arduinopwm
I have a project in which I need to drive 48VDC 90W fan and control it using it's PWM line.
I've found a number of resources including this, which seems to state that I can directly control the fan using a PWM pin from my microcontroller.
My concern however, is due to the much higher voltage of the fan, I feel like it would be unsafe for the fan to share a common ground with the microcontroller (5v).
What should one do to control such a setup safely?
Your setup is quite different from the standard TLC5940 use, but can be done. The key points are that the TCL5940 is open-drain, and that the actual current and high voltage will be driven by the TIP120.
Logic will be reversed, from the TLC5940's standpoint. Motor will be On, when the TLC Out is OFF. When the OUTx is set to Off, the line will be brought up to 5V through the R Pullup, which also limits the current to 2.4mA (5V - TIP120's 1.4v drop = 3.6V through the resistor, 3.6 / 1500 = 2.4mA). Based on the TIP120's gain, this should (2.4mA * 1000) 2.4 Amps. You can adjust the resistor for the needed gain. When the OUTx is set to On, the line is shorted to ground, turning off the Tip120.
I.E. Setting the TLC5940 to 65% ON, will result in an actual 35% ON, at the motor. Setting it to 25% ON will result in 75% ON at the motor.
By far the lowest component count solution to the problem of simulating the TACH signal from a fan is to use a small microcontroller (MCU) that can measure the PWM input signal duty cycle. The PWM signal can be measured two ways with the MCU either by using a timer to directly measure the high / low pulse widths OR by filtering the PWM via an R/C circuit to produce an analogue level that you read into the MCU via an A/D converter channel. The output TACH signal can be generated easily using an MCU timer that is capable of driving a pin. In fact I have even done TACH signal simulation using a timer that generated a periodic interrupt and then let the MCU software toggle an I/O pin in the interrupt service routine.
Best Answer
Without any special circumstances (such as large currents in the shared ground) sharing a ground with a high voltage is no problem, we do it all the time: we share the ground (literally) with all kinds of high-voltage power lines.