V in case of a short circuit in the fan. Integrated or discrete linear regulators Common voltage regulator ICs like the popular LM78xx series are sometimes used to provide variable or constant voltage to fans. When thermally bonded to the computer's chassis, one of these ICs can provide up to 1 A of current at a voltage of 6, 8, 9 or 10 V for the LM7806, LM7808, LM7809 and LM7810, respectively. Adjustable versions like the popular LM317 also exist; when combined with a potentiometer, these adjustable regulators allow the user to vary the fan speed of several fans at currents far in excess of what a standard potentiometer could handle. For higher currents, discrete linear regulators are relatively simple to construct using a power transistor or MOSFET and a small signal transistor or a Zener diode as a voltage reference. While discrete regulators require additional components (a minimum of two transistors, three resistors and a small capacitor), they allow for arbitrarily high currents, allowing for the regulation of additional fans and accessories. As with other linear regulators, the waste heat that is produced will be roughly . Pulse-width modulation Pulse-width modulation (PWM) is a common method of controlling computer fans. A PWM-capable fan is usually connected to a 4-pin connector (pinout: Ground, +12 V, sense, control). The sense pin is used to relay the rotation speed of the fan and the control pin is an open-drain or open-collector output, which requires a pull-up to 5 V or 3.3 V in the fan. Unlike linear voltage regulation, where the fan voltage is proportional to the speed, the fan is driven with a constant supply voltage; the speed control is performed by the fan based on the control signal. The control signal is a square wave operating at 25 kHz, with the duty cycle determining the fan speed. 25 kHz is used to raise the sound of the signal above the range of human hearing; use of a lower frequency could produce an audible hum or whine. Typically a fan can be driven between about 30% and 100% of the rated fan speed, using a signal with up to 100% duty cycle. The exact speed behavior at low control levels (linear, off until a threshold value, or a minimum speed until a threshold) is manufacturer dependent. Many motherboards feature firmware and software that regulates these fans based on processor and computer