description={a device that is connected to and works with a \gls{computer} in a some way, but is not essential to a \gls{computer}'s function}
}
\newglossaryentry{voltage}{
name=voltage,
description={also called electromotive force, is a quantitative expression of the potential difference in charge between two points in an electrical field}
}
\newglossaryentry{current}{
name=current,
description={(electric) is the flow of charged particles through a conducting medium}
jIn general physics terms, power is defined as the rate at which energy is transferred (or transformed). Electric energy in particular, begins as electric potential energy – what we commonly refer to as voltage. When electrons flow through that potential energy, it turns into electric energy. In most useful circuits, that electric energy transforms into some other form of energy. Electric power is measured by combining both how much electric energy is transferred, and how fast that transfer happens.
The electric power P is equal to the energy consumption E divided by the consumption time t
$$P =\frac Et$$
where P is the electric power in watt [W], E is the energy consumption in joule [J] and
t is the time in seconds [s].
Electrical Power, in a circuit is the amount of energy that is absorbed or produced within the circuit. A source of energy such as a voltage will produce or deliver power while the connected load absorbs it. Light bulbs and heaters for example, absorb electrical power and convert it into heat or light. The higher their value or rating in watts the more power they will consume.
\subsection{Alternating current (AC) circuits}
When a reactance (either inductive or capacitive) is present in an AC circuit, the previous formula does not apply. The product of \gls{voltage} and \gls{current} is, instead, expressed in volt-amperes (VA). This product is known as the \textit{apparent power}.
\textbf{Real power} (or true power) is the power that is used to do the work on the load:
$$P = V_{RMS}I_{RMS}\,cos\,\varphi$$
where P is the real power in watts (W), $V_{RMS}$ is the RMS voltage, defined as $V_{peak}/\sqrt{2}$ in Volts (V), $I_{RMS}$ is the RMS current, defined as $I_{peak}/\sqrt{2}$ in Amperes (A) and $\varphi$ is the impedance phase angle - phase difference between voltage and current.
\textbf{Reactive power} on the other hand, is the power that is wasted and not used to do work on the load. Curiously, it is defined as
$$Q = V_{RMS}I_{RMS}\,sin\,\varphi$$
with $Q$ being the reactive power in volt-ampere-reactive (VAR).
\textbf{Apparent power} is the power that is supplied to the circuit. Definition:
$$S = V_{RMS}I_{RMS}$$
where the unit of apparent power $S$ is volt-ampere (VA). It can be seen that it is not phase-angle dependent.
The relation all these three quantities are in is defined as
$$ P^2+ Q^2= S^2$$
however, again, nothing in the real world is perfect, and this relation only applies for a perfectly sinusoidal waveforms.
\subsubsection{Phasor}
A phasor is a constant complex number representing the complex amplitude (magnitude and phase) of a sinusoidal function of time. It is usually expressed in exponential form. Phasors are used in engineering to simplify computations involving sinusoids, where they can often reduce a differential equation problem to an algebraic one. The origin of the word phasor comes from phase + vector.
Phasor is a vector that represents a sinusoidally varying quantity, as a current or voltage, by means of a line rotating about a point in a plane, the magnitude of the quantity being proportional to the length of the line and the phase of the quantity being equal to the angle between the line and a reference line.
%\subsection{Power factor}
%
%
%\subsection{Measuring electric power with a microcontroller}
\subsection{Measuring the electric power}
Measuring the electric power makes most sense on the customer appliances. The first reason is, that they generally consume power that is purchased on contract. The energetic company measures all the power used up by the end customer, but customer has no easy way to see how much and how \textit{effectively} is power used by the appliances. The second important reason is that the appliances has a standardised connector (plug) that is guaranteed to fit in all the area using it, which is not a case for example on battery powered devices (batteries has different sizes, connectors and general properties.
When it comes to measuring the electrical power, the first and the most important thing to discuss is safety. Only after all the safety precautions had been made clear, the theory can be clarified and subsequently, the practice can be applied. The chapter focuses solely on the first two topics.
\subsection{Voltage ranges and safety disclaimer}
The \gls{iec} international standard \textbf{IEC 60038:1983} defines a set of standard \glspl{voltage} for use in AC electricity supply systems.
\begin{table}[ht!]
\centering
\caption{International standards grouping ranges of supply voltages to three categories}
\label{t:volt_ranges}
\tabcolsep=5pt
\renewcommand{\arraystretch}{1.3}
\begin{tabular}{|l|l|l|l|}
\hline
{\bf IEC voltage range}&{\bf AC}&{\bf DC}&{\bf defining risk}\\\hline
High voltage &\textgreater 1000 V$_{RMS}$&\textgreater 1500 V & electrical arcing \\\hline
Low voltage & 50–1000 V$_{RMS}$& 120–1500 V & electrical shock \\\hline
Extra-low voltage &\textless 50 V$_{RMS}$&\textless 120 V & low risk \\\hline
\end{tabular}
\end{table}
The appliances under test work on mains voltage. In Europe, the nominal voltage of mains line for appliances is 230 V / 50 Hz. This falls into the voltage range, with defining risk of \textbf{electrical shock} as described in the table \ref{t:volt_ranges}.
If not handled with care, operating or manipulating with voltage can cause permanent damage to appliance or health, or can cause fire or even death. Thus, respect, increased care and knowledge is necessary in all further practical steps involved.
