Although not commonly found in the average parts bin, transformers are nonetheless a fundamental electronic component that have some pretty interesting properties, namely converting, ehem, transforming, one voltage into another! Who'da thunk? In this instructable I'll briefly describe transformer design and operation, and then we'll use a transformer-based circuit scrapped from an old disposable camera to power a Nixie tube. High voltages, here we (safely) come!
Step 1: What Is a Transformer?
Transformers are everywhere, though they're well hidden. Chances are, if you have any electronic device or machine that plugs into a wall, then it likely has a transformer within to convert the high voltage 120 or 240 VAC into more modest AC voltages that are then rectified to power DC devices. Transformers form the bulk of the volume and mass of "wall warts" that power most modern electronics. Why are they so heavy? What do they do? How do they do it?!
A "typical" transformer has three main parts: the primary coil, secondary coil, and the core. The coils consist of tightly wound copper wires, the number of turns of said wires dictating what effect the the transformer will have on the voltage applied. Primary and secondary are relative terms when it comes to describing a transformer. The primary coil is the input coil, and the secondary is the output coil, swap the two and you can simply refer to them in reverse. While the labels aren't as important, the design of each coil does. Generally, the two coils will be wound with a ratiometric configuration, that is one coil typically has fewer turns of wire than the other. These coils are wrapped adjacent to each other around a usually laminated iron core (hence the heft)
Transformer operation relies on changing magnetic fields that induce a current from one coil to the other. A transformer requires an AC voltage across its primary coil in order to induce a current in the secondary coil. The rapidly changing magnetic field, the magnetic flux, is carried through the iron core from one coil to the next. The input is AC, thus the output is AC as well. The ratio of the coils determines whether the transformer will boost (increase) or buck (decrease) the voltage to the secondary. A low turn count in the primary and a high turn count in the secondary will result in a higher voltage across the secondary. You can use the equation: Vs = Vp * (Ns/Np) The voltage across the secondary is equal to the voltage across the primary multiplied by the number of turns in the secondary divided by the number of turns in the primary. For instance, if you ran 120 VAC across the primary coil with 200 turns and the secondary coil had 500 turns of wire, then ideally you'd have:
Vs = 120 * (500 / 200)
Vs = 120 * 2.5
Vs = 300
Also, although transformers are bidirectional, it is important to always use the manufacturer's recommended operation, as part of the transformer may not be rated for certain currents.
Step 2: High Voltage Safety
Transformers often run at very high voltages and currents, which can be lethal if not handled properly. It's important to be extremely prepared and cautious when working with any high (>120V) circuit. Wear proper safety equipment such as insulated gloves and goggles. High voltage sparks can quickly ignite objects into flame, so be mindfull of the nearest fire extinguisher. When in doubt, use the "one hand" rule when handling a HV circuit, so that, at the very least in the even the circuit discharges through your body, it will not have a conductive path across your heart, as is the case when you hold a circuit with both hands. We'll be working with a 300V circuit from the camera, which, while operating at low currents (the real danger) can cause large sparks and nasty burns. Stay alert!
Step 3: Types of Transformers
The most common variety of transformers, iron cores usually consist of an electrically separate pair of coils wrapped around laminated sheets of iron. The secondary coil also usually has one or more "center taps" to allow a variable selection of voltages at the output. Two independent coils means the circuits are electrically isolated from each other. Iron Core transformers can have higher ratios for boosting or bucking large voltages.
Wires wrapped around ceramic ferrites, these transformers operate in much the same way as iron cores, however they can be much smaller.
Autotransformers are unique in that the primary and secondary coil are wound from the same wire. The center tap of the transformer forming a common point between the two coils. This configuration is slightly more compact, but the circuits are no longer isolated. Autotransformers are use for smaller voltage shifts.
Variable Transformer (Variac)
Variable transformers are another center-taped transformer variety. Similar to the internal structure of a rotary potentiometer, a variac has an internal wiper as a center tap across the the secondary coil, which allows you to have a selectable AC voltage output.
Step 4: Making a Nixie Power Supply
Most disposable cameras simply snap together. Using your fingernail or a small screw driver to carefully pry apart the tabs along the seam of the camera shell. These are usually quick weak. Once you've removed the back, pop the front cover off to reveal the flash circuit. Pop out the battery and the PCB should fall right out when you tip it over. Check with a multimeter across the two pins of the capacitor to see if there is any latent charge, but be care not to short the pins as it will spark if the capacitor has been recently charged.
Theory of Operation
The camera flash circuit has a few key parts. The first half of the circuit consists of a rapidly oscillating circuit across the larger transformer that charges the capacitor whenever the power switch is on. This rapidly builds up to about 300V across the capacitor. When the trigger switch is closed, this charge runs through a secondary transformer that further boosts the voltage which is then high enough to trigger the flash tube. We're ignoring the second half of the circuit that discharges the flash, so you may wish to remove it entirely (the circuit will still run fine). We're only interested in the more modest 300V source from the capacitor. (Check out this video if you'd like to know precisely what's going on in the circuit)
*Note: This isn't a terribly efficient means of generating high voltage, and there are many efficient circuits dedicated to generating the ideal voltages for Nixie tubes, but this is a low cost option and does demonstrate the properties of a transformer. This definitely falls under the "neat experiment" category and shouldn't be used for long term projects.
Step 5: Nixie Driver Circuit
Step 6: Software
The sketch is fairly basic, and runs through the numbers by writing the four bits to the binary input pins on the nixie driver chip. Although you can turn on more than one cathode segment at a time, the nature of a Nixie tube means that only one number can be visible at a time. Throw in three more driver chips and Nixie tubes, and try your hand at a Nixie clock! They're quite the sight to behold and make for an awesome time piece.