WARNING : the high voltage supply involves lethal voltages & currents in some parts of it. It is assumed that the reader is experienced with mains wiring and high voltage circuits. The author cannot be held responsible for any damage, death or injury caused by or having any relation with the information presented below.
One of simple ways to build an EHT supply is by using small transformers connected back-to-back followed by a voltage multiplier. By using several small 230V units voltages in the order of 600Vrms are achieved easily before capacitor/diode ladder. Note that small transformers typically have poor load regulation, so
a combination of different trannies may be needed to obtain required output voltage. It is recommended to use vacuum cast short-circuit and fireproof small trannies for safety reasons.
a combination of different trannies may be needed to obtain required output voltage. It is recommended to use vacuum cast short-circuit and fireproof small trannies for safety reasons.
A Variable version of EHT supply.
While the circuit shown above is simple and reliable, experiments with panels often ask for adjustable output voltage. The circuit shown below is a modified version of simple EHT supply. Ability to adjust output is achieved by including a variable voltage sine wave generator before step-up transformer array. A NE5532 op amp is sufficient to provide enough current to drive a couple of small mains transformers. The first op-amp of NE5532 together with R3-R5 & C1-C3 form a phase-shift oscillator. The second one acts both as a variable gain stage and a buffer. Gain(thus the output voltage) is set by potentiometer VR1. It is then fed to two small 8/230V mains transformers. While such trannies are normally rated at 50 Hz no problems have been observed while running it at 650 Hertz(as measured). It is interesting to note that simulated frequency was around 500 Hz. The cause of this is unclear; perhaps it should be attributed to variations in component tolerances. The purpose of zener diodes D1 & D2 is to ensure that load inductance won't feed voltages higher that supply rails to op amp's output(i.e in case of short circuit, etc).
The frequency of operation has been chosen mainly because it's a good compromise between output current and reliability. The maximum current of voltage multiplier depends a lot on frequency. From the other side, dielectric strength of most dielectrics falls with increasing frequency, so too high a frequency could cause breakdowns in small trannies.
The real device has been split into two separate boards : one for high voltage circuitry and another containing power supply & sine wave generator. The choice of using two boards was not driven by any practical considerations. High voltage board was taken from previously built fixed voltage EHT unit. One note about output resistor Ro. It should be either high voltage type or a string of, lets say, 10x1 MOhm standard resistors, as single 300V resistor is likely to fail.
Measuring the output voltage.
Due to very high voltages involved and low currents direct measurements via simple voltmeter at HV output are not possible. There are two tricks in measuring output voltage:
a) Measure the input voltage of the multiplier ladder and then multiply by the number of stages. Its possible the reading could be somewhat inaccurate as frequency of interest is different from mains. Digital multimeters may show wrong readings for frequencies like 650 Hz. Furthermore, it might be difficult(and dangerous) to connect multimeter leads to already assembled board. Therefore a second approach might be preferable.
b) It's relatively simple to make a high voltage measurement device if the input impedance of the DMM is known . A very high Ohmic value HV resistor is required. The resistor is connected in series with voltmeter, forming a simple voltage divider. Typically, DMM's have input impedance of 10 MOhms or so. By connecting it in series with 5 GOhm HV resistor a 1:500 voltage multiplier is built. Such resistors are available from e-bay. It is important to choose high enough resistance values. Otherwise the HV supply will be overloaded, resulting in lower than actual voltage reading. For example a 1 GOhm resistance would already caused voltage to drop in the circuit shown above. If the same voltage multiplier is fed with low frequency signal(like 50 Hz) the voltage drop would be excessive.