\newpage
\section{Embedded system}
An embedded \gls{system} is some combination of \gls{computer}\gls{hw} and \gls{sw}, either fixed in capability or programmable, that is specifically designed for a particular function \cite{ganssle2008embedded}. Industrial machines, automobiles, medical equipment, cameras, household appliances, airplanes, vending machines and toys (as well as the more obvious cellular phone and \gls{pda}) are among the myriad possible hosts of an embedded \gls{system}. Embedded \glspl{system} that are programmable are provided with programming \glspl{interface}, and embedded \glspl{system} programming is a specialized occupation.
@ -145,90 +218,21 @@ TP-Link TL-WR703N \gls{router} is a popular choice among \gls{hw} customisation
Whole printed circuit board of TL-WR703N was remade by the GL.inet team to expose the unused \gls{gpio} pins on the \gls{soc}, utilize two \Gls{ethernet} port instead of one and utilize the \gls{usb} 2.0 port. Memory chips were replaced by their higher capacity alternatives.
%\caption{The back side of the GL.inet board exposing the \Gls{flash} memory and a main voltage regulator}\label{f:board_back}
%\end{figure}
\newpage
\section{Electric power measurement}
Measuring the electric power makes most sense on the customer appliances. The first reason is, that they generally consume power that is purchased on contract. The energetic company measures all the power used up by the end customer, but customer has no easy way to see how much and how \textit{effectively} is power used by the appliances. The second important reason is that the appliances has a standardised connector (plug) that is guaranteed to fit in all the area using it, which is not a case for example on battery powered devices (batteries has different sizes, connectors and general properties.
When it comes to measuring the electrical power, the first and the most important thing to discuss is safety. Only after all the safety precautions had been made clear, the theory can be clarified and subsequently, the practice can be applied. The chapter focuses solely on the first two topics.
\subsection{Voltage ranges and safety disclaimer}
The \gls{iec} international standard \textbf{IEC 60038:1983} defines a set of standard voltages for use in AC electricity supply systems.
\begin{table}[ht!]
\begin{figure}[ht!]
\centering
\caption{International standards grouping ranges of supply voltages to three categories}
\label{t:volt_ranges}
\tabcolsep=5pt
\renewcommand{\arraystretch}{1.3}
\begin{tabular}{|l|l|l|l|}
\hline
{\bf IEC voltage range}&{\bf AC}&{\bf DC}&{\bf defining risk}\\\hline
High voltage &\textgreater 1000 V$_{rms}$&\textgreater 1500 V & electrical arcing \\\hline
Low voltage & 50–1000 V$_{rms}$& 120–1500 V & electrical shock \\\hline
Extra-low voltage &\textless 50 V$_{rms}$&\textless 120 V & low risk \\\hline
\end{tabular}
\end{table}
The appliances under test work on mains voltage. In Europe, the nominal voltage of mains line for appliances is 230 V / 50 Hz. This falls into the voltage range, with defining risk of \textbf{electrical shock} as described in the table \ref{t:volt_ranges}.
If not handled with care, operating or manipulating with voltage can cause permanent damage to appliance or health, or can cause fire or even death. Thus, respect, increased care and knowledge is necessary in all further practical steps involved.
\subsection{Electric power as a physical quantity}
In general physics terms, power is defined as the rate at which energy is transferred (or transformed).Electric energy in particular, begins as electric potential energy – what we lovingly refer to as voltage. When electrons flow through that potential energy, it turns into electric energy. In most useful circuits, that electric energy transforms into some other form of energy. Electric power is measured by combining both how much electric energy is transferred, and how fast that transfer happens.
The electric power P is equal to the energy consumption E divided by the consumption time t
$$P =\frac Et$$
where P is the electric power in watt (W), E is the energy consumption in joule (J) and
t is the time in seconds (s).
\subsubsection{In resistive circuits}
In the case of purely resistive (Ohmic, or linear) loads, Joule's law can be combined with Ohm's law (V = I·R) to produce alternative expressions for the amount of power that is dissipated:
$$P = IV = I^2R =\frac{V^2}R$$
where R is the electrical resistance in ohms ($\Omega$).
\subsubsection{In alternating current (AC) circuits}
When a reactance (either inductive or capacitive) is present in an AC circuit, the previous formula does not apply. The product of voltage and current is, instead, expressed in volt-amperes (VA). This product is known as
the \textit{apparent power}.
\textbf{Real power} (or true power) is the power that is used to do the work on the load:
$$P = V_{rms}I_{rms}\,cos\,\varphi$$
where P is the real power in watts (W), $V_{rms}$ is the RMS voltage, defined as $V_{peak}/\sqrt{2}$ in Volts (V), $I_{rms}$ is the RMS current, defined as $I_{peak}/\sqrt{2}$ in Amperes (A) and $\varphi$ is the impedance phase angle - phase difference between voltage and current.
\textbf{Reactive power} on the other hand, is the power that is wasted and not used to do work on the load. Curiously, it is defined as
$$Q = V_{rms}I_{rms}\,sin\,\varphi$$
with $Q$ being the reactive power in volt-ampere-reactive (VAR).
\textbf{Apparent power} is the power that is supplied to the circuit. Definition:
$$S = V_{rms}I_{rms}$$
where the unit of apparent power $S$ is volt-ampere (VA). It can be seen that it is not phase-angle dependent.
Peter Babič
9 years ago