JACOBS_LADDER_HV_SUPPLY

Published:2009/7/16 21:51:00 Author:Jessie | From:SeekIC

The operation of the Jacobs's Ladder depends on the current-limited power supply that delivers 12,000 V at 40 mA from the 120-Vac line. The full-wave bridge BR1 rectifies the 120-Vac line input, and resistor R1 limits the dc charging current of capacitor C3 to a safe value. The drive circuit power is obtained by dropping the 120-Vac line through capacitor C4 and current-limiting resistor R2. Capacitor C4 and resistor R2 present a complex impedance, so most of the ac line voltage is dropped across the reactance. The 555 timer IC is configured as a square-wave oscillator. The output frequency of its square waves is determined by the setting of trimmer potentiometer R5 and capacitor C6. The frequency is about 25 kHz for the values of R5 and C6 shown. Potentiometer R5 can be used to adjust the circuit's power output. Increasing the frequency reduces the circuit's output by increasing the inductive reactance of the transformer leakage inductance. The output of IC1 on output pin 3 appears at the bases of the current source, NPN transistor Q1, and sink, PNP transistor Q2. The emitters of this transistor pair are ac-coupled through capacitor C8 and resistor R6 to drive the primary of driver-isolation transformer T1. This drive prevents dc from flowing through the primary. Transformer T1 is wound on a high-permeability core with as few turns as possible to eliminate leakage inductance. The gate circuits of MOSFETs Q3 and Q4 contain 27-Ω resistors (R7 and R9) to slow their switching times. This eliminates possible parasitic oscillations that could occur if the MOSFETs were switched at their speed limit. The primary of output transformer T2 contains 32 turns, but its secondary contains 2500 turns. The ratio of these turns is approximately 1 to 78. When this is multiplied by the rectified line voltage of 160 Vdc, an output of about 12,000 peak volts is obtained across the secondary. This 12,000-V output is the peak open-circuit voltage of the system, and it produces a short-circuit current of approximately 40 mA. This current is limited by the leakage inductance caused by the loose magnetic coupling between the primary and secondary circuits of transformer T2.



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