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30kV Flyback PSU

Introduction

This high voltage PSU utilises a high voltage flyback transformer (also known as a Line Output Transformer – LOPT) salvaged from an old black and white TV. It is capable of producing a variable DC output of up to approximately 30kV. This circuit uses a relatively simple transistor based oscillator circuit to drive the flyback transformer – there are far more complex driver circuits out there but this circuit can be built simply with minimal knowledge and parts, most of which can be salvaged from an old CRT TV. This PSU can be used to power many other small but interesting high voltage projects such as plasma globes (high voltage AC would work better), Jacobs ladders, Marx’s generators, and Ionocrafts.

Background Theory

Safety

Very high voltages of up to 40kV can be generated using the standard flyback transformers discussed here. 40kV is more than enough to ionise a relatively large air gap! Air has a breakdown voltage of approximately 30kV per centimetre. The nominal output current is relatively low <10mA, but can still be dangerous.

The output voltage of a flyback transformer has a high frequency, usually around 20kHz. Our nerves are not very sensitive to high frequency currents and therefore little or no sensation of shock is felt at high frequency, even if a significant current at high voltage flows through your body, contrary to expectations. You will however feel flesh burns due to high current ohmic heating and/or the extremely high temperature of plasma arcs, the former being less important in this example due to relatively low output currents for flyback transformers.

Plasma arcs have temperatures in the thousands of degrees and can easily set flammable materials on fire and cause flesh burns.

Flyback Transformers

Flyback transformers (FBT) or Line Output Transformers (LOPT) are special step-up transformers designed to generate high voltages at high frequencies. They were invented for use in cathode ray tubes (CRT), commonly found in CRT TV’s and PC monitors.

FBT’s come in all shapes and sizes and can usually generate voltages between 20kV and 50kV and operating at frequencies between 15kHz and 50+kHz depending on the FBT.

Standard operation of a FBT uses a low voltage saw-tooth input at high frequency applied to the primary windings of the FBT. The fast switching of the input current and large step-up turn ratio between the primary windings and the secondary windings induce a very large voltage in the secondary winding.

There are three main FBTThere are three main FBT variants which can be obtained by salvaging them from various electrical appliances.

1. Old Black & White TV FBT:

This is regarded as the most ‘useful’ FBT. This is mainly due to the transformer consisting solely of primary windings and a secondary winding with no high voltage rectification or high voltage smoothing capacitor being incorporated into it as a unit. They have an exposed ferrite core which allows easy access and space to wind you own sets of primary and feedback windings. Not having the high voltage rectification allows for a full-wave AC output and not having the high voltage smoothing capacitor allows one to pull longer continuous arcs rather than high frequency arc pulses. For a high voltage AC current the correct driver circuit must be used to produce an AC current on the FBT secondary; the simple driver circuit I have used described on this page will not allow for a high voltage AC current output. The above advantages allows one more options regarding experimentation with high voltage experiments such as plasma globes, Jacobs ladders etc, which all require or perform better with high voltage AC. If a DC supply is required for lifters or ion motors for example, the AC output can be rectified and smoothed using external components. The old B&W FBT’s generally output a lower voltage (15kV to 25kV) but with higher current capabilities than the modern colour TV or PC monitor FBT’s (20kV to 40kV).

2. Colour TV FBT:

These are the next best thing if an old B&W FBT is not available to you. These usually incorporate high voltage diode rectification across multiple secondary output stages producing a full wave rectified output but with no smoothing capacitor which allows one to pull longer continuous arcs. These are known as diode-split FBT’s. These FBT’s can be ‘pushed’ further than the old B&W TV FBT’s and can output up to 60kV with the correct driver circuit. These FBT’s are great if you want to experiment with lifters (ionocrafts) and ion motors that require a DC output.

There are old colour TV FBT’s which are known as cascade (or voltage multiplier) FBT’s. They output around 8kV AC at the secondary winding and use a voltage multiplier (tripler) to step the voltage up to ~25kV DC.

These FBT’s are generally slimmer than the monitor FBT’s as they don’t have the built in smoothing capacitor. The units consist of a horseshoe shaped set of pins encased in a black/grey plastic case with a thick red high voltage wire coming out of the top.

3. PC monitor FBT:

These are the least ‘useful’ FBT’s. They are very similar in design to the colour TV FBT’s except that they include a built-in high voltage smoothing capacitor making drawing arcs difficult and therefore not useful for experiments such as Jacob’s ladders.

