tend to come with no regulation. When powered
from their nominal voltage, they provide high airflow, but also high noise.
Cooling fans tend to come with no regulation. When powered
from their nominal voltage, they provide high airflow, but also high noise.
Even modest airflow rate is often adequate for sufficient cooling. The fan can be slowed down with e.g. a suitably dimensioned series resistor (usually 10-100Ω for 12V fans).
This is however still not ideal. An active regulation with fan speed and therefore airflow dependent on the actual cooling need can provide superior quietness when little cooling is needed, and full airflow when necessary.
A linear regulator can be used in variable output voltage configuration.
The fixed voltage (1.2 volts typically for adjustable versions, 3.3V can also
be used) is between the "ground" pin and the output. If the "ground" is raised
above the load's ground, the output voltage rises correspondingly. This raise
can be done with diodes, LEDs, a Zener diode, or a resistor divider.
The resistor divider approach was chosen, with a thermistor in the
divider. A 100k NTC resistor, a little glass bead used commonly for 3d printer
heads, was chosen on the basis of size, cost and availability.
 schematics |  board layout |  circuitboard with sensor |  circuitboard with sensor |
A HT7333 chip, a linear low-drop 3.3V regulator, was chosen for its ability to go to 12 volts on input and ability to source 250 milliamps. This should be sufficient for modest sized fans. With rising output voltage the fan's power input current rises. The voltage across the regulator however decreases, which keeps the power dissipated on the regulator in check.
Other chips can be chosen, e.g. LM1117 or LM317.
The minimal output voltage is the sum of the regulator's own output voltage plus the voltage across the bottom resistor of the divider with the maximum applicable (usually room temperature) resistance of the thermistor. For a 3.3V regulator the practical room temperature output voltage is about 4 volts. A 1.2V regulator, LM1117-ADJ, is intended to be tested.
An optional resistor can be attached in parallel to the thermistor, to adjust the desired temperature-airflow characteristics.
Tests shown that usual computer cooling fans often start spinning at voltage as low as 3 to 3.5 volts. The R2 resistor is omitted, R1 of 22kΩ or 33kΩ can be used. The higher R1, the higher the room temp output voltage and the higher the temperature of maximum output voltage.
The voltage for the fan stopping is lower than for the fan starting, as for spinup the initial mechanical resistance of the rotor bearings has to be overcome.
With 3.3V regulator, the room temp output voltage can be too high (above 4 volts). Ordinary silicon diodes, in series with the fan, can be used to lower the voltage, about 0.7 volts per diode; 1N4007 diodes were used.
The thermistor is attached to the board with a pair of enameled wires.
Two regulator boards were assembled. One was attached to a 80x80mm fan assigned to the 3d printer's stepper drivers, with R1=22kΩ and two series diodes. The other, with R1=33kΩ and three series diodes, was attached to the 3d printer's main head.
The thermistor for the stepper drivers was pushed in between the fins of the Z stepper heatsink.
The temperatures for the stepper weren't measured. The fan is stopped at room temperature. On enabling the steppers, the heatsinks start heating up and the fan kicks in rapidly and silently; the fan rotor first starts subtly twitching, twitching intensifies, then the rotor spins up. On disabling the steppers, the fan continues running until the heatsink temperature drops sufficiently for the fan to stop again.
The thermistor for the head was placed in the middle of the aluminium heatsink.
The printer head was heated to a range of temperatures. Time was provided for the equilibriation of temperatures. The tests were performed at room temperature of about 24°C. Measurements of temperature were made with a K-type thermocouple on a multimeter.
At room temperature, the fan is not spinning. The fan spins up at heatsink temperature of 39°C, and cools it down to 37°C; the temperature then rises again to the equilibrium point.
On cooling, the fan stops at heatsink temperature of about 28-29°C.
The printer head was set to temperature range of 200°C to 300°C, in 25°C intervals. The equilibrium temperatures were:
This indicates a nicely linear dependence of 1.4°C per each 25°C of head temperature, or 0.056°C per °C of head.
The performance of the regulator, especially in relation to the parts cost, is astonishingly good and markedly exceeed expectations.
 board on 3d printer head |  board on 3d printer head |  sensor position on 3d printer head |  sensor position on 3d printer head |
The variables are:
All resistance values can be in kiloohms to avoid unnecessary zeroes.
The high-side divider resistance is
Rt = 1/(1/R2+1/Rtherm) Rt = Rtherm
The output voltage is
Vout = Vref * ( Rt + R1 ) / R1A calculation script, using qalc, was written to facilitate automatic what-if calculations.
- calculating script