Friday, February 24, 2017

Fun with self-oscillating TV flyback transformer circuits, arcs and high voltage

A few weeks ago I ordered a few things from The Electronic Goldmine and one of the items that I picked up was a small flyback transformer (Stock #:  G20787, manufacturer part number BSH12-N406L) as would have been used in a small CRT (Cathode Ray Tube) television.  In perusing the internet I was able to determine that this transformer was originally intended for a small Black-and-White TV with a nominal anode voltage of around 12kV.

Figure 1:
Drawing a 1/2" (1cm) arc from the contraption.
Click on the image for a larger version.
Having a back-burner project that will need 8-12 kVDC at a very low current I decided to mess about with a simple, self-exciting oscillator circuit.  Before I go on, I need to throw out a few "weasel words":

This project deals with high voltages - possibly in excess of 12 kV.  While the current is low, it is still possible for the output to cause fire, injury - directly or indirectly - or even death.
Any experimentation or use of the circuit(s) described on this page should be done with extreme caution and only by persons familiar with high voltage safety.

You have been warned!
In a television these transformers are driven externally at a specific horizontal frequency - usually between 15.6-15.8 kHz - but with a small number of components a self-contained "power" oscillator can be assembled, operating over a wider range of frequencies and capable of producing high voltages.

The circuit and (my arbitrary) pin connection for this transformer is shown in Figure 2, below.

Figure 2:
Self-oscillating flyback transformer driver.  Like most modern flyback transformers, this unit contains a high-voltage rectifier - which may also be part of an internal capacitor-diode voltage multiplier.  Capacitor C1 is semi-optional, but is highly recommended to reduce the amount of switching frequency energy from appearing on the V+ line.  The pin-out diagram is specifically for the BSH12-N406L flyback transformer (Electronic Goldmine P/N:  G20787).
This circuit operated from about 3 to 15 volts with higher supply voltages yielding greater high-voltage output:  R1 and R2 would be tweaked for optimal operation at the desired supply voltage and the specific transistor used for Q1.
Click on the image for a different-sized version.

The pin-out in Figure 2 is specific to this particular transformer but similar arrangements may be divined with most other flybacks from solid-state televisions with an ohmmeter and the use of clip-leads to find the optimal connections.  What is common to most flybacks is that one or more of the pins on the bottom will appear to not be connected to anything else, but one of these will probably be the bottom end of the high voltage winding with one or more of the others may be used for focus voltage or similar.

The starting values for R1 and R2 would be 1k and 270 ohms, respectively, but this would be adjusted for best performance with the operating voltage, expected load, specific transistor and flyback transformer that was used.  In testing, these resistor values were found to work between 4 and 16 volts - albeit, not necessarily optimally.  The use of capacitor C1 is strongly recommended and it is suggested that a "Low ESR" type as found in switching supplies be used.

Transistor Q1 was a 2SC4130 pulled from a junked switching power supply and was used because it was free.  Because this is an oscillator and this transistor's original use was in a switching supply - and with its high voltage rating - it was particularly suitable for this application.  The specific transistor isn't particularly important and almost any NPN power device will work, preferably one that is rated for over 100 volts, but some seem to work better than others for reasons that aren't immediately obvious so it's worth trying a few different devices.  No matter which transistor you use it is a good idea to heat sink it if it will be operated under any load for more than a few seconds.

For what purposes would one use this sort of circuit?

Aside from making pretty arcs or producing coronas and lots of ozone, these sorts of voltage (6-12kV) at the low currents of which a set-up like this is capable could be used for "lighting up" an image intensifier (a.k.a. "night vision") tube, for "electrostatic wind" experiments, to mildly charge objects so that they are attracted to each other (e.g. paint, glitter, etc.), to "strike" and light small HeNe laser tubes (with the appropriate ballast resistor) or to briefly test gas discharge tubes such as neon displays to verify their seal integrity.

What's the voltage, Kenneth?

Voltages like this aren't particularly difficult to measure, rather they are awkward.  They are far too high for all but the most specialized of voltmeters (you risk damage if you try!) so the most appropriate tool for this would be a high voltage probe as is used to measure voltage on a cathode ray tube.  Usually around a foot (25cm) long and with a separate ground lead, these may be had second-hand, particularly now that cathode-ray devices are becoming a rarity.

It is possible to use resistors to make a divider to measure this voltage, but there's a catch:  Most common resistors are rated for only 250-1000 volts (at most!) drop across them, the rating depending both on how they are made and the wattage/physical size.  As an example, if you wanted to use 10 Megohm, 1/2 watt resistors, you'd need to wire at least twenty of them in series to achieve a nominal 10kV safety rating, assuming a 500 volt rating per resistor:  Check the specs!

In my case I rummaged about and found a bunch of 10-20 Megohm, 2 watt carbon composition resistors (safe for1000-1500 volts, each) and wired them in as a divider to get an approximate voltage measurement.  Even though the resistance was in the 100+ Megohm range, I could tell by the reduction of the arc length and the amount of current being drawn from the power supply that this was loading the output and significantly reducing the voltage meaning that with no load at all, the voltage was higher, still.

Remember:  For whatever purpose you intend to use it, be careful!


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