AEDs pose the additional challenge of size- and cost-reduced components, all defibrillators need stable and repeatable measurement of the charging voltage, which determines the amount of electrical energy delivered to the patient. The defibrillator-charging circuit uses high-voltage resistors, with a high-value resistor, normally in the range of 5 MΩ to 50 MΩ, and a low-value resistor providing a potential divider for voltage feedback.
Critical features of such high-voltage resistors are linearity (expressed as voltage coefficient or VCR), temperature coefficient (TCR), and long-term stability under voltage stress. Thick-film resistors best suit this application. Their temperature characteristic is typically “U” shaped with limits expressed by the TCR, normally from ±25 to ±100 ppm/°C. The TCR error can be reduced by choosing the highest-possible ohmic value, which lowers self-heating, and by designing layouts that avoid proximity to heat-generating components.
The voltage characteristic, by contrast, only ever has a negative gradient, with a limit expressed by the VCR, typically between –1 and –5 ppm/V. High-voltage resistors use special design techniques to minimize VCR, but this needs to be traded off against product size. As the gradient increases at high voltage, only operating the resistor at up to 75% of the full rated voltage can reduce VCR error. Designers need to choose resistors with both a low VCR and a high voltage rating. Furthermore, if the nominal VCR is known, compensation is relatively simple.
Environmental stability describes the limits of non-reversible resistance change under given loading and environmental conditions. The most demanding condition is high humidity, but some devices use a specially formulated high-density epoxy material to achieve typical resistance changes of less than 0.25% after 56 days at 95% relative humidity and 40ºC.
