Series and Parallel Circuits


All hams should have at least a limited understanding of basic circuits, and this means being able to differentiate between series and parallel components.

Besides numerous license exam questions (dozens below in green boxes; knowing helps you pass the exams), some technical discussions in ham radio will throw the terms around so let’s explore the matter here.  In addition to our own presentation, some excellent web references are given at the end for further (and often more interesting) information.


Before jumping into circuits, let’s discuss series and parallel connections.  Visualizing this will help us understand series and parallel circuits.

As the name suggests, series connections are lined up end-to-end.

series res

We’re demonstrating with resistors but the principle applies to any two-terminal component: capacitors,  inductors, diodes, cells/batteries, and light bulbs can all be wired in series with two or more of each (or a mix of different parts).  Lining them up terminal to terminal makes a series connection.


Schematically, 3 parts in series looks like this:

Series schem

From this simple schematic we intuitively see that the current flowing through a series string has to be the same though the chain; there is nowhere else for electrons to flow (current).


Equal current is one way of defining a series circuit.


Also as the term suggests, parallel connections are side-by-side.

parallel res

Again, demonstrating with resistors and again, the principle applies to any two terminal component.  Arranging components across each other makes a parallel connection.


Schematically, 3 parts in parallel looks like this:

Parallel schem

From this simple schematic we intuitively see that the voltage across parallel components must be the same.


Equal voltage is one way of defining a parallel circuit.


We just learned that current is the same through components in series, and voltage is the same across components in parallel.  What about  the voltage across series components, and current through parallel components? Continue reading

Dummy Load

Radio amateurs should be familiar with the term dummy load, which is a RF-friendly substitute for an antenna when testing a transmitter or piece of equipment such as a Watt meter.

Dummy1   Dummy2   Dummy3

A dummy load is somewhat generic, also having industrial and commercial uses.  As applied in ham radio, it electrically simulates an antenna to allow a transmitter to be tested without radiating radio waves, typically at 50Ω to match transmitter output impedance.


Dummy loads are rather simple—  just a big resistor and some way to dissipate heat, all in a package that must be non-reactive, meaning it provides insignificant capacitance and inductance.


Why must a dummy load be non-inductive?  Because of impedance (practically speaking, AC resistance), which increases with frequency based on the formula of inductive reactance XL=2πfL.


Most common power resistors are wire-wound, which have significant inductance.  So RF dummy loads must use resistors with little or no inductance.

As an example, this four-resistor series combination using common Dale metal-clad resistors measures 49.4Ω at 0Hz (DC).  Sounds like a perfect dummy load, right?


Unfortunately  it also has Continue reading