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?
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
Note also that we can have multiple secondaries. Any
• High Voltage, Low Cost
• N, P, or What?
n FIGURE 1. Example AC high
AC HV 2
AC HV 1
12 July/August 2018