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. This will need to be a high power wirewound type, and make sure that the terminations are properly insulated. I will have to leave it to you to experiment to find a suitable value resistor.Figure 2 - AC Fan ControllerThis circuit is based on the thermo-fan design, but now switches a TRIAC, using an opto isolator specially designed for the purpose. I experimented with simpler circuits, but was highly unimpressed with the results, so in the interests of making a reliable and predictable protection unit, decided on the slightly more complex solution. See Figure 4 for the pinouts for the opto-coupler and TRIAC. To calculate the value of R5, use the for the thermistor version (substitute R5 for R3, otherwise the formula is the same).If You Can Get ThermistorsThe simplest way of all is to use a thermistor, which can be almost anything you can get hold of, but in Australia, none of the popular retail suppliers has any thermistors that I have found, and they are not at all common any more. Pity, because they are very easy to use. If you can get them, Figure 3 is by far the simplest way to control the fans, but it will require some degree of experimentation, since I cannot predict what sort of thermistors may be available where you live.The NTC (Negative Temperature Coefficient) thermistors are RT1 to RT3, but you can add more or use fewer than this as needed. The trimpots make it easy to make fine adjustments for each thermistor. If you can, try to get thermistors with a nominal value of about 10k (at 20 degrees C). A trimpot and diode is needed for each thermistor to prevent interaction between the sensing circuits.Figure 3 - Thermistor VersionI still used the TRIAC isolator / trigger IC, as this is the easiest way to trigger the TRIAC reliably and maintain safety. The thermistors supply the bias to Q1 as the resistance of any one of the thermistors falls with increasing temperature. Q1 turns on Q2, which supplies LED current to the MOC3021.The zener current needs to be about 20mA. Knowing this, R3 can be calculated -R3 = (+Ve - 12) / 0.02 where +Ve is the amplifier supplyAs an example, assume that your amp has +/-50V supply rails. We will also calculate the power rating for R3 -R3 = (50 - 12) / 0.02 = 38 / 0.02 = 1900 Ohms (use 1k8)R3 = V2 / R = 382 / 1800 = 1444 / 1800 = 0.8W (use at least 2W)Electrical WiringElectrical safety is (as always) critical. There should be no track material between the pins of the MOC3021, and a completely bare section of board (whether PCB, perforated or Veroboard) must be left between all low voltage circuits and high voltage circuits. This safety zone must 5mm MINIMUM.Likewise, there should be 5mm minimum between any live (mains) board wiring and chassis.All mains wiring should be shrouded with heatshrink tubing or plastic insulating sheet to ensure that human contact is not possible.The TRIAC and MOC3021 (US readers can use the lower voltage MOC3020) are shown in Figure 4.Figure 4 - MOC3021 and TRIAC ConnectionsThe terminal marked * must not be connected on the MOC3021. Unlike transistors or FETs, TRIACs do not have a sensible designation for the main terminals, and they are referred to simply as MT1 and MT2.AirflowThe direction of airflow is important to ensure maximum cooling. Computers do it the wrong way around, by sucking air across the power supply. This was done for aesthetic reasons, and has nothing to do with efficiency. Someone decided (perhaps not unwisely) that air coming through disk drive slots and other orifices in the computer cases would be annoying to users, so the fans were reversed.If you want to cool a heatsink - blow air onto the surface. This creates turbulence that disrupts the laminar airflow, and allows cooler air to come into direct contact with the surface of the heatsink. If air is sucked past the heatsink, this is nowhere near as effective. If your spoonful of soup is too hot, do you blow or suck air across the spoon? I rest my case
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