BUILD A DIRECT CURRENT CONVERTER

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To have a Direct Current Converter on your work bench increases greatly your capability of experimenting within an ample range of electronic components that need over 100 volts to be driven. Among these components we find neon lamps, nixie display numerical tubes, and some kind of triode and pentode valves. Neon lamps open a broad field to build audio oscillators, blinkers and flashers, polarity finders, display decorations, hot chassis checkers...etc. Nixie tubes allow you to construct displays for digital counters, and various types of clocks; and with electronics valves many kind of radio transmitters and receivers can be mounted, among a huge lot of other electronics instruments and equipment. Some of these devices work with voltages between more or less 100 and 200 VDC.

You can find, in books or the web, several circuits to implement a DC converter. Most of them use a transformer which primary windings are used to create an oscillator...so that the magnetic field that arises in its' nucleus induces a current and a voltage in its secondary winding. Though these circuits are not extremely complicated, there is a more simple way of building a DC converter with the use of integrated circuits. We have chosen the CMOS 4093 IC to build an oscillator that drives an audio power transistor to pilot the primary of a small transformer...which output give us about 200 to 220 VDC.

1 Small transformer. Primary 9 volts; secondary 220 volts	1 Integrated circuit CMOS 4093
1 Socket for the 4093 IC	1 Transistor, BDY20, or 2N3055, TIP 120, IRF 640
1 Adjustable 100 K ohm resistor	1 Resistor 10 K ohm 1/2 watts
1 0.22 uF, ceramic or polyester, capacitor	1 Electrolytic 100 uF, 25 Volts, capacitor
1 Rectifier diode 1N540 or equivalent	1 Electrolytic 3.3 uF, 450 Volts
1 2.2 K ohm resistor	..........................empty space....................................

We will also need the common electronic accessories to implement our circuit; such as connection cables, solder, small bolts and nuts, terminal binding posts and an appropriate panel board.

We don't need a special transformer for this project. I salvaged a transformer from one of these feeding sources from a calculator machine that we plugged into the main outlet. This transformer has a coil with 3.3 k ohms, measured in DC current, and another one with only 7.8 ohms.

In this last one is injected the oscillatory current coming from the digital oscillator 4093 IC circuit, after being previously amplified in the same IC and later in the power transistor. From the 3.3 k ohms transformer coil we get the 220 dc volts, after rectification of the current and voltage induced in this coil...with the 1N540 diode.

The energy obtain is accumulated in an electrolytic 3.3 uF capacitor for further use. This project can be built with other types of transformers...always having in mind the rating between primary and secondary windings. The less resistance windings, measured with a multimeter, around 8 or 10 ohms, are to be connected to the power transistor collector. The high resistance windings, about 3300 ohms, deliver the high voltages. So, feel free to use the transformer you have handy if it meets more or less the above specifications.

This is a widely used IC in digital electronics. There are included in its capsule four Schmitt Nand gates of two inputs for each one. These gates can be used independently, which allows us to use the first one as an oscillator, and the three remaining gates as buffers or preamplifiers before entering the oscillations obtained by the power transistor.

The feeding voltage to the 4093 IC can be from 3 to 15 volts, being the most appropriated 9 or 12 volts. The current taken from the source in standby is only about 0.8 milliamps. To energize this IC connect pin # 14 to the positive rail of your project and pin #7 to the negative rail or ground. The remaining pins must be connected as shown in the appropriate project schematic.

As the primary coil of the transformer has only about 8 ohms, a current of 1.2 to 1.5 amperes runs in this part of the circuit when it is fed between 9 or 12 volts.

An appropriated transistor to support this current is needed. I chose the BDY 20 TR that was found in my component stock. This is a robust transistor which endures up to 15 amps, 100 volts emitter-collector, and 117 watts. Of course we don't need to have this component suffer these extremes values. Our direct converter is fed with only 9 or 12 volts, which means an easy life for the BDY 20 transistor. But it is not mandatory to use that transistor exclusively in this project. There are many other modern semi-conductors that perform its' task as well. So we have the famous 2N3055, the Darlington TIP 120 or the IRF 640. Any one of them is a good substitute for the BDY 20 transistor. A good constructive practice is providing a heat-sink for any power transistor. In my prototype I did not apply the heat sink because the transistor works far from the limit current admitted in its' specifications. Nevertheless, allow me to advise you using a heat sink in your project.

While the construction of this project we test individually each one of its components. We have to be sure of the resistors and capacitors values checking them with a DVM. There are two electrolytic capacitors which polarity must be respected.

