Nuts and Volts - July/August 2018

In this column, Kristen answers questions about all aspects of electronics, including computer hardware, software, circuits, electronic theory, troubleshooting, and anything else of interest to the hobbyist. Feel free to participate with your questions, comments, or suggestions. Send all questions and comments to: Q&A@nutsvolts.com.

n WITH KRISTEN A. McINTYRE

High Voltage, Low Cost

Q: I’m running an experiment for my high school science project that requires a 5,000V power supply. The commercial supplies are crazy expensive. Is there an inexpensive DIY way to generate a few thousand volts at only a few microamps?

Greta Chivers Boca Raton, FL

A: Before we start on this question, I want to mention that 5KV — even at microamps — can be dangerous. Please be cautious when dealing with high voltage in any form. Keeping the current limited to a few microamps is difficult at all points in a circuit like this, so it’s possible to induce a fairly high voltage across your body if you touch the wrong thing.

There are a few different approaches one could take to such a design, but all the ones that are relatively easy involve an oscillator that is based on positive feedback through a transformer. This serves the dual purpose of providing an easy feedback path that is naturally oscillatory, plus gives us an opportunity to get some immediate voltage multiplication through the turns ratio of the transformer.

Let’s take a quick look at how transformers work. One warning, though: I’m going to play fast and loose with causality for the sake of clarity in this particular discussion.

Transformers work on the principle of Faraday’s Law of Induction and Ampere’s Law from Maxwell’s Equations. We can have a time-varying voltage produce a time-varying magnetic flux and vice versa. Magnetic flux is the magnetic field that passes through a given surface; for instance, the surface covering a loop or loops of wire. If, for example, that magnetic flux is changing with time, then we get a corresponding voltage around the loop. Conversely, if we have a time-varying voltage, we can “induce” a time-varying magnetic flux. Our job is to exploit this property. The number of wire turns that are around the surface through which the magnetic field passes are additive. In other words, the more turns, the more voltage. If we have a turns ratio of, say, 10 to 1, then the voltage on one turn on what we often call the primary of a transformer will be multiplied by 10 and appear on the secondary.

Of course, there’s no free lunch, so the current will be reduced by a factor of 10 (at least) on the secondary so that energy is conserved. So, Pin >= Pout. Another way to think of that is:

Iin • Vin >= Iout • Vout The voltages that appear on these wire turns are not referenced to anything except themselves. This means that we get to choose the reference by what we connect those wires to. For this circuit, we’ll choose to reverse the voltage by connecting one secondary up ‘backwards,’ reversing the voltage.