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G8MNY  > TECH     18.08.18 08:30l 111 Lines 4564 Bytes #8 (0) @ WW
BID : 34125_GB7CIP
Subj: PA & PSU Fan Noise
Sent: 180818/0627Z @:GB7CIP.#32.GBR.EURO #:34125 [Caterham Surrey GBR]

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By G8MNY                                 (Updated May 15)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

I have often noticed fan noise from PSUs & powerful RF PAs, this can be quite
distracting on Rx & the fan is normally not necessary in Rx mode.

Quiet fans are made & usually have feathered slots in the blade leading edges.

 normal             Quiet
 _.--._           _.--._
|       |         |      =<
 \     /Leading    \    =< Deep
  \ _ / Edge        \ _=< V Slots
  /~ ~\             /~ ~\

There is little loss in efficiency, but nearby stationary support bars now
produce a softer blurred sound pulse considerably reducing the blade pitch

Permanently reducing the fan seed will reduce the noise, & for some over cooled
systems this works well. But for applications where maximum cooling is
occasionally needed a variable speed solution is appropriate.

There are several options.
1/ Underrun the fan in Rx mode.
   This needs a series R that is shorted out when on Tx. This can be done with
   a relay contact on a mains fan, & to keep the R power down the mains fan
   should be Power Factor tuned with a mains C, or use a lower mains voltage.

    LĿ    +12VĿ
     === Fan     Ŀ __         LĿ230V
     C---      Relay /_\            ||)____ \ 
          Ŀ          OR      ||)150V  
          R   /                  ||)     Fan
    N     PTT               N

2/ Temperature sense.
   By adding a thermal switch a DC or mains fan can be kept off, or running
   slow most of the time.

           ON OFF
    LĿ  MODE              SPEED UP     LĿ
        Fan                     MODE           Fan
          /Thermal                              R   /Thermal
    N Switch                          N Switch
       (With suppression)

3/ Variable controlled DC a fan.   
   This offers the quietest fan control.

                 FAN ===100u
                       e\_Old PNP
                   \    / Germanium
                       In heatsink
             0V  NPN

   If an old germanium PNP is not available, it can be a suitable NTC
   thermistor & the NPN can have a hold off R base to emitter to set the
   operation temperature. The cap is needed on DC fans to handle the pulse
   currents & make the motor start better. A permanent quiet run R can be put
   across the NPN if wanted.

Feedback from Osvaldo LW1DSE..
In place of a resistor to drop the voltage for the AC motor fan, I used a 1F
275VAC, keeping in mind that this cap not resonates with the motor inductance.
I used 1F, but any other cap will do the job, and increasing its value,
reduces the capacitive reactance (Xc = 1/ 2 * F * C) and then increases motor
speed. If this cap have infinite value, the motor will run at normal direct
speed. And it doesn't discipates heat!

For DC motors, the solution is simpler. There exists a device called LM35 from
National Semiconductor in  TO92 case, the LM35 that is a thermal sensor
directly calibrated inside the chip to give an output of 10mV/C, then, at 30C
it will give about 300mV. Then, letting this voltage to a comparator like LM311
& putting the wiper of a potentiometer in the other input as voltage
Temperature reference, & giving some hysteresis using a positive feedback, you
can do a precise thermal control, in the form of an on/off circuit. As the
power consumption of a normal fan is in order of tens of mA, the internal LM311
transistor is sufficient to drive this motor directly, the most of them being
brushless, & then the freeweeling diode is unnecesary. Or, you can feed this
voltage to the input of a PWM controller like one of the voltage mode PWM IC's,
& do a PWM analog speed control with low power drain.

See my TECH Buls "Variable Speed Fan for AF PA" & "Variable Speed Thermal Fan"

Why Don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP

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