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Maxim > Design Support > Technical Documents > Application Notes > Temperature Sensors and Thermal Management > APP 1814
Keywords: fan speed control, isolated -48V fans, fan speed regulator, IC, integrated circuits, fan control, speed
control
APPLICATION NOTE 1814
Speed Control for Isolated -48V Fans
By: Jim Christensen, Strategic Applications Engineer
Dec 10, 2002
Abstract: A circuit is shown that allows a MAX6650 fan speed regulator IC to control a high-voltage fan on the
other side of an isolation barrier.
Chips are available to monitor and control ventilation fans. Less common are fan controllers for telecom
applications, which must operate in electrically isolated environments with a supply voltage in the range 36V
to 72V. As an example, the MAX6650 has the numerous control and monitor functions useful to system
designers, but it resides on the secondary (low-voltage) side of the isolation barrier, and thus lacks the pulse
width modulated (PWM) output often used to control high voltage fans.
It is possible to place a MAX6650 on the high-voltage side of the barrier and translate bidirectional I²C signals
across to the low-voltage side, but an easier alternative is to keep the MAX6650 on the low-voltage side and
use opto-isolators to translate two unidirectional signals (control and tachometer pulses) across the barrier
(Figure 1).
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