|OddMix TECHNOLOGY - HIGH VOLTAGE|
High Voltage DC-DC Converter 60-80 Volt Solid State Power Supply
|Warning! - Higher voltages are dangerous! - Never use both hands at the same time while working on high voltage circuits! - Disconnect power from a circuit when work on it! Think carefully when around high voltages!|
|Figure 1. Schematic of HV DC to DC Converter|
Experimenting with vacuum tubes requires a (relatively) high voltage (B+) power supply for the anode and power grid voltages. Many years ago, tubes and high voltage transformers were popular components and widely available. Nowadays, finding a high voltage transformer is difficult and costly. There are some ways to enjoy the tube type circuit's unique benefits without high voltages. Space charge tubes and techniques work with many but not all circuits but tubes are difficult to find. An easy solution to the high voltage problem is with a simple DC to DC converter circuit.
For electron tube type car radios, high voltages for operation of tubes were usually made using electromechanical choppers called vibrators. These were good when new, but in a few short hours the repeated sparking wear the contacts out. Vibrators were developed that were able to rectify the stepped up voltages. Now, all voltage conversions can be made simply, without moving parts, using some semiconductor devices.
|Figure 2. 1 KHz Waveforms of HV DC to DC Converter|
|Figure 3. 200 Hz Waveforms of HV DC to DC Converter|
This DC-DC circuit uses an audio output, speaker transformer in reverse. The transformer used in this circuit has a low turn ratio of only about six to one. Thus the low output voltage. The transformer is driven by the power transistor switch Q1 that chops up the DC input at the frequency determined by generator IC1 - Figure 1. The design uses many common, readily available parts and it is easy to build or change as needed. The output of this converter is in the 60-80 volt range depending on the type of the output transformer. The output waveform on the oscilloscope display has a 10,000 ohm load resistor connected - Figure 2.
This power converter has a fix frequency oscillator that is set to produce a 1,000 Hz rectangular - nearly square - wave with a near 50% duty cycle. That frequency is well within the range of the audio transformers, but could be a problem when using power transformers. The drive available from a 555 timer IC may be used direct. When a higher load current is needed, the external NPN transistor switch Q1 is used. Q1 is a saturated switch that operates safe and cool even while switches significant power. The stepped up voltage from the transformer's original primary windings is connected to the full wave rectifier diodes D1 and D2. The C4 filter capacitor removes most of the ripple. A voltage regulator can also be added.
The output of Q1 drives the speaker side of the audio output transformer. Such transformers best obtainable from old radios. If it is from a tube type radio, it would be able to produce higher voltages, but a transistor radio output transformer will be almost as good. If neither output transformers were available, a small, 120 VAC to 12 VAC line transformer could be used. Since these power transformers were designed for 60Hz operation many uses low-grade iron for their core. These power transformers may be happier with a lower oscillator frequency. Changing C1 to 0.5 uF is that necessary to reduce the frequency to 200 Hz, but depending on the load the value of the C4 filter capacitor has to increase to a 100 uF or larger - Figure 3.
Parts list for High Voltage DC to DC Converter: B1 - Battery, 12 Volt C1,C2 - Capacitor 0.1 uF, 25V, Disc C3 - Capacitor 1 uF, 15V, Disc or Film C4 - Capacitor 10 -100 uF, 250V, Electrolytic D1, D2 - Diode 1N914 or 1N4004 IC1 - 555, Integrated Circuit, Timer Q1 - Transistor, 2N2102, NPN, HS Switch R1 - Resistor 1K, 5%, 1/4W, CC R2 - Resistor 6.8K, 5%, 1/4W, CC TR1 - Transformer Audio, output or power 120VAC-12VAC