Simple High Voltage Generator:
Low Voltage DC In, up to 30 kV Out

Version 1.25a

Copyright © 1994-2004
Samuel M. Goldwasser
--- All Rights Reserved ---

For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page.

Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:

This notice is included in its entirety at the beginning.
There is no charge except to cover the costs of copying.

Go (Back) to Various Schematics and Diagrams.

Preface
- Author and Copyright
- DISCLAIMER
Introduction
- Simple High Voltage Generator
- Safety Considerations
High Voltage Inverter
Low Voltage Power Supply
- LV Power Supply Description
- LV Power Supply Parts List
Typical Flyback Schematic

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Preface

Author and Copyright

Author: Samuel M. Goldwasser

For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page.

Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:

1.This notice is included in its entirety at the beginning.
2.There is no charge except to cover the costs of copying.

We will not be responsible for damage to equipment, your ego, blown parts, county wide power outages, spontaneously generated mini (or larger) black holes, planetary disruptions, or personal injury that may result from the use of this material.

Back to Simple High Voltage Generator Table of Contents.

Introduction

Simple High Voltage Generator

The basic circuit described in this document is capable of generating up to 30 kilovolts or more from a low voltage DC source using the flyback (LOPT) transformer salvaged from a B/W or color TV or computer monitor. Typical output with a 12 VDC 2 A power supply or battery will be 12,000 V. Maximum output current at full voltage is typically around 1 to 2 mA. Higher currents are available but the output voltage will drop. At 2 kV, more than 10 mA may be possible depending on your particular flyback transformer input voltage and current.

Safety Considerations

Before thinking about experimenting with anything using or producing high voltages, see the document: Safety Guidelines for High Voltage and/or Line Powered Equipment. While the circuit described below isn't likely to be lethal using the suggested input voltage and components, who knows how you might 'enhance' it! :-)

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High Voltage Inverter

HV Inverter Circuit Diagram

As you can see from the schematic below, it doesn't get much simpler than this!


   +Vcc     Q1   +----------------+
     o           |                 )::
     |       B |/ C                )::
     |  +------|    2N3055         )::
     |  |      |\ E           5 T  ):: +------|>|----------o  +HV
     |  |        |                 )::(   HV Diode, usually
     |  |       -_-                )::(    built in.
     |  |                          )::(
     +--|-------------------------+ ::(
     |  |   Q2  _-_                )::(
     |  |        |                 )::( Secondary (HV) winding,
     |  |    B |/ E           5 T  )::(  intact.
     |  |  ----|    2N3055         )::(
     |  |  |   |\ C                )::(
     |  |  |     |                 )::(
     |  |  |     +----------------+ ::(
     |  |  |                        ::(
     |  |  -----------------------+ :: +------------------o  -HV
     |  |                     2 T  )::
     |  |               +---------+ ::
     |  |               |     2 T  ):: T1 - Flyback transformer from B/W or
     |  +-------------------------+         color TV or computer monitor.
     |                  |
     |            R1    |    R2
     +----------/\/\/\--+--/\/\/\--+
                  110        27   _|_
                  5 W        5 W   -

Pinout for TO3 metal can transistor:

               _
             / O \         View from bottom (pin side).
           / o   o \
          (  B   E  )      B = Base, E = Emitter, C = Collector.
           \       /
             \ O / C       The metal case is the Collector.

Pinout for the TO220 or TOP3 plastic case transistor:


                    TO3Pn
       TO220        _____
       _____       /     \
      |  o  |     |   O   |      View from front (label side).
      |-----|     |-------|
      |Label|     |       |      B = Base, E = Emitter, C = Collector.
      |_____|     | Label |
       | | |      |_______|      If there is an exposed metal tab, that is
       | | |        | | |         the Collector as well.
       B C E        | | |  
                    B C E

A slightly modified version of this basic circuit which I use as an RF source to excite a glow discharge in helium-neon laser and other gas discharge tubes is shown in: Flyback Based RF Source. This one uses a flyback transformer without a high voltage rectifier (or with the rectifier removed). The inductor, L1, is an addition that should reduce the stress on the transistors and power supply by limiting current at the time each of the transistors go into saturation just before the base drive switches to the opposite side. I have not specifically tested this circuit with the inductor but have used it with similar inverters.

These designs are similar to circuits found in: "Build your own working Fiberoptic, Infrared, and Laser Space-Age Projects", Robert E. Iannini, TAB books, 1987, ISBN 0-8306-2724-3 and many other places.

For larger (e.g., color TV or monitor) flybacks, or use with more than 12 VDC in, transistors with higher power ratings may be needed for sustained operation in addition to a good heat sink. An alternative is to parallel more than one power transistor along with small (e.g., .05 ohm, 2 W) current balancing resistors in series with their emitters.

HV Inverter Assembly

Read the following in its entirety! This assumes the basic circuit using a small flyback and input voltage of 12 VDC or less. Some modifications may be needed when using larger flybacks and higher input voltages.

