Waveform

The waveform describes the shape of one cycle of the voltage or current. Current can be generated as an alternating current (AC), where the direction of the current flow alternates around zero with positive and negative direction (bipolar; Figure 7a,c,e,f). Alternatively, a direct current (DC) flows in only one direction (unipolar), either the positive or the negative (Figure 7b,d). DC currents are typically pulsed (pDC), meaning the current is turned off (zero amplitude) for a proportion of the cycle time. The resulting waveform can be expressed by the mark:space ratio (where the mark is the time the current is ‘on’ and the space is the time the current is ‘off’, ie at zero). An alternative description of this is the duty cycle, where the duration of the mark is expressed as a percentage of the duration of the cycle time.

The way an AC or DC current flows can be examined over time to reveal the shape/form of the wave. For example, waves can be smooth undulating curves (sinusoidal or sine), square or rectangular, sawtooth or triangular. Current can also be modified to produce different waveforms. For example, AC waves can be rectified to different degrees (eg half or fully) to produce DC waves (eg pulsed or constant, respectively). Or a wave can be clipped to produce various different shapes. A variety of waveforms (including some of those shown in Figure 7) have been used in electrical waterbath stunners, mainly to attempt to reduce carcass damage or to improve the efficacy of the electrical pulse. However, so far, scientific research indicates that carcass quality is not necessarily improved when waveforms are altered and a sine AC appears to produce the most effective stuns for animal welfare.

Figure 7. A selection of AC and DC waveforms. Time is typically described in milliseconds (ms).

(Note: this figure is for descriptive purposes only, to enable understanding of electrical terminology; the waveforms shown are not necessarily appropriate for humane electrical stunning of animals.)

Frequency

The frequency of a current is the number of repetitions of one complete cycle of the waveform per second and it is measured in hertz (Hz). For example, ‘standard’ mains electricity in Europe is characterised by a sinusoidal waveform with a frequency of 50 Hz, ie it repeats 50 times per second (Figure 8a). If there are 50 cycles every second, that means one full cycle is completed in 20 milliseconds; this duration is known as the period of the current. Waveforms that repeat one full cycle a greater number of times per second will have a higher frequency, eg the current in Figure 8b has a frequency four times greater than the current in Figure 8a.

Figure 8. Top: a sinusoidal 50 Hz wave. 5 cycles in 1/10th sec = 50 cycles per second.

                 Bottom: a sinusoidal 200 Hz wave. 20 cycles in 1/10th sec = 200 cycles per second.

Next: Descriptive units of current & voltage