The established in some apartments gas water heater (column) is often fraught with inconvenience. If, for example, a column posted on the kitchen, to check and adjust the temperature of the water filling the tub, has to repeatedly go from the bathroom to the kitchen and back. It is advisable to install a water temperature gauge in place of its consumption, and the indicator next to the column. In this case, you can adjust the temperature, not departing from the speakers.
Electronic thermometer which is schematically depicted in Fig. 1 solves this problem. Temperature sensor are three semiconductor connected in series diode (VD1 - VD3). They can be any type germanium or silicon. It is only important that the diodes were small, this will reduce the lag device. The stabilized voltage supply of 5 V served from any suitable source. Current consumption less than 10 mA. I used the PSU BP-3 kit calculator BZ-34.
The action of the thermometer based on the fact that the voltage drop across the semiconductor the diode at a constant direct current is linearly dependent on the temperature of its p-n transition. If at zero temperature the engine tuning resistor R5 to be mounted so that the voltage between it and a minus power source equal to the voltage drop across the diodes VD1 - VD3, the current through the microammeter PA1 in these conditions missing. With the heating of the diodes the voltage on them is reduced and through microammeter current to flow proportional to the temperature.
Frame used by the author of the microammeter resistance Rp=617 Ohms made of copper wire (temperature coefficient of resistance α=4,3-10-3 °C-1). When the temperature of the frame at ΔT=20 °C the growth of its resistance will be
Since the dynamic resistance of the diodes VD1-VD3 and little resistor values R4-R6 are small, the current through the microammeter RA1 at a predetermined temperature, depends mainly on the sum of the resistances R2, R3, and Rp. In this case it approximately equal to 7 ohms. The proportion 7000 Ohm - 100 %, 53 Ohm, x %, find
Consequently, the temperature fluctuations at the site of installation of the microammeter RA1 at 20 °C the thermometer will change by less than 1 %. But if leave in the scheme of one diode, to save the old device sensitivity have to reduce the total resistance of the resistors and frame three times, therefore, the error will triple.
Graduate thermometer in the following sequence:
- plunging diodes VD1 - VD3 in a vessel with melting ice (snow), trimmer R5 set the arrow of the microammeter RA1 to the zero mark of the scale;
- putting diodes in a vessel of boiling water, a trimming resistor R3 sets the arrow on the mark of 100 °C.
It remains to attach the diodes to the pipe through which flows hot water, as closer to the place where need to control its temperature. Possible option the attachment shown in Fig. 2. So the temperature of the diodes 2 did not differ from the temperature of the pipe 1 and the water in it, the site should be covered with asbestos insulation 3, and atop of it with aluminum foil 4. Wires 5, the length of which is almost not limited to, diodes 2 connected with the rest of the thermometer, which can be placed near the knob of the gas burner, even located in another the room.
Author: V. Burkov, Ivanovo