Building the HVPS—High Voltage Power Supply Introduction Voltages higher than the LVPS provides—kilovolts—are needed in later experiments to get strong electric fields and to generate microwaves. The high-voltage power supply makes such experiments possible. You will have voltages from 100V to 1000V available to you, at a safe current of less than a milliampere. The HVPS is built on the empty space of the LVPS board and is powered by it. Some experiments in this course will need both the HVPS and another device, an amplifier, which will be built on the board of a second LVPS. You and your partner will have the necessary pair of LVPS’s between you; if you are working alone, you will eventually need two LVPS’s. Background To get adjustable high voltage, we will convert the adjustable dc from the LVPS into adjustable ac at a frequency of about , by means of an LC oscillator. Then, a step-up transformer will be used to get high voltage ac, which can be turned into dc with a half-wave voltage-doubler rectifier. Here is the block diagram: Figure 1: HVPS block diagram Figure 2: HVPS circuit diagram 1The oscillator is powered by the 2N3055 transistor. The main feature of a transistor is that a small current from base to emitter results in a large current from collector to emitter (larger by a factor of order 100). Then the emitter current flows through an six turn primary winding on a transformer made by using the windings of a 5 millihenry inductor as the secondary, as shown below. The transformer is a device that converts an ac voltage across the primary coil winding into a different ac voltage across the secondary coil winding. This is accomplished by using two closely spaced coils of different number of turns, one is called the primary, and the other, the secondary. The primary coil has a number of turns denoted by NP , and the secondary coil has N number of turns. Then the voltage across the primary, VP , is related to the voltage across the ssecondary, VS , according to VSVP = .NP NS The secondary has about 300 turns, so the secondary voltage is about 50 times the primary voltage. Figure 3: LC oscillator The secondary winding of the transformer is part of a parallel LC circuit with a resonant frequency f = 1 2π 1 LC The inductance is 5mH and the equivalent capacitance is 91 pF , so the resonant frequency is f = 1 2π 1 LC = 1 2π 1 (5×10−3 H )(9.1×10−11 F ) =2.4×105 Hz . 2Figure 4: capacitor voltage divider The two capacitors shown above make a voltage divider, such that 1/11 of the total voltage across the coil appears across the 1000 pF (.001 microfarad) capacitor. This voltage is fed back to the base of the 2N3055. If the transformer windings are properly connected, an increasing current through the transistor will produce an emf in the secondary of the resonant circuit which, when part of it is fed back to the base of the transistor, has the correct polarity to cause yet more current to flow in the 2N3055. This is positive feedback, rather than the negative feedback utilized in the LVPS regulator. The positive feedback tries to make the transistor current go to infinity, which, of course, is not possible. The voltage across the primary coil does increase to almost the positive power supply voltage, at which point it must stop increasing, and the voltage induced in the transformer secondary goes through zero and changes sign. Then the positive feedback makes the current through the transistor plunge in the opposite direction to zero. Thus, an alternating voltage almost equal to the LVPS voltage is produced at the primary, at the resonant frequency of the LC circuit of which the transformer is a part. This ac is stepped up to 1000V or more, peak-to-peak (maximum to minimum), by the transformer action. The high voltage ac is rectified so as to produce an output dc voltage approximately equal to its peak-to-peak voltage, by using two high voltage diodes in a voltage doubler circuit: Figure 5: voltage doubler 3Here is how is how the voltage doubler works: if the sine wave voltage across the resonant circuit has a peak-to-peak value 2V0, then an alternating voltage of amplitude V0 appears across the circuit composed of C1 and D1. Thus C1 and D1 act as a half-wave rectifier, which charges C1 to V0. As a result the sine wave voltage at point A causes the voltage at A to oscillate from 0 to 2V0. The second diode, acting as a half-wave rectifier, will then charge to a voltage approximately equal to 2V0. Hence the name ‘doubler’. Following the voltage doubler, there is a filter capacitor, C2, and then each output lead is in series with a one megohm resistor. These are to isolate the supply from the long leads you may connect to it, and to reduce the intensity of the shock you can get by putting yourself across the output. The supply is safe, because the stored charge in the small filter capacitance is too small to be dangerous, and the steady output the supply can deliver is also too small to hurt you. However, if you touch it lightly you can feel the burn of the resulting spark. HVPS Parts • LVPS, already built • resistor, 8.2 kΩ 12W • 2N3055T transistor, TO-220 type • TO-220 white transistor socket • clip on heat sink • 2 - 1N5062 diodes, (max 800V peak inverse voltage, 750 mA max forward current) • ferrite core inductor (5mH ), 3 segment • 1 ft - #26 insulated solid wire • 1 ft - #22 bare solid wire • 1 ft - #22 red insulated solid wire • 2 -1 MΩ resistors, 12W ceramic disc capacitors: • 100 pF , 1kV , [1 pF = 1 picofarad = 10−12 F .] • 1000 pF , 500V • 2 -470 pF , 1kV 4Recognizing the Parts Figure 6: Top view of HVPS Transistor: The transistor package looks like the regulator of the LVPS, and since it also produces heat that must be led away, you should mount a heat sink on it. The transistor is an amplifier of current, and in conjunction with the transformer produces ac at a frequency of about 240 kHz . Figure 7: 2N3055T Transistor Inductor: Your inductor consists of three connected coils wound on a ferrite core (a magnetic sintered powder); the coils are about 45 feet in length, something under 500 turns of thin insulated wire. You will change the inductor into a “step-up” transformer converting low alternating voltage at relatively high current to high voltage at low current. (Your wall transformer, converting 120V ac to 12V ac, is a “step-down” transformer.) Figure
View Full Document
Unlocking...