It is advisable to start building the 4093 IC oscillator and check if it is working... which we can do with a crystal earphone. With one terminal of the earphone contacting IC's pin #3, and the other terminal at ground, you will hear a tone in the audio region. Thus, we know the oscillator is working. Finally, when the circuit is finished, we measure the voltage between the binding posts of the 3.3 uF capacitor. We can find there a tension around the 200 volts, which depends of the input voltage applied.

Please keep in mind that the high voltage coming from your DC converter can give you a shock. So take all necessary measures to not receive an electric discharge in carelessly managing this piece of equipment .

The best test we can do to be sure that our converter works properly is performing some experiments with neon lamps.

Neon lamps, as nixie tubes and electronics valves, need limiting resistors to save theirs lives. Never connect one of these components directly to a high voltage source. The neon gas inside of these kinds of lamps or nixies ionizes progressively and if there is not a current limiting resistor, the component will destroyed. So a current limiting resistor over the 100 k ohms is a must! Now let us perform some interesting experiments:

This is one of the simplest oscillators in the whole world of electronics. It is a so-called relaxation oscillator and is made up of only with three components...the neon lamp, a resistor and a capacitor.

Once it is connected, as shown in the appropriate schematic, to a voltage source over 90 volts, the capacitor starts charging through the resistor up to the neon ionization voltage. In this moment the lamp conduces and the capacitor is discharge through the lamp. Now the capacitor starts again charging and the cycle repeats indefinitely while the source voltage is connected.

The oscillation frequency depends entirely of the resistor and capacitor. By changing the values of these components you can get a very wide range of frequencies, but never forget that the resistor must have at least 100 k ohms if you want to preserve the life of your neon lamp!

This circuit also stands up because of its simplicity. While you can build an astable oscillator with two LED diodes, you will also need at least two transistors, two capacitors and four resistors to implement such an oscillator...a lot of components compare with the two neons, two resistors and a capacitor that make up this other type of astable circuit.

To change the circuit frequency, it is best and easier to experiment with capacitor values from 0, 1 uF to some microfarads.not forgetting that the capacitor voltage must be superior to at least 250 volts, being advisable not to use electrolytic capacitors which are expensive and require polarity attention. Ceramic, mylar or polyester capacitor are more appropriate.

Still we can fire up three neon lamps in a sequence. This circuit remembers the phase shifter oscillator built with a transistor, some resistors and three capacitors. But we don' t need so many components, as you can see studying the circuit.

Building the circuit with capacitors of several microfarads, you can easily watch the three lamps firing in a slow sequence. This invites you to add more lamps, resistors and capacitors to this flasher with the aim of creating a long row of lamps flashing sequencely. But unfortunately a circuit with more than 4 neon lamps will work in a random and unpredictable way.

Variable frequency tone oscillators can be easily constructed and connected to audio amplifiers...as shown in the schematic below. No special components are needed. Only a small audio transformer to allow the oscillator driving the amplifier. These audio transformers are found in transistor radios manufactured some years ago. The primary winding of these transformers has about 100 ohms and the secondary winding between 2 and 10 ohms. We use this secondary winding to excite the audio amplifier.

Before installing the transformer in the circuit it is important to check the isolation between the two windings. We must be absolutely sure that high voltage coming from the direct converter never reaches the audio amplifier input. To avoid damage to the amplifier a 0, 1 uF capacitor must be inserted in the amplifier input.

The oscillator frequency is modified with the 1 M potentiometer and the capacitor can be changed to other values to obtain different ranges of frequencies. These capacitors must have voltage ratings over 250 volts or even 450 volts would be much better.

Finally, never forget that if you experiment with these circuits you are liable to an electric shock. This direct current converter project can reach up to 320 volts when feed with only 12 volts from cells or a battery !!!

The table below shows the different 'output' voltages the direct current converter can accomplish...when one supplies the device with a certain 'input' voltage.

INPUT VOLTAGE (Volts)	INPUT CURRENT (mA)	OUTPUT VOLTAGE (Volts)
3.0	27	46
4.5	42	95 (High Voltage)
6.0	63	158 (High Voltage)
9.0	88	240 (High Voltage)
12.0	141	320 (High Voltage)

So, be extremely cautious and don' t forget that we are not working with transistors or integrated circuits...we are working with high voltage equipment!

Having all this in mind I wish you full success and happy hours at your bench enjoying this very special field of electronics.