A 2nd order thermal sigma-delta modulator for flow sensing

A low-cost thermal anemometer with digital output has been realized. It uses a self-heated bipolar transistor as its sensing element and thus circumvents the packaging problems associated with micro-machined sensors. The transistor is heated by its own collector current, while its temperature is determined by measuring its base-emitter voltage. To compensate for ambient temperature variations, a 2nd order thermal sigma-delta modulator is used as a control loop, which maintains the transistor at a constant temperature (~5K) above ambient temperature. The output of the modulator is a bit-stream whose average value is proportional to the transistor´s flow-dependent heat loss. Wind tunnel tests show that an off-the-shelf transistor in a SOT-89 package can be used as the basis of a low-cost flow sensor