PWM Fan controllers

       I had a few of the Nidec hi-CFM fans from gutted projectors and it has a PWM control pin without the need for PWM switching devices to switch the main power input to the fan. Info on 4wire fans can be seen here: 4-Wire Fans and here: Fan PWM Spec sheet.

Nidec fans with PWM control on the right.

       There are two circuits featured here. One is a fixed PWM circuit based on a 555 timer IC and another that is temperature controlled using a TL494.

555 Based schematic.

       First one above is a generic 555 PWM circuit. This version runs at about 21kHz on the prototype. Pot adjusts duty cycle for a Min and Max duty cycle of about 10% and 90% respectively. This version just runs the fan on a reduced speed (which the user sets) and stays there.

TL494 Based schematic.

       Next one above uses a TL494 for the PWM generation and is basically a chopped up version of my DC-DC converter board. The schematic designations are also the same but with a few changes. The Voltage Feedback line, soft start and deadtime controls are disabled and instead of an RC loop compensation circuit, a negative feedback loop (R2) is inserted on the error amp and comp output. This allows the fan to gradually speed up as the temperature increases rather than toggling between stopped and full speed. Increasing the value of R2 increases the rate of change of fan speed vs temperature change.

555 circuit on proto board and installed in my amprack.

TL494 version top side (PWM oscillator section of DC-DC converter board).

TL494 bottom side.

Temp controlled version mounted on amprack.

Page created and copyright R.Quan © 24 Feb 2014.