C4.3 - Low Power Design for Wireless Instrumentation

An intrinsically safe and self-supplied wireless adapter has been developed for HART devices. The adapter works with a supply voltage down to 1.4V and a supply current as low as 3mA. To achieve this, analogue and digital low power design approaches, a new intrinsic safety concept, and a novel control scheme are used. The proposed adapter is connected serially to the 4...20mA loop and harvests energy thereof to supply itself. The voltage drop generated by the adapter should be minimised because the wiring is often close to the maximal allowed length. The voltage available to the field device is minimal at 20mA and maximal at 4mA. All in all, the maximum power that can be used by the adapter is just a few milliwatts. On the one hand, this paper presents techniques to optimise the power consumption such as: efficient and adaptive low voltage and low power conversion scheme based on off-the-shelf components; leakage-optimised large capacitive banks (in our case, 6mF). On the other hand, we also present our innovative solution for low voltage intrinsic safety barriers. At -40°C, the latter achieves a voltage loss of about 200mV compared to the 800mV required for the conventional diode-based solution.