4-Wire Pulse Width Modulation (PWM) Controlled Fans Specification

Electrical Specifications

Figure 1. Screen Shot of PWM Duty Cycle

Figure 1 shows a PWM output operating at 25.3 kHz with a 77.9% duty cycle output with a fan

having a ~ 5V pull up on the PWM signal.

A 100% duty cycle would be a constant 5V signal with this fan and Hardware Monitor device.

2.4 Considerations for Motherboard Designers

1. The trace from PWM output to the fan header must not have a pull up or pull down. The pull

up is located in the fan hub. The presence of a pull up on the motherboard will alter the fan

response to the PWM Duty Cycle. In some cases this may prevent the fan from achieving

full speed even with the Hardware Monitor device issuing a 100% duty cycle.

2. If driving multiple fans with a single PWM output, an open-drain / open collector output

buffer circuit is required. Consult your Hardware Monitor vendor for layout suggestions.