AC, DC and Electrical Signals

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) always flows in the same direction, but it may increase and decrease.

A DC voltage is always positive (or always negative), but it may increase and decrease.

Electronic circuits normally require a steady DC supply which is constant at one value or a smooth DC supply which has only 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.

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 below 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 the properties apply to any signal with a constantly repeating 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.

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 (V_RMS) which is 0.7 of the peak voltage (V_peak):

and

These equations also apply to current.

It is important to note that these equations are only true for sine waves (the most common type of AC) because the 0.7 and 1.4 factors 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 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 a typical 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.

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 8.4V. The UK mains supply is 230V AC, this means 230V RMS so the peak voltage of the mains is about 320V.

Pulse Width Modulation (PWM) is the technique of varying the proportion of a digital (on/off) signal which is on or high (the pulse width) to control an analogue quantity such as the brightness of an LED or the speed of a motor.

The pulse width is the mark time (Tm) and the off or low part of the signal is the space time (Ts). The pulse width (Tm) proportion of the complete on/off cycle (Tm + Ts) is called the duty cycle and is usually given as a percentage. The diagram shows examples of 90% (mostly on), 50% (equal on and off times) and 10% (mostly off) duty cycles.

PWM is a useful way to convert a digital signal to an analogue quantity determined by the average time the digital signal is on (or high) over a complete cycle.

The frequency of the on/off pulses must be sufficiently high for the output to appear continuous, for example at low frequencies an LED will simply flicker but at 1kHz (1000 flashes per second) our eyes cannot see the rapid flashes and instead we see an average brightness determined by the duty cycle. For controlling motor speed the frequency can be lower such as 100Hz because mechanical inertia in the system smooths out the pulses.