Obtain flyback transformer with known good HV secondary winding. primary may be left intact if it is known to be in good condition - non shorted. A flyback removed due to failure may be used if it was the primary that failed and the primary turns can be removed without damaging the HV secondary or losing the secondary return connection! Flybacks fail in both ways (primary and secondary).
Locate the return for the high voltage winding. This may be a different color wire than the low voltage winding or may exit from the potted part of the flyback in a different place. It is not possible to use an ohmmeter to locate the return for the high voltage winding if your flyback has a built-in HV rectifier or multiplier as the forward voltage drop of the rectifier diodes is much greater than the battery voltage used in your multimeter. However, a winding connection that has infinite resistance to every other terminal is likely to be the HV return. On flybacks with no HV rectifier or multiplier, the return is easily located by measuring resistance between the HV output and all other terminals. The HV winding will have a resistance of 100s-1000s of ohms compared to single digit readings or less for all the other windings.
Wind 10 turn center tapped drive winding and 4 turn centertapped feedback winding using #16 to 20 gauge insulated wire. Make sure both halves of each coil are wound in same direction. Connect centertap in each case at the winding - do not bring out a loop. Insulate well with electrical tape.
Vcc should typically be in the range 12 to 24 volts at a couple of amps. Circuit should start oscillating at around a Vcc of 5 V or so. If you do not get any HV out, interchange the connections to the transistor bases. Heat sinks are advised for the transistors. Be aware of the capability of your flyback (B/W monitors up to 15 kV, color up to 30 kV). You risk destroying the secondary windings and/or HV rectifier if you get carried away. Running this on 24 volts will probably cause an internal arc-over in a small flyback, at which point you start over with more caution and a new flyback.
Actual output will depend on turns ratio of the flyback you have.
- For a typical small B/W TV, monochrome computer monitor, or video display terminal, you should be able to get around 12,000 volts with 12 VDC input.
  I built one from a dead Mac-Plus flyback from which I removed the (dead) primary windings.
- With a large color TV or color monitor flyback, 30,000 V or more will be possible using a 24 VDC power supply.
The frequency of operation will be in the kHz to 10s of kHz range depending on Vcc, load, and specific flyback characteristics.
You can experiment with the number of turns, resistor values, etc. to optimize operation and power output for you needs.
CAUTION: contact with output will be painful, though probably not particularly dangerous due to low (a few mA) current availability.
HOWEVER, if you add a high voltage capacitor to store the charge, don't even think about going near the HV!

HV Inverter Parts List

None of the component values are critical. It is quite likely that everything needed is already patiently waiting in your junkbox. If not, except for the flyback, most if not all of the parts should be available from Radio Shack. See the section: "Low voltage power supply" for a simple design to use with this inverter.

Some experimenting with different value resistors and even the number of turns on each winding may improve performance for your particular flyback.

Q1, Q2 - 2N3055 or similar NPN power transistors (reverse polarity of Vcc if using PNP transistors.) Maximum stress on transistors are about 2 to 3 times VCC. Heat sinks will be needed for continuous operation.
R1 - 110 ohms, 2 W resistor (5 W for Vcc of 24 V). This provides base current to get circuit started.
R2 - 27 ohms, 5W resistor. This provides return path for base feedback during operation.
T1 - Flyback transformer from/for B/W TV, video display terminal, color TV, computer monitor, etc., modified according to text above.
Most modern flybacks include built-in HV rectifier diode(s) and/or voltage multiplier (tripler) so output without additional components will be high voltage positive or somewhat smoothed HV DC.
Note: this kind of flyback transformer drives the CRT directly and uses its glass envelope as the main high voltage filter capacitor. (A foot square piece of 1/8 inch Plexiglas with Aluminum foil plates makes an filter capacitor.)
Wire - a couple of feet of #16-#20 hookup wire, magnet wire, or any other insulated wire for home made primaries. Use electrical tape to fix windings to core. Wind feedback winding on top of drive winding.

Suggested Enhancements

Here are some minor changes that should improve the efficiency of this circuit.

(From: Robert (rrtcj@hotmail.com).)

Use a more modern transistor like the 2N3773 instead of the 2N3055. It has higher current (30 A) and higher voltage (140 V) ratings as well as higher Hfe thus requiring less base current.
Use a single feedback winding (2 turns) with each end connected to the base of one transistor and over a 150 ohm resistor connected to Vcc. (I'm not quite sure exactly what this means but my guess is to connect a 150 ohm resistor from one of the bases to Vcc to provide the startup base drive. --- Sam.)
Add an inductor (about 0.4 mH, can be more but not less) in series to the flyback circuit. Since both transistor are switched on for a little moment (when there is no voltage in the feedback), so there will be high currents (causing losses resulting in excessive power disspation in the transistors) The coil prevents the current spike without wasting power like a resistor.

Measurements: With an input voltage of 16 VDC, the output voltage is about 12 kV. Without the filter choke, the input current with no load is 5 A; with the filter choke it is only 3.2 A. With the output shorted (arcing), the current is about 9 A in both cases.

