a heat shrink tube. The inductor can be made independently by winding a winding with an inductance of at least 0,5 μH on a suitable ferrite rod with an insulated wire with a diameter of 0,7-40 mm, or use the finished DM-2 series. The L2 inductor winding (B26 magnetic circuit made of 2000NM1 ferrite with a non-magnetic gap of 1 mm) consists of 160 turns of PEV-2 0,43 wire.
Transformer. The magnetic circuit of the transformer T1 is an armored BZO made of ferrite 2000NM1, assembled without a gap. Winding I (two sections of 12 turns each) is wound with a PEV-2 0,74 wire folded in half and reliably insulated with varnished cloth from winding II, consisting of 160 turns of PEV-2 0,35 wire. Every two layers of the windings of the transformer T1 and the inductor L2 also lay insulation - a layer of varnished cloth. The end of one of the winding sections I of the transformer T1 is connected to the beginning of its other section - this is the middle output. The transformer and inductor L2 are attached to the printed circuit board with M2,5 screws through the central holes of the magnetic circuits. Ballast check. When checking the electronic ballast, an increased heating of the capacitor C9 was noted, so it is advisable to choose it with a maximum operating temperature of 105 ° C. Capacitors SU and SI - film, respectively K73-17 and K78-2, for the voltage indicated in the diagram. The rest (except oxide) - any ceramic or film. Diodes KD522B can be replaced with 1N4148 or other low-power silicon ones. The undervoltage detector KR1171SP10 can be replaced with another one with a lower threshold voltage. But the detector input in this case must be connected to the storage battery through a resistive voltage divider. When selecting a replacement, please note that some detectors (for example, MC34064R) differ in pin assignments. The domestic voltage stabilizer KR1157EN802 is similar to the imported 78L08. Adjustment. The establishment of electronic ballasts begins by breaking the power circuit of the field-effect transistors VT2 and VT3, for example, without mounting the inductor L1 on the board. The supply voltage to the remaining components of the electronic ballast can be temporarily supplied from any low-power DC voltage source of 12 V. First of all, set (roughly - by selecting the capacitor C3, precisely - by selecting the resistor R7) the required frequency of the clock generator fT = 616 kHz, which corresponds to the output frequency in operation 616/18 = 34,2 (kHz). Notethat the frequency division factor (18) is taken twice as large as the factor indicated in the datasheet. The fact is that the tabular values \u1211b\u1bof this coefficient given there do not take into account the division of the frequency by two in the output driver of the KRXNUMXEUXNUMX microcircuit. There is an error (an extra zero after the decimal point in the numerator) in the formula recommended by these sources for calculating the elements of the frequency-setting circuit of the clock generator of the microcircuit. The correct formula looks like this Ft = 0,7 / R7 C3 Having installed the L1 inductor in place, connect the electronic ballast with the EL1 lamp to the battery (you can use a sealed lead-acid 12 V with a capacity of 7 Ah) through an ammeter and measure the current consumed. He must be:
An ammeter with low internal resistance is needed. For example, when trying to measure the current with the M-890D multimeter, after a single flash of the lamp, the electronic ballast turned off, because with an increased current consumption at the time of ignition, the voltage drop on the measuring device led to the voltage drop detector being triggered. Council. It is desirable to check the correct operation of the undervoltage protection by connecting in series with a healthy and charged battery an auxiliary rheostat with a maximum resistance of several ohms. Electronic ballasts are switched on at zero resistance of the rheostat, and then, by controlling the supply voltage of the device with a voltmeter, gradually, until the protection is triggered, increase the resistance. At a voltage of 10-10,5 V, the lamp should go out, and the HL1 LED should turn on. Next, the electronic ballast is disconnected from the battery, the EL1 lamp is removed from the armature and, having again applied the rated voltage to the electronic ballast, they immediately check with an oscilloscope the presence of pulses on the drain (heat sink) of one of the field-effect transistors. After 5. s after switching on, the impulses should stop. A second check can be carried out only after the self-discharge of the capacitor C2 (which takes at least a minute), or by forcibly discharging this capacitor. After installing the lamp, the device is ready for operation. This electronic ballast can work with any fluorescent lamps with a power of not more than 20 W, including imported ones. As a rule, it is enough to change the inductance of the inductor L2. Calculation in Ballast Designer. Use the Ballast Designer CAD software to find the required value. In the first design step after its launch, specify the supply voltage "80 to 140VAC/300VDC". This option is closest to the lamp operation mode in our electronic ballast. At the second step, select the lamp of the used type or its close analogue from the list offered by the program. The third step is to choose any of the offered controllers, for example, IR21571. The parameters we are interested in do not depend on the controller type. Specify the "Single lamp / current-mode heating" lamp switching scheme at the fourth step, at the end (fifth step) give the "Design Ballast" command. Of the results obtained by the program, we are interested in:
As a rule, the calculated capacitance of the SI capacitor remains equal to 0,01 μF, so only the L2 inductor has to be replaced. The non-magnetic gap between the halves of the magnetic circuit in most cases can be left equal to 1 mm, which is equivalent to a gap of 2 mm on its central rod. With such a gap, saturation of the inductor magnetic circuit even at the moment of ignition is unlikely, which is due to the increased internal resistance of the transformer voltage source compared to the mains half-bridge. When converting electronic ballasts to work with a 7 W TC-EL lamp (this is the closest analogue of the existing F6T5 / 54 lamp) with the same capacitance of the SI capacitor, the inductance of the inductor L2 increased to 3,7 mH. The calculated value of the operating frequency for this lamp is 34,8 kHz, which is only 0,6 kHz more than the previously set 34,2 kHz. It was decided not to change the frequency-setting circuit of the controller, limiting itself to replacing the inductor. On a magnetic circuit similar to that used in the T1 transformer, 170 turns of PEV-2 0,35 wire were wound. The measured inductance of the inductor turned out to be 4,1 μH (more than calculated). However, before checking the performance of the electronic ballast, it was decided not to rewind the throttle. All other electronic ballast elements were left without any changes. Test run. Trial switching on showed effective heating and confident ignition of the lamp, a clear operation of the protection when simulating faults, as well as a fairly good coincidence of the operating mode with the nominal one (deviation - no more than 10%). The current consumed from the battery is approximately 0,7 A, which allows you to leave the emergency lighting on all night without fear of a complete discharge of the battery. Plateau. The manufactured electronic ballast is placed in a case 155x67,5x40 mm soldered from foil fiberglass, which simultaneously serves as a stand for the battery. Author: Kosenko S.I.
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