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

Table of Contents

• Introduction
• Construction
• Inverter Parts List (Excluding Low Voltage Power Supply)
• Low Voltage Power Supply
• Low Voltage Power Supply Parts List
• Typical Flyback Schematic

Introduction

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   -

Construction

Read the following in its entirety!

1. 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).

2. 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.

3. 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.

4. 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.

5. 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.

6. The frequency of operation will be in the KHz to 10s of KHz range depending on Vcc, load, and specific flyback characteristics.

7. You can experiment with the number of turns, resistor values, etc. to optimize operation and power output for you needs.

8. 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!

Inverter Parts List (Excluding Low Voltage Power Supply)

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.

Low Voltage Power Supply

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---------------------+                                        -

Low Voltage Power Supply Parts List

All of these are 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.

Typical Flyback Schematic

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
                                +----------------+

-- end V1.21 --