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PA FAN temperature controlled speed Homemade

Well this PA VLA200 from RM Italy is a nice but also super HOT PA stage.
So I wanted to make a good temperature regulated FAN speed controller,
To keep the temperature as low as possible but also the audio noise as low as possible,
The FAN must be able to stop if the unit is cold enough,

Input  Output Ampere  WattUse  Efficiency
3      50 W   15.7    216 W    23%
6      100 W  21.8    300 W    33%
12     150 W  27.3    376 W    40%
24     200 W  32.2    444 W    45%
35     225 W  34.7    478 W    47%

The most normal FAN speed regulators have a minimum speed.
so they always run and always make noise, even if the unit is cold.

if the curve goes like this to ZERO, then the fan will stop if PA=cold
BUT the internal parts of all brushless fans make a funny noise at voltages lower than running.

so the most optimal curve will be something like this.

Here is the schematic, I found out that R1 22k must be changed to 18k, then the fan stops at a nice almost cold level.
also the FAN is controlled using a regulated constant voltage, much better audio noise performance compared to all me old current mode regulators.
Circuit discription:
When the temperature rise at the NTC, the resistance gets smaller, 25C = 47k
the voltage at C1 will then rise, and when it is over the ON/OFF setpoint at C2 the fan will start to run
the FAN start voltage can be adjusted with R7/R8, the temperature gain to FAN speed is set with R6
R4 sets the on/off hysterise level, it is only a few C, but works fine to prevent the FAN from getting any analog
from 0-4V, if no R4 is mounted, the FAN will get almost analog like levels in thar region, and the FAN will make noise.

If other opamp types are used, you might need to add a series resistor to the FET gate, to prevent oscilation,
not all opamp types can handle capacitive loading at the output, else just use a FET with much lower gate charge.
Any highvoltage low current FET will have a much lower gate charge. Since the FET is used lineary as a voltage regulator,
the internal resistance is uncritical and can be several ohms. If the FAN is big, the FET will get hot,
if it gets too hot, use an isolated type and just glue it onto the heatsink you have anyway :-)

The PCB is made using SMD components, R0 is a 0R 1206 jumper, all other ressistors are 0805 size.
See the PCB layout BIG size

The R9 NTC resistor is glued onto the heatsink directly where one of the power transistors are mounted on the other side,
this will give the fastest fan speed response. Also Epoxy is quite good to transfer heat

Prototype

Prototype close up

Prototype testing

Prototype more testing

Well I did not waste too much brain power figuring out how to mount the FAN :-)
After one night of testing I am convinced that this regulator is my new master piece !!
It really stops and starts and regulate the speed perfectly according to the PA temperature,

This page is updated June 2005