These FBT’s are generally fatter than the colour TV FBT’s as they include a built-in smoothing capacitor which can be identified from the outer plastic casing as a long cylindrical extrusion on the side of the unit near to the thick red high voltage out put wire. The units consist of a horseshoe shaped set of pins encased in a black plastic case with a thick red high voltage wire coming out of the top very similar to the colour TV FBT’s.

flybackpsu_2

Driver Circuit

There are many different driver circuits out there that can be used with a FBT. There are some very simple and some very complex designs which allow you to control driving frequency, current and voltage supply etc – see here for some good driver circuit variations. I decided to use one of the simplest designs based on a resonating transistor based circuit. The circuit schematic is shown below.

flybackpsu_3

This circuit operates as follows:

When a DC voltage is applied at “+” and “–”, the transistor will conduct current through the FBT primary via R1 and R2. This induces a current in the secondary windings and the feedback winding. The feedback coil will generate a current, which will trigger the transistor to stop conducting. The magnetic field in the ferrite core collapses and a HV spike will appear on the secondary windings by induction. There will no longer be any feedback current on the feedback windings and therefore the transistor is allowed to conduct again. This cycle repeats several thousand times a second, and the cycle will repeat at the natural frequency of the FBT.

By having a circuit using such a feedback system, adjustments are minimal, and the circuit becomes dynamic i.e. the operating frequency automatically settles at the natural frequency of the FBT. This can be observed when arcs are drawn; the frequency of the arc increases as the arc is drawn longer.

Depending on the ratings of the transistor used, this circuit can supply a relatively large current; but the transistor will need a large heat sink.

Flyback Transformer windings

To use a FBT one needs to either find or wind their own primary and feedback (and sometimes even secondary) windings.

Custom primary and feedback windings:

Winding your own primary and feedback windings allows you to supply a much larger input current than can be supplied to the commercial FBT’s that use very fine gage wire which can be damaged by Ohmic heating. Secondly you will have full control over the turn ratio between the primary and secondary windings and between the primary and feedback windings. Usually winding a new primary and feedback winding on the ferrite core and utilising the high turn secondary of a commercial FBT is adequate for good output results. The usual number of turns for a custom primary and feedback is between four and six turns for the primary and between two and four. However depending on the FBT and transistor used, these numbers need to be experimented with to get optimal performance. When winding and testing the primary and feedback windings you should be aware of the direction of each winding – if you can’t get an output try switching the connections across either the feedback or primary windings.

I could never quite get as good results by winding my own primary and feedback windings (probably needed to experiment more), and so, I stuck with the primary windings already present on the FBT.

Finding the primary and feedback windings on a commercial FBT:

Finding the appropriate pairs of pins to serve as your primary and feedback windings can be a somewhat tricky task. I have played with various modern commercial FBT’s and they usually have a very similar pin layout and pin designation. This site has a good guide that can be very useful for finding appropriate pairs of pins for the windings using a standard DVM.

The first thing to know is that in general the ‘useful’ pins are located on the horse shoe shaped array of pins (primary, feedback pairs and HV GND pin). The pins aligned in a straight line at the edge are not usually useful and can be ignored (they are usually connected to focus and screen adjustments).

When finding/selecting pin-out pairs your first need to work out which pins have continuity with other pins using a DVM on continuity mode. After this taking resistance measurements between pairs of pins showing continuity will give an indication of the relative number of windings (not necessarily correct as the internals of FBT’s are more complex than simply tapped windings). The feedback winding should have less winding than the primary and the primary should have as fewer windings as possible to allow a higher output voltage. Generally the lowest resistance between pins is between adjacent pairs.

flybackpsu_4

I found the primary and feedback pin-out pairs by experimenting on a trial and error basis, whilst using the knowledge from the sites mentioned above and from various other site that I have found that document FBT pin-outs and diagnostics (other flybacks will likely have a slightly different pinout).

I have noticed that the pin-out pairs which work best on one particular FBT, commonly occur in similar positions on various other FBT’s (be it a type 2 or type 3 FBT variant, mentioned in the Flyback Transformers section).

The standard layout and pin-out pairs that I have found to work most effectively and are generally commmon between FBT variants is shown in the above diagram. This diagram could be wrong for some FBT’s but it should provide as a first basis starting point to test a new FBT. A bit of experimentation with various pairs of pins and swapping the polarity of the connections to each pin-out pair will ultimately provide one with the most appropriate pin-out pairs for a particular FBT.

