Th1, 50K thermistor is a standard type. Mine was a rectangular bar or search thingy. Available from Tandy / Radio Shack. Almost any type will do. I experimented with different models of 22K to 100K
and all worked fine after replacing the trimmer pot. That used in the above diagram has been a model of 50K. The 50K was measured at exactly 25 ° C with a tolerance of 10%. The resistance increases with decreasing temperature. Tolerance for my application (cooling a power coolrib large) is 10%. Another name for this thing is "NTC". NTC stands for "negative temperature coefficient" which means that when the ambient temperature decreases the resistance of the thermistor will increase. I replaced my thermistor tightly to a 60K
type of glass sealed from the environment so that my application may contain corrosive particles that can affect performance at a later date. P1 is a normal lawn mower Bourns and adjusts a wide temperature range for this circuit.
I used the 10-turn type for a little finer adjustment, but the type of regular work for your application.
R1 is a resistance of "security" in case the pot trimmer P1 is set all the way to 0 ohms. When does the thermistor would receive the full 12 volts and it will be so hot that he puts blisters on your fingers ... : R3 feeds some hysteresis in the op-amp to eliminate "chatter" of the relay when the temperature of the thermistor reaches its threshold. Depending on your application and the type you use for Q1 and Re1, start with 330K or more and adjust its value down until your satisfied. The value of 150K shown in the diagram worked for me. Decrease in the value of R2 means more hysteresis, just do not use more then necessary. Or temporarily use a trimmer pot and read the value. 120K worked for me.
The transistor Q1 may be a 2N2222 (A), 2N3904, NTE123A, ECG123A, etc. No criticism at all. It only acts as a switch for the relay so almost any type of work will be, as it can provide the current necessary to activate the relay coil. D1, the 1N4148, acts as a spark arrestor when the relay contacts open and eliminates nuisance tripping. My application for the 1N4148 was quite good, since the relay was little I used only 1 amp.
However, you can use a variety of diodes here, my next choice would be a regular goal 1N4001 or something and should be used if your type of relay can handle more than 1 amp. If you like making your own PCB, try the above. The PCB is provided with holes for the relay, but can not meet your particular relay. It was designed for an aromatic HB1-DC12V.
The variety and the relay model is just great. How to get next? Well, I left a lot of space on the PCB to mount your relay. You can even go up upside down and connect the son individually. Use of the silicone adhesive, cyanoacrylate ester (crazy glue) or double-sided tape to hold the relay in place. Note that the platinum and the provision is inconsistent with the circuit diagram with respect to the connection of fans. The CPC is about 1.5 x 3 inches (4.8 x 7.6 mm) If you print the plate on an inkjet printer, it is probably not to scale. Try to fit a socket 8-pin IC on the printed copy to ensure it is appropriate before making the printed circuit ...
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