Home
Q) Pgs. 9 and 10: resistor #3. By enlarging Page 10, it appears this resistor is 220 ohms, 20 watts. Is this correct?
A) In a typical unit which runs well at low speed, I verified it is a 10 W, 390 ohms resistor in series with the coil to get the lower speed. The center contact of the switch is going to the printed circuit (blue cable) and the other blue cable goes, via one terminal of the resistor 3, to one end of the motor coil, the white cable goes from the printed board to the thermal switch on the motor, and one black cable from the power cord goes to the printed board too. Hope it is clear so.
Q) Pg 17: Schematic does not show switch or resistor in the circuit. I can guess how these are connected, but it would be nice to have the specific diagram.
A) Indeed, you are absolutely right, a further connection diagram representing the external connections to/from the control printed circuit, and incorporating also the switch, the power resistor 3 and the thermal fuse, had to be joined to the manual, where this info is not sufficiently clear. Sorry... you can guide yourself with the pictures on the pages 25 to 27, and 44.
Q) Nomenclature issues -- "NTC 1" -- I believe this is a 1/2 watt trimmer. 15S200L indicates a wire wound potentiometer, very costly. Will a cermet trimmer work as well? Also, the resistance value is what? 200 ohms? I'm not clear about its function. Looks like a current limiter to the optical emitter, but a fixed resistor would fulfill that purpose, so what does this variable resistor control?
A) The NTC1 resistor, type 15 S200L or equivalent is a Negative Temperature Coefficient whose purpose is to limit the inrush current when closing the switch. Its cold resistance value is about 30 ohms and in the steady state condition it has about 5 ohms, depending on the ambient temperature. IT IS NOT A 1/2 W trimmer. If this type number corresponds also to a wire wound potentiometer, it is pure coincidence, sorry. From the oriental manufacturer we used in the prototypes, that type number designates a NTC resistor.
A fixed value resistor would not be optimum, as to be able to limit the inrush current, it had to be in the 20...30 ohms range, and this would lead to an unnecessary power dissipation and a reduced max. speed.
NOTE - CORRECTION: Due to the lack of rated power, not indicated in the data sheet, it had to be calculated. The power dissipation of that NTC can reach up to 4 W, because it must be calculated this way:
P = I^2 * R (square of the current, times the resistance) this yields at Imax = 4A, 16 times 0.248 = 4 W. this means about 150 degrees C elevation, very hard for the size of this component. Somewhat optimistic as rated power. This would be an extreme case, we are running with a much lower RMS current in this NTC resistor, about 1 A typically, but the resistance of the NTC is also higher, typically 1 ohm at that condition, this means about 1 W leading to what I said, about 39 degrees elevation.
Please download the NTC THERMISTOR JNR data sheet here: http://tinyurl.com/n2ng6me
Q) The UF 5807 is hard to find in the US -- I assume it's the same as 1N 5807? 50 Volt, 3 Amp, fast recovery?
A) 1N5807 is a 50 V fast diode and this number is not correct, sorry, it came from a confusion with 1N5407 (3A, 800 V diode). This would be the right diode for 220-240 V mains operation. For 115 or 127 V, a 1N5406 (3A, 600 V) is sufficient. The purpose of this diode is to isolate part of the control circuit from the back voltage induced in the motor coil by the rotating magnet during part of the cycle.
Q) PHCT 203 optical switch is difficult to get in the US. There are any number of nice optical switches available, could you suggest a substitute, or list any critical parameters?
A) OK, this optical switch is not critical, just a transfer ratio larger than 10, preferably between 15 and 20 @ 20 mA LED current, and phototransistor voltage of 25 V minimum, would be perfect. I must look for an equivalent number. Please take care with the pins location, an electrical equivalent may not be a pin for pin replacement.
Q) Pg 45: Timing angle is illustrated but not specified. Is there a recommended angle?
A) An approximate optimum is to have the leading edge of the shutter (blinds) entering in the optical sensor at an angle of about 15 to 20 degrees after the magnet axis aligning the coil axis, so that the trailing edge of the shutter will go out the sensor about 40 degrees before the next magnet-coil axis alignment. But the best is to adjust for an optimum (max speed) experimentally, then gluing the shutter.
Q) The first, most important question is, why would you want your fan to be slower? R3 simply absorbs some of the power that would otherwise go to the stator coil, and converts this energy into heat -- not, presumably, the goal. I'm very puzzled!
A) The reason to offer a reduced speed is to comply with noise requirements (bedrooms, hospitals, etc) where silence is of paramount importance and where just a slight breeze is frequently sufficient. We developed also a continuously adjustable speed control, but it is electronically more complicated and not commercialized yet. For the sake of simplicity we adopted that series resistor #3 as a trade-off, even conscious it is not that efficient on the lower speed.
Q) Thank you for the clarification re: the NTC resistor. Makes perfect sense. What is its wattage rating?
A) The wattage must be 4 A (max) x 0.248 (ohms @ Imax) = 1 Watt. This seems consistent with 1 W / 26 (mW/degree) = about 39 degrees temp. elevation above ambient.
Q) Also, the 600 V diode's purpose is clear, thanks.
One other minor question, the thermal fuse inside the winding. In a commercial product I understand why this is necessary -- safety and fire prevention. But, if it's a fuse, one over-temp would require re-winding the motor. Or is this a circuit breaker that will re-set itself when temperature lowers?
A) Normally, such a fuse should automatically re-close the circuit when the temperature decreases, this is not 100% reliable though, for such a low cost component. Now in case of short circuit, it normally opens forever. But it is held in a sleeve under a few wire turns of the coil, so that it should be relatively easily replaced, without to rewind the motor, if the coil did not burn.
