Radiator sensor

by Stefano Purchiaroni

A sensor with a flashing led signalling that the radiator temperature is growing. No batteries are required, since it is feed by a Peltier cell, using an LTC3801 Linear ic to generate the needed voltage from just 20mV of input.

Linear, world leader in the field of Energy Harvesting, has in the catalog the LTC3108 ic, available in TSOP format (0.65 mm), for the direct use or the accumulation of energy coming from very low voltage sources. This integrated circuit, also available as a free sample, is already active at 20 mV input, and can be configured to generate a voltage between 2.2 and 5 volts with the Switching technique, which can be used to directly feed a load, or can be stored in a super-capacitor to have available energy to be used later. This scheme comes from the datasheet and shows a typical use. The definitive scheme of the sensor is instead reported later.

In the chosen implementation, the Chip is configured by acting on the VS1 and VS2 pins to generate 3.3V against the energy collected by the Peltier Cell, using it directly for the power supply of a Flashing blue led connected directly to the Vout. No Super-Cap is used in this case. The 1: 100 LPR6235-752SML micro-transformer recommended by Linear is fundamental, and can be obtained as a free sample from Coilcraftdalla Coilcraft

The thermo-electric conversion (Seebeck effect ) takes place in the presence of a temperature gradient, when the two sides of the cell are subjected to different temperatures. Leaning on the wall of a radiator, however, even the initially cold face after a while warms up, causing the LED to turn off in a few minutes, with thermal equilibrium reached. The flashing will resume at a subsequent re-increase in the temperature of the radiator body, following the ignition cycles of the system. A heat sink on the cold side slows the heating down, thus increasing the duration of the flash. We can summarize with the following formula the relationship between temperature and obtained voltage:

Considering constant the coefficients of Seebeck Sa and Sb (thermoelectric power) of the two materials used for the thermocouples, we obtain the linear relation V = (Sb-Sa) * (T2-T1). The values of the coefficients vary a lot between producers, costs and "nobility" of the materials used. The TEC-12706 cell is the cheapest on the market (3 euro) and generates about 12 mV per degree centigrade of thermal difference, with its 127 Bismuto-Tellurio thermocouples (Bi2Te3) organized in series on a 40 mm ceramic tile side. The use of the LTC3801 allows to start the signal as soon as the radiator starts to heat up. Any other technique would introduce a delay, due to the need to overcome the threshold voltage of the semiconductors used to raise the voltage to the minimum 2.4 volts required by the flashing LED, and therefore only starting from about 40 degrees of thermal difference, using Germanium transistors, or 60 for those at Silicon..

Schematic:

Prototype realized with TSOP-DIP adapters both for the integrated and for the micro-transformer, and pre-drilled "Meccano" type supports with clips for hooking to the front wall of the radiator. The article measures 6 x 3 cm, but in a pure SMD realization it is possible to further reduce its dimensions:

Circuit installed on the front side of a radiator, being operative as its temperature begin to grow:

A short video: