Auto Fan

"This circuit was designed to automatically activate a set of three or four small DC fans to cool a large cool-rib for a 10 Amp powersupply. Can be used in a variety of other applications as well."

Table 1
Part Description Radio Shack Digi Key Newark Notes
IC1 LM741 Op-Amp 276-007 LM741CN-ND LM741CN NE741,µA741, etc.
Q1 2N2222A transistor 276-2009 2N2222A-ND 2N3904 See text
D1 1N4148 Diode 276-1122 1N4148GICT-ND 1N4001 1N4001, or others
Th1 50K Thermistor KC005T-ND 96F3309 KC005T in prototype
Re1 12V Relay 275-249 Z753-ND 83F8057 RS is 1A
R1 15K, 5% resistor 84N2487 brown-green-orange
R2,R5 10K, 5% resistor 84N2479 brown-black-orange
R3 150K, 5% resistor 84N2485 brown-green-yellow
R4 4K7, 5% resistor 271-1330 50N1628 yellow-purple-red
R6 1K, 5% resistor 271-1321 50N6012 brown-black-red
R7 1K8, 5% resistor brown-gray-red
P1 100K Trimmer Pot Bourns
C1 10uF/25V Capacitor Electrolytic
C2 0.01uF, Capacitor Ceramic
Led Red, 3mm Light Emitting Diode

Replacement Parts:
Q1 = 2N3053, 2N3904, NTE123A, ECG123A, NTE128, ECG128, etc.
D1 = 1N4001, NTE519, ECG519, NTE116 etc.
Th1 = Thermistor, 22K - 100K. Used 50K in prototype.
Re1 = Relay, type 842-1C-C from "Mode Electronics". Order # 50-333-0 ($1.55)
A reed relay will work fine also.

Newark Electronics
Digi-Key
Radio Shack/Tandy

Radio Shack's pittyful selection of parts these days is a real headache.
So I'm no longer gonna waste my time looking for partnumbers. Unless I'm sure
they carry the part. Too bad...

Couple Notes:
Th1, the 50K thermistor, is a standard type. Mine was a bar or rectangular looking thingy. Available from Tandy/Radio-Shack. Almost any type will do. I experimented with different models from 22K to 100K and all worked fine after replacing the trimmer pot and a little bit of tuning.
The one used in the above circuit diagram was a 50K model made by Fenwal (#197-503LAG-A01). This 50K was measured at exactly 25 °C and with 10% tolerance. The resistance increases as the surrounding temperature decreases. Tolerance for my application (cooling a large powersupply coolrib) is 10%. Another name for this thing is 'NTC'. NTC stands for "Negative Temperature Coefficient" which means when the surrounding temperature decreases the resistance of this thermistor will increase. You may have to shop around to get the cheapest price. Some thermistors can be had for as little as $4.00 but as much as $55.00 Canadian currency for the glass encapsulated type (the best).
I replaced my thermistor for a 60K hermetically sealed glass type since the environment for my application may contain corrosive particles which may affect performance on a future date.

P1 is a regular Bourns trimmer and adjusts a wide range of temperatures for this circuit. I used the 10-turn type for a bit finer adjustment but the regular type will work for your application.

R1 is a 'security' resistor just in case the trimmer pot P1 is adjusted all the way to '0' ohms. At which time the thermistor would get the full 12 volt and it will get so hot that it puts blisters on your fingers... :-)
R3 feeds a bit of hysteresis back into the op-amp to eliminate relay 'chatter' when the temperature of the thermistor reaches its threshold point. Depending on your application and the type you use for Q1 and Re1, start with 330K or so and adjust its value downwards until your satisfied. The value of 150K shown in the diagram worked for me. Decreasing the value of R2 means more hysteresis, just don't use more then necessary. Or temporarily use a trimmer pot and read off the value. 120K worked for me.

Transistor Q1 can be a 2N2222(A), 2N3904, NTE123A, ECG123A, etc. Not critical at all. It acts only as a switch for the relay so almost any type will work, as long as it can provide the current needed to activate the relay's coil.

D1, the 1N4148, acts as a spark arrestor when the contacts of the relay open and eliminates false triggering. For my application the 1N4148 was good enough since the tiny relay I used was only 1 amp. However, you can use a large variety of diodes here, my next choice would be a regular purpose 1N4001 or something and should be used if your relay type can handle more then 1 amp.

Circuit digagram, PCB, and Layout are updated with C1, C2, R7, and Led. C1 and C2 are new to eliminate switching noise from the relay in some applications. The optional Led and resistor have been added to the circuit board and layot for your convenience. The relay used here and in the KIT is a small 12V/0.5A type. If the relay rattles increase the feedback resistor R3 to 180K or a little higher, and small ceramic cap over the coil connections in parallel with diode D1. A 1N4148 diode is specified for a small relay. If you use a larger relay used a 1N4001 or 1N4004.