In fact I don't need big heatsinks. With the 2N3773, I can drive this circuit continuously with one medium-size heatsink and the transistors get only warm!!!

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Low Voltage Power Supply

LV Power Supply Description

The power supply (12 to 24 V) doesn't need to be anything fancy. Regulation is not needed so a simple power transformer-bridge rectifier-filter capacitor design will be fine. The circuit described below will provide about 15 VDC at up to 3 A. Unless you are going for maximum output, this should be adequate.

During initial testing at least, a Variac on the input (or variable voltage power supply) is highly desirable to avoid blowing anything should your wiring or parts not be quite right and to gain a feel for the capabilities of your circuit before it is too late! In neither of these is available, use a 10 ohm 25 W power resistor or 100 W light bulb in series with the load (inverter) to limit current to a safe value - one that won't fry too many things too quickly.

A typical circuit is shown below:


                  _         T1
  H o-----o/ o---- _------+                5 A diodes
       S1 Power  F1 Fuse   )||             or bridge
                  1 A      )|| +---------+----|>|-------+-------+-----o +Vcc
                           )||(         ~|        D1    |+      |
                           )||(          +----|<|----+  |     +_|_ C1
 115 VAC                   )||( 12 VAC            D2 |  |      ___ 20,000 uF
                           )||(          +----|>|----|--+     - |  25 V
                           )||(          |        D3 |          |
                           )|| +---------+----|<|----+----------+--+--o Gnd
                           )||          ~         D4     -        _|_
  N o---------------------+                                        -

LV Power Supply Parts List

All of these parts should be readily available:

T1 - 12 V, 3 A power transformer.
S1 - SPST toggle switch.
F1 - Fuse, 1 A.
D1-4 - Silicon rectifier diodes, 5 A minimum. Or, 5 A bridge rectifier.
C1 - Electrolytic filter capacitor, 20,000 uF or more, 25 V minimum.

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Typical Flyback Schematic

This diagram shows a typical flyback that might be found in a direct view color television or computer monitor. Resistances are included for illustrative purposes only and may be quite different on your flyback!

The high voltage section on the right may actually be constructed as a voltage multiplier rather than a single winding with multiple HV diodes. The rectifiers or multiplier, and/or focus/screen divider may be external to the flyback transformer in some models.

Flyback transformers used in black-and-white TVs and monochrome computer monitors do not have a focus and screen divider network.

The ferrite core of a flyback transformer is constructed with a precision gap usually formed by some plastic spacers or pieces of tape. Don't lose them if you need to disassemble the core. The ferrite core is also relatively fragile, so take care.

The focus and screen divider network uses potentiometers and resistors (not shown) with values in the 10s to 100s of M ohms so they may not register at all on your multimeter. The high voltage rectifiers (CR1 to CR3 on this diagram) are composed of many silicon diodes in series and will read open on a typical VOM or DMM.

Note that there is no standardization to the color code. However, the fat wire to the CRT is most often red but could also be black. Of course, you cannot miss it with the suction cup-like insulator at the CRT anode end.


                                        +--|>|-----------o  HV to CRT
              _                   1  ::(   CR1             (25 to 30 kV,
             |   B+  o-------------+ ::(                    suction cup on
    Drive    |                      )::(                    fat red wire)
    winding <                       ):: +-------+
             |             1.32     )::         |
             |                    2 ):: +--|>|--+
             |_  HOT o-------------+ ::(   CR2
              _                   3  ::( 
             |   50  o-------------+ ::(
             |                      ):: +-------+
             |              .11   4 )::         |
             |   35  o-------------+ :: +--|>|--+
  Various    |                      )::(   CR3  |
  auxiliary <               .28     )::(        /
  windings   |                    5 )::(        \<-------o  Focus 
             |   16  o-------------+ ::(        /          (3 to 10 kV,
             |                      )::(        \           orange wire)
             |              .12   6 )::(        |
             |_   0  o----------+--+ ::(  9     |
              _                 | 7  :: +--+    /
             |   H1  o----------)--+ ::    |    \<-------o  Screen
 CRT Heater <               .08 | 8 )::    |    /          (200 to 800 V,
             |_  H2  o----------+--+       |    \           brown wire)
                                |          |    |      
                                |          +----|--------o  To CRT DAG
                                |               |            ground
                                +---------------+

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Go (Back) to Various Schematics and Diagrams.

-- end V1.25a --

Simple High Voltage Generator: Low Voltage DC In, up to 30 kV Out

Version 1.25a

Copyright © 1994-2004 Samuel M. Goldwasser --- All Rights Reserved --- For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page.

Table of Contents

Preface

Introduction

High Voltage Inverter

Low Voltage Power Supply

Typical Flyback Schematic

Simple High Voltage Generator:
Low Voltage DC In, up to 30 kV Out

Copyright © 1994-2004
Samuel M. Goldwasser
--- All Rights Reserved ---

For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page.