Battery operated high voltage source, 9/10-500 volts 1,5 milliamps. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Battery operated high voltage source, 9/10-500 volts 1,5 milliamps. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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In amateur radio practice, as well as when repairing equipment, a portable high-voltage current source, battery-powered, can be useful. Such a device can be useful when checking the reverse voltage of a diode, the stabilization voltage of a high-voltage zener diode, the ignition voltage of neon lamps, and also for testing high-voltage transistors.

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The following is a description of a portable high-voltage source, the output voltage of which can be continuously adjusted from 10 to 500 V. The output current depends on the voltage (the higher the voltage, the lower the current). At the maximum voltage, the current is 1,5 mA. The generator is powered by the "Krona" (galvanic battery with a voltage of 9 V), having no connection with the mains. And, nevertheless, when working with it, you need to take precautions (it won’t kill, but it can shake).

The power source is the G1 battery. A voltage of 9 V through the VD1 diode (serves to protect against accidental incorrect power connection) is supplied to the DC-DC converter with a transformer output on an A1 microcircuit of the MC34063 type. This microcircuit is designed for DC-DC converter circuits of low power, or higher power, but with an additional key on a powerful transistor. Here, the source is low-power, therefore, the microcircuit's own output key is used.

The operation of microcircuits of the MC34063 type has been repeatedly and in detail described in various literature. Let me just remind you that this is a pulse generator with a variable width, which can be adjusted using pin 5. This pin is used for the stabilization circuit for the output final (secondary) voltage.

Resistor R1 works in the circuit for protecting the output of the microcircuit from overcurrent. When the voltage across R1 exceeds the control value, the output stage turns off.

The conversion frequency is set by the capacitance of the capacitor C2, which operates in the frequency-setting circuit of the generator.

Microchip loaded. A1 is the primary winding of a step-up high-frequency pulse transformer T1. The alternating voltage from the secondary winding is supplied to the rectifier on the diode VD2.

The R6-R5-R4 circuit is used to keep the output constant voltage stable and adjust the output voltage. It uses the internal output voltage stabilization / setting circuit available in A1. Its essence is that the microcircuit changes the width of the output pulses so that the voltage at its pin 5 is equal to 1,25 V. That is, if the voltage at pin 5 is less than 1,25 V, the width of the output pulses supplied to the primary winding of the transformer T1 will be increase, and if the voltage at pin 5 is greater than 1,25 V, the latitude will decrease.

Thus, the PWM circuit will work so that pin 5 maintains 1,25 V. Now you need to make the voltage at pin 5 depend on the voltage at the output of the transformer (on its secondary winding). The purpose of R4-R5-R6, which is an adjustable voltage divider, is used to set this ratio of the dependence of the output voltage on the voltage at the pin. 5.

The HL1 LED should not burn, in its place it would be possible to put a 1,8 ... 2 V stabistor, but it is easier to purchase an LED. In this circuit, it performs the functions of a stabistor that limits the maximum voltage at pin 5 A1. The need for such a limiter arose after one copy of the MC34063 chip was damaged when the handle of the resistor R5 was turned too quickly. The problem is that the output voltage adjustment range here is very wide, and with a quick adjustment, the voltage across the capacitors C4 and C5 does not have time to change accordingly. This is especially noticeable at idle or when working on a high-resistance load. As a result, at some point in time, the voltage at pin 5 A1 may be too high and damage the input of the comparator of this microcircuit. To prevent this from happening, there is a VD3-HL1-C3-R3 circuit. In practice, this is a parametric stabilizer that does not allow the voltage at pin 5 A1 to rise above 2,5 V. Moreover, with a sharp adjustment to reduce the output voltage, this circuit creates an additional discharge current for capacitors C4 and C5 (at some point in quick adjustment, the LED may even flash ).

Variable resistor R7 serves to increase the output impedance of the source. This may be required when testing diodes for reverse breakdown. You connect the diode to terminals X1 in the opposite direction, connect a multimeter to terminals X2 (which will show 10 times less voltage than on the diode) and begin to gradually increase the voltage. As soon as a breakdown occurs, the voltage that the multimeter shows stops growing or drops, despite the increase in resistor R5. Thus, R7 is a current limiting resistor in the circuit under test. The limit value can be set by adjusting R7, and if no limit is needed, turn its knob to the minimum position.

Transformer T1 is wound on a ferrite ring with an outer diameter of 28 mm. The ferrite ring must be processed before winding, - to give its edges roundness with sandpaper, and then cover the ring with a thin layer of epoxy varnish. After the pack has dried, check the surface of the ring for nicks and sharp edges (for example, due to defects during the hardening of the varnish). All scratches and edges must be smoothed and, if necessary, varnished again.

After the final hardening of the varnish, wind the secondary winding. It contains 2000 turns of PEV 0,12 wire wound in bulk evenly around the ring, but so as to leave a small gap between the beginning and end of the winding. Winding should be done like this. so that its sections with a large difference in the number of turns do not touch. That is, wind in bulk, but evenly moving in one direction, and not back and forth. After winding the secondary winding, it is necessary to cover it with a layer of varnished cloth or fluoroplastic film and wind the primary winding on this surface - 15 turns of PEV 0,61 wire (or another diameter from 0,5 to 1 mm). Distribute the winding evenly over the surface of the secondary winding. Wind both windings in the same direction. The diagram shows how they need to be phased.

Author: Karavkin V.

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