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I am at present compiling some useful circuits, the page will be added to on a fairly regular basis, but it will depend to some extent on me building and evaluating new circuits, and doing some further work on some that I have found useful in the past. I will try and ensure that only commonly available components are used, and where something is a little unusual I will identify possible sources.
Probably one of the first things a would be high voltage enthusiast will build is a circuit based upon a TV or monitor flyback transformer. All TVs and monitors that use a cathode ray tube (CRT) need to generate high voltages, for a monochrome system this is usually in the range of 8 to 11kV. In colour monitors and TVs the same type of circuit is used with a few extra components to multiply the voltage further.
Below is a simple circuit, it is not optimal but it works! Using it you should be able to get about 10kV, and remember this will produce a lethal current, so treat it with great respect.
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First of all get hold of your LOPT, you could well find a scrap TV or monitor, try your local repair shop. Personally I have never bought a LOPT, mine have all come from sets thrown out by neighbours, sometimes I have found a couple in one week. It is easiest if you get an old monochrome set, as the more recent colour sets have other components encapsulated as part of a whole flyback circuit. Don't even worry if its an old valve set, the coils work just as well, if not better. If you have not looked inside a TV before, then you will need to identify the component that you need. On the side of the tube you will find a soft plastic connector, this will have a thick HT cable, follow the lead back somewhere there will be a high voltage capacitor and a rectifier stick, this latter is often mounted on the LOPT. The LOPT is usually a rectangular ring of ferrite material with several coils mounted on it. Remove the LOPT from the circuit board using a reasonable size soldering iron. Take care, the ferrite is fragile. Also remove the stick rectifier, the capacitor and HT cable (do not throw them away they are well worth keeping).
It is necessary to modify the LOPT first. Fortunately most of them dismantle quite easily, I can't give details for each case, but often there are just a couple of long bolts through the device. Don't worry if it won't come apart. If you can remove the low voltage windings do so; they will have several connections. The HT winding will only have two, one of which is grounded, the other will have been connected to the rectifier stick mentioned earlier. Be careful when dismantling the LOPT, as there will probably be two thin plastic spacers between the ends of the two ferrite components, they are important, and must be replaced when you reassemble the transformer, if you lose them all is not lost, just replace them with a thin piece of adhesive tape. Now make a new primary winding, if you were unable to disassemble the unit just use the space near the earth end of the HT coil. Wind on fourteen turns of hook-up wire making a centre tap at turn seven. Tape the coil in place, or use hot glue. Reassemble the coil and carry on with the rest of the circuit. Remember to keep the low voltage side away from the HT output.
The components are not too critical, almost any pnp power transistor capable of handling 5amps or more will do. The transistors must be mounted on a suitable heatsink. If the mounting tabs are connected internally (they almost always are), then the appropriate mounting kits should be used. If the transistors get too hot to touch during use, then you need a bigger heatsink. The resistors should be rated at a couple of watts.
This circuit needs about 3amps @ 12-18volts. Don't use a fancy power supply, the more sophisticated it is, the more likely it is to end up being a problem. A simple circuit comprising a 100VA 12V transformer with a bridge rectifier rated at 10A 50V, and a smoothing capacitor rated at something like 4,700mfd 25V. I know I've rated this conservatively for this use, but such a power supply is a very useful thing to have. You can actually use a higher voltage transformer, up to about 18V is ok, but remember that this circuit can draw up to 6A when the output is drawing a short arc.
Another Flyback Circuit
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I actually prefer this circuit slightly over the other, although I cannot give any particular reason. It is no more efficient or reliable. Although the 2N3055 is probably not the best choice of device, it does have the advantage of being cheap, but then it has been around for over thirty years and has been a staple for designers of power supplies. The key problem is that in this circuit it is being used close to the edges of the safe envelope.
All that has been said about the other circuit applies to this one at least as far as construction is concerned. Large heatsinks are essential. The feedback turns can be of thinner gauge wire than the primary turns. If the circuit doesn't produce any output, check your wiring. If all is ok, then switch over the base connections to the feedback windings. The output diode can be either a TV flyback output diode, or a string of 1N4007s (15 is about right).
Capacitance Meter
Frequently coilers build their own capacitors. My own experience is that some that I have built have not been of the pre-calculated value. In most cases the values needed are only approximate, at least in primary tank circuits. The instrument described below is not a laboratory standard one, but with careful calibration it is certainly very useful. It can also be used to directly compare the unknown with components of known value.
There is nothing very critical about the circuit, except that 1% resistors should be sufficient. Six ranges are set by the paired variable resistors VR1 - VR6 and fixed resistors R5 - R10. Leads around the test terminals and IC2 should be kept reasonably short.
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R1 15k, R2 220R, R3 1k2, R4 380R, R5 47R, R6 470R, R7 4k7, R8 47k, R9 470k, R10 4m7
VR1 - VR6 100R, VR7 4k7
C1 100nF, C2 10nF, C3 10nF, C4 100nF
IC1 & IC2 NE555 or equivalent, IC3 7805
The 1mA fsd meter should be as large as possible, don't bother to purchase one specially uless you have to, 1mA is a common value for instrumentation meters, and can often be salvaged from faulty or surplus equipment, for example the one in my own instrument came from a laboratory mercury sniffer! You may well need to rescale it although a linear scaled meter reading 1, 10 or 100 fsd doesn't really need it.
Ranges are selected using the 2pole 6way switch. When VR1 and its associated fixed resistor are selected the range is 0 - 100pF, each subsequent position selects a decade higher.
