DC Fan - A proportional controller

DC Fan Controller Takes Bare Bones Approach

Alan Stummer

Fans can be necessary for circuit cooling, but continually running a fan at a rated voltage will cause mechanical failure, usually in the bushings or bearings. By running it only when and as fast as needed, you can greatly extend its useful life as well as the life of the equipment it is cooling.

A simple on/off fan control will work, but it can cause switching transients and similar problems as the fan cycles. A proportional controller that starts the fan when circuit temperature passes a threshold, speeds up the fan as the temperature rises, and slows down and stops the fan when the circuit cools down is more elegant.

Most proportional fan speed controllers are far fancier than required, though, because circuit cooling isn’t really a precise science. This simple circuit is just as effective as a fancy design and has been used many times with great success (Fig. 1). It requires only a thermistor for temperature sensing, a FET to control the fan, one pull-down resistor, and a bypass capacitor to keep the purists happy. It assumes the thermistor is the more common negative temperature coefficient (NTC) type. If you wish to use a positive temperature coefficient (PTC), then reverse the thermistor and pull-up resistor.

Figure 1.	A thermistor and FET provide a bare-bones proportional controller for a dc cooling fan.

At room temperatures, the FET gate voltage is below its typical threshold value V_gs(th), so the drain current is zero and the fan is off. As the temperature rises, the thermistor’s resistance drops, causing the gate voltage to rise above V_gs(th), and the FET begins to conduct. At still higher temperatures, the FET saturates and the fan runs at full speed. In practice, the temperature has to increase approximately 5°C for the fan to go from off to fully on.

The pull-down resistor R1 determines the threshold temperature at which the fan starts running. For example, ON Semiconductor’s NTD4959NH FET in a D-Pak package has a gate threshold of 2.0 ±0.5 V.Panasonic’s ERTJ1VR103H thermistor is in a 0603 (metric 1608) surface-mount (SMT) package and is nominally 10 kΩ at 25°C. To set the threshold temperature at 40°C, with a +12-V bias from the fan supply, the resistor would be:

R1 = R_Thermistor × V_gs(th)/(12 V – V_gs(th))

Using 2.0 V as the typical value of V_gs(th) and (from the data sheet) R_Thermistor at 40°C = 5.067 kΩ, you get R1 = 1.00 kΩ to the nearest standard 1% value.

With a fixed resistor, there will be some production variations in the actual threshold temperature because of variability in V_gs(th). For low production volumes, you could replace the resistor with a trimming potentiometer to allow adjustment of the threshold temperature. But for lowest cost, you may have to simply tolerate the variations.

Serendipitously, N-channel MOSFETs have gate thresholds with a negative temperature coefficient, which will help to mitigate the effects of V_gs(th) variability. Still, to ensure this circuit will work, you need to check that the temperature threshold range is acceptable in your system.

Working backwards with the extreme upper and lower values for V_gs(th) from the datasheet provides the worst case temperature thresholds:

V_gs(th)min = 1.5 V and R1 = 1.00 kΩ

So, the fan starts when:

R_Thermistor = 1.00 kΩ × (12 V – 1.5 V)/1.5 V = 7.00 kΩ

which occurs at a temperature of 33°C according to the datasheet. Similarly, the highest threshold temperature would occur when the thermistor is 3.80 kΩ at 46°C. Because most MOSFETs will have gate thresholds in the middle of the datasheet range, a production threshold temperature of 40°C ±3°C is a reasonable expectation.

A few things to consider: First, this design applies only to dc “muffin” type fans. It would be inefficient for large fans or arrays of fans and could not work for ac fans. Further, it requires fans that can automatically restart their movement. Most fans can, but be sure to verify with the datasheet. If your design requires the fan to be always running at some minimum speed, bypass the FET's drain to source with a resistor.

Also, realize that the FET may dissipate a significant amount of power when it is when operating in its linear region where the fan is not at full speed. Because this occurs when the fan is running, however, placing the FET in the fan’s airflow handily solves the problem.

Fanii pot fi necesare pentru răcire de circuit, dar care rulează în permanenţă un fan la o tensiune nominală va provoca insuficienţă mecanice, de obicei, în bucse sau rulmenti. Prin rularea doar atunci cand si cat de repede este necesar, se poate prelungi foarte multdurata de viaţă utilă, precum şi durata de viaţă a echipamentelor ITeste de răcire.
Un simplu on / off de control al ventilatorului va funcţiona, daraceasta poate provoca tranzitorii de comutaţie şi probleme similareşi ciclurile de ventilator. Un controler proporţional, care începeventilatorul atunci când temperatura de circuit trece un prag, viteze de până ventilator ar fi temperatura creşte, şi încetineşte şi se opreşte ventilatorul atunci când circuitul se raceste este mult mai elegant.
Controlorilor de cea mai proporţională, viteza ventilatorului sunt multcrescator decât este necesar, deşi, pentru că circuitul de răcire nueste cu adevărat o ştiinţă exactă. Acest circuit simplu este la fel deeficace ca un design de lux si a fost folosit de mai multe ori cu mare succes (fig. 1). Este nevoie de doar un termistor pentru senzor de temperatură, un FET pentru a controla ventilator, un rezistor pull-down, şi un condensator by-pass pentru a menţine puristii fericit.Aceasta presupune termistor este mult mai comun negativcoeficientul de temperatură (NTC) de tip. Dacă doriţi să utilizaţi un coeficient de temperatură pozitiv (PTC), atunci inversa termistoruluişi trageţi-up rezistor.

Use a PWM Fan Controller in an EMI-Susceptible Circuit

Microchip Technology offers a family of cooling-fan speed controllers that operate in PWM mode for use with brushless dc fans (Reference 1). To control fan speed using the PWM waveform's duty cycle, you can use either an external NTC (negative-temperature-coefficient) thermistor or one of Microchip's PIC microcontrollers and its SMBus serial-data bus. Figure 1 illustrates a typical application that the data sheet describes for the TC664 and TC665 controllers (Reference 2). Using a frequency-control capacitor, CF, with a value of 1 µF, fan-controller IC1 generates a PWM pulse train with a nominal frequency of 30 Hz and a temperature- or command-dependent duty cycle that varies from 30 to 100%. 

Although using the controller in PWM mode reduces power dissipation in transistor QA, which drives the fan, the 100-mA, square-wave motor-drive current can cause unwanted interference in a nearby high-sensitivity audio circuit. The circuit in Figure 2 solves the problem. An additional driver transistor, Q1, and an RC network comprising C3 and R3 form a simple PWM-to-linear converter. You can also use another PWM-to-linear-conversion circuit, such as an integrator based on an operational amplifier. 

Figure 3 shows a graph of the dc voltage at Q2's collector versus IC1's PWM drive-output waveform's duty cycle. The voltage applied to the fan corresponds to the difference between Q2's collector voltage and the 12 V supply voltage. Even though a steady voltage appears across the fan, current pulses that the fan motor's commutation produces still develop a voltage across current-sense resistor RSENSE that connect to Q2's emitter, and all of IC1's protective and advisory features remain available. 

The listed component values are valid for a 100-mA, 12 V, brushless fan. Use a general-purpose NPN transistor such as the 2N2222 for driver-transistor Q1 and an NPN transistor, such as Fairchild Semiconductor's PZT2222A, that can dissipate one-third of the fan's maximum power consumption for Q2. Note that you can vary the PWM's nominal frequency over a range of 15 to 35 Hz by altering the value of CF.