Schematic Reference

To drive nixie tubes or neon lamps you need a DC power supply of around 170 volts for consistent and fast tube striking. But where to get such a supply? This simple switchmode converter provides the required voltage with enough current to supply most projects.

This kit allows you to make a very simple switchmode boost (step-up) converter for powering nixie tubes and similar displays that require voltages between 100 and 200 Volt DC. It uses very readily available parts which can be bought from almost any electronics components store should the need arise for replacements.

How it works :

The circuit is designed around one of the most common ICs on the planet—the 555 timer chip. While several decades old, this chip still finds uses in a great many areas, including simple converters like this one. The 555 is configured as an astable oscillator which oscillates at a frequency (around 45kHz) determined by resistors R1 and R2, and capacitor C2. The 555’output directly drives a high voltage MOSFET, Q1, that switches the current through an inductor, L1.

When the FET is on, current flows from V+ through the inductor to ground. As the current in the inductor builds up, the FET then turns off.When the FET turns off, the current flowing through the inductor tries to continue to flow but can’t flow through the FET, so the voltage on the FET’s drain terminal rises until the ultrafast diode, D1 becomes forward biased. This allows the energy contained in the coil to be dumped into the high voltage capacitor, C4.

This cycle continues until the voltage across C4 reaches a value set by trimpot VR1. R4, R5 and VR1 form the feedback divider for the circuit, and are chosen such that the voltage across C4 is divided down to around 0.7 volts. This voltage is applied to transistor Q2, a BC547. When the voltage across C4 rises to the point where the voltage from the divider is enough to turn Q2 on, then Q2 pulls down the Control pin of the 555, stopping it from oscillating and shutting down the converter. As soon as this happens, the voltage across C4 begins to fall, as does the voltage into the base of Q2, and so Q2 turns off, allowing the converter to restart. This is how the circuit regulates the voltage, and in practice it works quite well. Resistor R6 and capacitor C3 form a simple snubber network, while C1 is the main reservoir cap for the circuit. And that’s about all there is to it!

Building it :

Well, there is not much to say here, just get the components in their appropriate places and the right way around (for the polarised components like the diode, electrolytic capacitors, transistors and IC) and solder them into place. Make sure there are no shorts between IC pins or other places on the board. When you have checked and double checked your work, you can then give the unit a test.

The smoke test :

Place the board on an insulating surace (clean, dry wood or plastic is fine) and, after making sure the trimpot is centred, connect a 9 to 12 volt DC source to the input terminal block. It is best to use a current limited supply to start with, in case there is a problem. Do not connect the circuit to a battery or other high current supply without having tested it first! Now, take your multimeter and check the output voltage. It could be anywhere between 100 and 200 Volt DC. If there is no voltage on the output, check that the 555 is oscillating (you do have a logic probe of course!). If not, find the problem and fix it. Once you get a high voltage output,make sure it is adjustable from around 100 volts to 200 volts. The lower limit varies somewhat with different components, one prototype couldn’t be adjusted below 145 volts.

Note that, as the kit is supplied with a 400 volt MOSFET, the output can exceed 200 volts, The final test involves connecting a load to the supply, such as a nixie tube via a 22k resistor. If the supply drives the load, then you’re in business! Of course, turn the supply off and allow the voltage to drop to a safe level before making any connections! Note that the voltage will vary a bit with load, but it should not swing by more than a few volts from 0 to 10mA load.but don’t use the kit for voltages above this.

This circuit, while not as dangerous as mains power, provides enough energy to potential kill a person should they be connected between the high voltage output and ground in the right way, so be careful when working on this circuit! Always, turn off the power and wait for the high voltage output to drop before making any changes to the circuit. When completed, the circuit must be mounted inside an insulated case with properly insulated terminals to prevent anyone accidentally coming into contact with the high voltage output!