[time-nuts] Re: 20221128: A question re fans and low-noise measurements / techniques

[Lux, Jim]

On 11/28/22 3:02 PM, Andrew Kalman via time-nuts wrote:
> All:
>
> I'm wondering if anyone has some wisdom backed up with experience on the
> best practices for implementing cooling fans inside of low-noise
> instruments. Other than "Don't do it!" :-)
>
> Maybe someone encountered an instrument that had a particularly elegant /
> efficient / quiet fan implementation.
>
> I recently repaired the fan circuit on an SRS SR620 (it uses a PTC to
> control fan speed through two bipolar transistors via a typical dc
> brushless motor fan, PCB had some burnt traces), and in doing so it
> occurred to my (naive) mind that there might be advantages to using such
> "simple", "analog" fan speed control over temperature compared to using
> more modern PWM controllers, from a conducted (and radiated?) noise
> perspective.

You're going to get noise regardless, unless the fan is an AC fan with a 
shaded pole or PSC winding.

Analog control of a DC brushed motor is going to get noise from the 
commutator. The advantage of a brushless is that at least you *might* 
have narrowband interference.

I suppose you could cobble up something that generates clean sine waves 
and drive a stepper or synchro as a "brushless" motor - generate the 
sine waves with a oscillator and phase shift network (or pick the right 
series capacitor to shift the phase - that's the PSC motor approach).

PSC motors can be speed controlled with variable AC voltage (a variac, 
for instance) - I suppose you could work up some sort of light dependent 
resistor scheme.


Tektronix had a very quiet "fan" that was a piece of piezo film that 
flapped back and forth.  I don't know what the EMI signture was, though.

What about a fixed speed blower, with a hose, and variable valve?