Here are a couple of internal schematic diagrams of some commercial FBT’s to help with understanding the pin-out design.

flybackpsu_6 flybackpsu_5

Here are some more helpful links worth reading:

FBT pin-out: http://www.electronicrepairguide.com/flyback-transformer-pinout.html

CRT diagnostics: http://www.electronicrepairguide.com/high-voltage-circuit.html

Making of my variable 30kV PSU

Circuit Design

I decided that it would be convenient to power this device by the mains as this would eliminate the need for an external DC power supply. For this I required a mains transformer and a bridge rectifier with smoothing capacitor. To allow me to vary the DC voltage into the flyback driver circuit I decided to use a voltage regulator to produce a variable DC voltage from 1.25 to 27.95 volts (see this very useful LM317 calculator). The next stage of the circuit incorporates some protection from the driver circuit as I have found high voltage currents tend leak back towards the power supply end of the circuit. For this I have incorporated a transient voltage suppression diode constructed using two standard zener diodes connected as shown in the circuit diagram. I have also included inductors used as chokes to suppress any voltage spikes from the driver circuit. Finally, I have put two in-line diodes to stop any AC currents flowing back up from the driver circuit. The last part of the circuit is the flyback driver circuit as seen previously and is separated form the DC supply via a switch.

flybackpsu_7

Component list:

V1 Mains power, 240VAC RMS, 50HZ.
S1 Rocker switch, 250VAC, 6A.
Tr1 Mains Transformer, 240VAC primary, 12-0-12V secondary centre tapped, 250mA.
D1 Bridge rectifier (KBPC104), 280VAC RMS, 1A.
C1 Polarised electrolytic capacitor, 330uF, 50V.
Reg1 Voltage regulator (LM317T).
R1 Carbon film resistor, 220Ω 5%, 0.25W.
POT1 Linear carbon track potentiometer, 4.7kΩ.
C2 Polarised electrolytic capacitor, 1uF, 50V.
D2, D3 Zener diode (BZV85C 36V), 36V 5%, 1.3W.
L1, L2 Ferrite core inductor, 20mH.
D4, D5 Rectifier diode (1N4005S), 600V surge and peek reverse voltage, 1A max forward current.
S2 Rocker switch, 250VAC, 6A.
R2 Ceramic power resistor, 235Ω, 14W (parallel pair of 470Ω, 7W).
R3 Ceramic power resistor, 27Ω, 3W.
Tr1 NPN Power transistor (C5387).
TR2 Flyback transformer.

My Flyback Transformer

I managed to salvage an old styled FBT from an old CRT TV. IT was relatively easy to find primary and feedback pin-out pairs. The two wires/pins on the plastic base seen in the front of the photo below are the primary input pins and on the other side were six other pins relating to tapped output of the secondary winding. The HV ground pin can be seen on the far right of the plastic base in the photo. I did have a go at winding my own primary and feed back windings however I couldn’t get the FBT to produce a reasonable output. I therefore resorted to using the existing windings and pin-outs.

flybackpsu_8

Designing and Soldering the Circuit

I like to use MS excel to layout my circuit components and connections when I’m using strip board. I set up a matrix of small square cells representing individual holes on a sheet of strip board. I mark soldered pins as pink cells and red for track breaks. I also colour in yellow blocks of cells approximating to each of the component sizes and shapes to help with compacting and arranging the layout of all the components.

I then started to solder all the components on a sheet of strip board cut to the required size. The voltage regulator required a heat sink and the power transistor required a very large heat sink. These were attached to the components using self tapping mounting screws and thermal paste. The heat sinks were salvaged from an old TV and had mounting pins which could be soldered to the strip board. The voltage regulator potentiometer and switch S2 were mounted in the lid of the project box with wires connecting directly to the circuit board. The mains power was connected to a fused and switched IEC euro connector module that was mounted in the side of the lid of the project box. Wire to the the mains transformer was soldered directly to the module’s live and neutral terminals.

flybackpsu_10 flybackpsu_9

The FBT was mounted on a sheet of rigid polystyrene (heat moulded with a 90° bend in it) using some rather convenient threaded mounting holes on the FBT base. Connections to the FBT primary and feedback windings were soldered directly to the FBT pins and connected to screw clamped terminal posts on the circuit board (the terminal posts allow the connections to be removed easily if ever needed). The HV (hot) output wire was extended (as it had been cut short) with some spare HV wire from other FBT’s that I had salvaged. Some more spare HV wire was soldered to the HV GND pin of the FBT. These HV wires were connected to two terminal posts mounted on the lid of the project box.

Some pictures of the finished PSU:

flybackpsu_12 flybackpsu_11