Constant voltage versus constant current

The commercial electrical waterbath systems presently in use, operate using a constant voltage. Constant voltage stunners are designed to apply an equal voltage to each bird passing through a waterbath, whether it accommodates a single bird or multiple birds.

The minimum total current required for a waterbath will be the maximum number of birds that can simultaneously be in the water, multiplied by the minimum required current per bird, eg Figure 9A. However, this is, at best, an estimate because it assumes all birds have equal resistances, which, in reality, they do not.

When a pre-determined, constant voltage is applied to a group of birds, each bird’s different level of resistance will cause it to receive an associated different current amplitude (Figure 9B). This means a bird with a lower-than-average resistance may receive more current than the operator intends and therefore may be stunned (but may also experience more damage to the carcass); whilst a bird with a higher-than-average resistance might receive less current than the operator intends and may not be adequately stunned. Therefore, attempting to deliver to every bird, the minimum recommended current per bird, using a multi-bird constant voltage stunner, is extremely difficult.

A more preferable electrical stunning system for animal welfare and meat quality will operate using a constant current. The challenge of the natural, multi-factorial variation in individual birds’ resistance, and across the range of available electrical parameters, is met by automatic variation in the applied voltage, to ultimately produce a very similar current amplitude for each bird, ie sufficient current for bird welfare but not too much current for meat quality, eg Figure 9D. Unfortunately, whilst constant current systems have been used since the 1990s for scientific research, no large-scale commercial waterbath systems are yet available to industry.

Figure 9. An example of how constant voltage and constant current systems work and the current and voltage used to attempt to supply each bird with a prescribed current of 100 mA. A voltmeter and ammeter are displayed on the right and the parameters per bird are shown above and/or underneath each bird. A) If it is assumed that all birds have an average resistance of 1600 Ω.

B) In reality, each bird may have a different resistance which causes them to each receive a different current amplitude in a constant voltage system. The warning triangles indicate birds that receive less than 100 mA and, as a result, may not be effectively stunned.

C) To combat this animal welfare problem, the voltage of a constant voltage stunner should be increased to ensure all birds receive at least 100 mA.

D) Alternatively, a constant current stunner can alter, as necessary, the voltage applied to each bird, in order to deliver the same prescribed current to each bird.

(Note: all figures are examples; the voltages shown should not be assumed to be the necessary voltage to achieve these current amplitudes in practice. For guidance on appropriate parameters, read the section ‘Parameters for stunning’.)

Please refer to the HSA Guidance Notes No.7 on ‘Electrical Waterbath Stunning of Poultry’ if additional detail is required.

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