Alternating Current (AC)
Alternating Current (AC) flows one way, then the other way, continually reversing direction. An AC voltage is continually changing between positive (+) and negative (-).The rate of changing direction is called the frequency of the AC and it is measured in hertz (Hz) which is the number of forwards-backwards cycles per second.
Mains electricity in the UK has a frequency of 50Hz.
See below for more details of signal properties.
An AC supply is suitable for powering some devices such as lamps and heaters but almost all electronic circuits require a steady DC supply (see below).
Direct Current (DC)
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| Steady DC from a battery or regulated power supply, this is ideal for electronic circuits. |
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| Smooth DC from a smoothed power supply, this is suitable for some electronics. |
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| Varying DC from a power supply without smoothing, this is not suitable for electronics. |
Electronic circuits normally require a steady DC supply which is constant at one value or a smooth DC supply which has a small variation called ripple.
Cells, batteries and regulated power supplies provide steady DC which is ideal for electronic circuits.
Power supplies contain a transformer which converts the mains AC supply to a safe low voltage AC. Then the AC is converted to DC by a bridge rectifier but the output is varying DC which is unsuitable for electronic circuits.
Some power supplies include a capacitor to provide smooth DC which is suitable for less-sensitive electronic circuits, including most of the projects on this website.
Lamps, heaters and motors will work with any DC supply.
Please see the Power Supplies page for further information.
Power supplies are also covered by the Electronics in Meccano website.
Properties of electrical signals
An electrical signal is a voltage or current which conveys information, usually it means a voltage. The term can be used for any voltage or current in a circuit. The voltage-time graph on the right shows various properties of an electrical signal. In addition to the properties labelled on the graph, there is frequency which is the number of cycles per second. The diagram shows a sine wave but these properties apply to any signal with a constant shape.
- Amplitude is the maximum voltage reached by the signal.
It is measured in volts, V. - Peak voltage is another name for amplitude.
- Peak-peak voltage is twice the peak voltage (amplitude). When reading an oscilloscope trace it is usual to measure peak-peak voltage.
- Time period is the time taken for the signal to complete one cycle.
It is measured in seconds (s), but time periods tend to be short so milliseconds (ms) and microseconds (µs) are often used. 1ms = 0.001s and 1µs = 0.000001s. - Frequency is the number of cycles per second.
It is measured in hertz (Hz), but frequencies tend to be high so kilohertz (kHz) and megahertz (MHz) are often used. 1kHz = 1000Hz and 1MHz = 1000000Hz.
Mains electricity in the UK has a frequency of 50Hz,frequency = 1 and time period = 1 time period frequency
so it has a time period of 1/50 = 0.02s = 20ms.
Root Mean Square (RMS) Values
The value of an AC voltage is continually changing from zero up to the positive peak, through zero to the negative peak and back to zero again. Clearly for most of the time it is less than the peak voltage, so this is not a good measure of its real effect. Instead we use the root mean square voltage (VRMS) which is 0.7 of the peak voltage (Vpeak): They are only true for sine waves (the most common type of AC) because the 0.7 and 1.4 are different values for other shapes.
The RMS value is the effective value of a varying voltage or current. It is the equivalent steady DC (constant) value which gives the same effect.
For example a lamp connected to a 6V RMS AC supply will light with the same brightness when connected to a steady 6V DC supply. However, the lamp will be dimmer if connected to a 6V peak AC supply because the RMS value of this is only 4.2V (it is equivalent to a steady 4.2V DC).
You may find it helps to think of the RMS value as a sort of average, but please remember that it is NOT really the average! In fact the average voltage (or current) of an AC signal is zero because the positive and negative parts exactly cancel out!
What do AC meters show, is it the RMS or peak voltage?
AC voltmeters and ammeters show the RMS value of the voltage or current. DC meters also show the RMS value when connected to varying DC providing the DC is varying quickly, if the frequency is less than about 10Hz you will see the meter reading fluctuating instead.What does '6V AC' really mean, is it the RMS or peak voltage?
If the peak value is meant it should be clearly stated, otherwise assume it is the RMS value. In everyday use AC voltages (and currents) are always given as RMS values because this allows a sensible comparison to be made with steady DC voltages (and currents), such as from a battery. For example a '6V AC supply' means 6V RMS, the peak voltage is 8.6V. The UK mains supply is 230V AC, this means 230V RMS so the peak voltage of the mains is about 320V!So what does root mean square (RMS) really mean?
First square all the values, then find the average (mean) of these square values over a complete cycle, and find the square root of this average. That is the RMS value. Confused? Ignore the maths (it looks more complicated than it really is), just accept that RMS values for voltage and current are a much more useful quantity than peak values.In alternating current (AC or ac) the movement of electric charge periodically reverses direction. In direct current (DC), the flow of electric charge is only in one direction.
The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current or voltage.
AC is the form in which electric power is delivered to businesses and residences. The usual waveform of an AC power circuit is a sine wave. In certain applications, different waveforms are used, such as triangular or square waves. Audio and radio signals carried on electrical wires are also examples of alternating current. In these applications, an important goal is often the recovery of information encoded (or modulated) onto the AC signal.
AC DC DYNAMICS
Established in 1984, AC/DC Dynamics is the leading manufacturer, importer and distributor of quality products in the electrical, electronics, pumps and tools industries. Employing over 400 staff, AC/DC Dynamics has its Head Office in Edenvale, Johannesburg with branches in Germiston, Cape Town and Pinetown in KZN.
Manufacturing includes, transformers, power supplies, electronic timers, motors starters, MCCs and power factor correction systems.
Since October 2006, AC/DC Dynamics has manufactured and distributed Rhomberg Electronics, a long established premium brand whose products include timers, counters, temperature controllers, relays and sensors. These products are manufactured in Cape Town and distributed world-wide through the Rhomberg distributor network. The manufacturing facilities form part of our Cape Town branch in Paarden Eiland and have recently been awarded ISO9001:2008 by SABS.
AC/DC Dynamics markets to specifiers, consulting engineers and architects, pulling back to distribute its products through OEMs, Franchises and electrical wholesalers throughout Southern Africa. 60 of the leading electrical wholesalers have been appointed as Official Distributors who keep stocks and have trained staff to support AC/DC Dynamics products. End users, electrical contractors and small OEMSs are referred to Official Distributors and Franchises. This places AC/DC Dynamics products within easy reach anyone in Southern Africa. AC/DC Dynamics products are specified in projects ranging from stadiums to mines, hotels to homes and from farms to breweries.
AC/DC Dynamics continues to develop Distributors and Franchises in Southern Africa. A Franchise is a business system, an independently owned electrical wholesaler whose products are primarily sourced from AC/DC Dynamics and trades with the brand AC/DC Express.
The company is committed to providing fast, efficient and friendly service. For this reason the company provides national sales and technical sales call centres. Orders and enquiries may be processed through this website. A new and improved website will be launched late 2011. New training courses are being provided ranging from fundementals to advanced product applications.
One of the company’s many strengths is its’ catalogue which is widely recognised as a benchmark in the electrical industry. The new 2011 catalogue is over 50%larger than the previous one, It was launched at the Electra Mining Exhibition on October 4th 2010. The Rhomberg catalogue is additional to this. Many new product ranges include solar and wind generator systems, tools and adhesives, drives and soft starts, and the most comprehensive range of energy saving lighting using CFL, inductive and LED technologies.
