In an effort to reduce the time-to-defibrillation interval
and improve cardiac arrest survival, health service providers have increasingly
turned to a strategy of wider access to defibrillation to augment emergency
medical services. In some countries this is being implemented by providing automated
external defibrillators (AED) for use by police, first aid volunteers, and even
members of the general public.
Two of the main improvements required to adapt hospital-based
equipment were the miniaturization of the defibrillator to a small, portable
device and its ability to withstand harsher temperature and humidity
environments without affecting reliability.
To help find a viable solution to these requirements, the Welwyn Applications Team was approached by
one of the market leaders in the development and manufacture of AEDs. The
specific requirements of the project were to provide a high voltage resistor to
be used in the defibrillator charge control circuit (Figure 1).
The stable and repeatable measurement of the charging
voltage is a critical function of this circuit, as this determines the amount
of electrical energy delivered to the patient. In this instance, R1 was a high
voltage resistor with a value of 20 M and 1% tolerance, which, together with a standard
chip resistor R2, forms a potential divider for voltage feedback.
In addition to the requirements for a physically smaller component
and better environmental performance, the high voltage resistor needed to have
excellent linearity, expressed by voltage coefficient and temperature
coefficient, and long-term stability under voltage stress.
To overcome these problems, the design team at Welwyn
developed a new, high-voltage planar resistor incorporating a printed thick-film
resistor ink, based on a ruthenium oxide/glass system that is subsequently
encapsulated with a specially formulated high-density epoxy protection that
gives improved environmental performance over the traditional porous conformal
coat protection.
Martin Birkett is an applications engineer at Welwyn
Components Ltd.