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. Also, HP has/had many instruments that just run AC fans at full blast, irrespective of any temperatures that might be sensed. That's great, except it's killing my hearing. So, I guess I'm wondering whether there are any general statements about best practices (other than "Do lots of filtering") if one needs to implement a fan in a low-electrical-noise instrument. The main fan schemes I can think of are: - AC fan (full speed, all the time). - PTC or similar temperature control over brushless fan voltage. - PWM-based fan driver with sensed temperature affecting PWM duty cycle. Thanks, --Andrew -------------------------------- Andrew E. Kalman, Ph.D.

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?