Regulated HV power supply

This variable high voltage power supply has been a useful tool for a lot of experiments with valves an nixies. Its specifications are:

• Regulated output voltage: 60-300 V (325 V unloaded)
• Output current: 0-60 mA
• Regulation: better than 1 V for outputs up to 250 V and load up to 60 mA or for loads up to 40 mA and output voltages up to 300 V.
• Heater output voltage: 4V/1A and 6.3V/1.5 A 50 Hz, unregulated.

Circuit diagram

If you're familiar with regulated power supplies, it is easy to see how it works. There is an EL84 used as a power stage in common cathode. Since the EL84 can only handle 60 mA, this is the maximum output current. An EF184 pentode is used as error amplifier. It is actually used as a triode here. It's reference is the voltage of the neon stabiliser in its cathode. The neon stabiliser is supplied from a negative supply to get a -72 V reference voltage.

I thought I needed some kind of short-circuit protection. To achieve this, I added an output relay. This is powered by a pushing a button. After that, the relay gets only its holding current through a resistor and the neon warning light atthe front. When the "off" button is pressed, the relay coil is shorted and it turns off. Besides the "off" switch there is a reed relay that can short the output relay and switch off. The output current goes through the reed relay's coil. The sensitivity of the current limitation is set with potentiometer R4 so that it energises at 65 mA.

What it looks like

As you can see on the photograph, the power transformer is one of those tar-impregnated black transformers from an old Philips radio.

The neon stabiliser tube is rather uncommon. It is a Japanese tube on a 4-pin base, probably very old. I got it from a friend of my father who was a PTT technician and electronics hobbyist. As you can see on the photographs, it gives nice visual effects to the inside of the power supply.

The case is home made. The sides are pine, the center frame and the front plate are sheet aluminium and the top, bottom and back cover are perforated aluminium. I embellished the front plate by anodizing it in a bath of diluted sulphuric acid. Worked quite well and resulted in a hard matt finish.

Design error

There is one error in this design. I was made aware of this by someone who wrote in my guestbook that the EL84 has a U_fk rating of only 100V. As the cathode in this design can be as high as 330V, the heater should not be connected to ground but to a separate heater winding on the transformer, strapped to the output by a large resistor. He was right, of course. Because there is no spare heater winding, I'll have to use an extra transformer.