My New PWM (4-wire) Fans
JMC 1225-12HBHA-PWM with control circuit visible in the background.
120x25mm PWM fan
40 – 135CFM
4-wire fans are gaining in popularity and offer several significant advantages over the more common 3-wire variety. With typical 3-wire fans running with a PWM fan controller, the operating voltage is switched off and on to control speed. This presents a problem as power is not on long enough to complete an entire tach cycle. The PWM drive in effect chops the tach signal output an may produce incorrect readings. To obtain an accurate measurement it is necessary to occasionally provide a longer pulse. This technique is known as pulse stretching and can contribute to commutator noise.
In contrast, 4-wire fans provide constant power to the fan. Electronics continue to function at full power even as the drive coils are pulsed resulting in acoustics almost as good as linear (constant dc) power. The component count in circuits used to drive and monitor 4-wire fans is substantially reduced. Driving the coils at rates greater than 20 kHz moves the noise outside of the audible range, so typical PWM fan-drive signals use a rather high frequency (>20 kHz). My circuit consists of a low voltage power supply, a thermistor with an input to a microprocessor's analog to digital converter (ADC) and a built in PWM generator. A “C” program burned into the microprocessor runs continuously sampling the thermistor voltage and setting the PWM duty cycle accordingly. For testing the PWM duty cycle can be varied with a potentiometer. The circuit is very simple and the use of a microprocessor makes changes easy without circuit modifications.
JMC 7015-HBA-PWM with 4-pin connector.
70x15mm PWM fan
Black = GND
Yellow = 12V
Green = Tach
Blue = PWM
4-wire fan control circuit.
I connected a Nexus 120 with a fan-mate to a molex connector. I compared the two fans by running the Nexus at 11.65V ~ 1000rpm, and adjusting the potentiometer of the PWM controller to drive the JMC at roughly the same CFM as the Nexus. I haven't been able to monitor rpm so far and the voltage at the fan is a constant 12V.
I then set the Nexus 120 voltage to 5.00V and adjusted the JMC accordingly. At both settings the noise seemed equal. I noticed small variations in whirl pooling between the relatively flat JMC blades and the curved blades of the Nexus. This is impressive as the JMC PWM fan is rated at 3600rpm, yet weighs no more than a 1600rpm low speed model. PWM fans seem to be offered in only high speed models as they can easily function at low speed with a 10% duty cycle.
This is a broadband noise source generated by air separation from the blade surface and trailing edge. It can be partially controlled by good blade profile design, proper pitch angle and notched or serrated trailing blade edges
The JMC produces noise levels comparable to the Nexus at both 5.00v and 11.65V when creating approximately the same air flow. I attribute this performance to the use of high frequency PWM. Noise is slightly lower that my 3-wire JMC 1225-12L low speed driven through a T-Balancer. The JMC 3-wire fans in my experience do not exhibit PWM howling. It is also worth noting that this fan has a nominal speed of 3600rpm and a current draw of .55a. So far, it does seem that this fan is capable of very low noise levels at low to moderate speeds. Of course anything over 1500rpm will be noisy for a quiet fan. - FG
1. Application of PWM Fan Control by Michael Huang
http://www.coolingzone.com/Guest/News/N ... _2003.html
2. Analog Devices Analog Dialog by Mary Burke Vol. 38, Feb. 2004
http://www.analog.com/library/analogDia ... speed.html