PWM to analogue filtering - Page 1
Forum section: electronics / beginners

author: Simon
author_rank: Unknown
timestamp: September 20, 2015, 06:39:22 pm
content: I have devised a circuit to send a signal to a fan to tell the fan how fast to run.Attached is a diagram of the input circuitry. This is obviously simply converting the PWM into an analog voltage in fact these fans could be controlled off a potentiometer. Another page of the data sheet shows that the input control is from a highside switch. But that leaves me wondering, if I am low pass filtering a PWM signal and the input impedance of the circuit reading the voltage on the capacitor is very high how will the capacitor ever discharge so it would just charge higher and higher with each pulse. Isn't the correct way to drive such a filter with a totem pole output so that there is a time when the capacitor is charged and a time when it is discharged thus providing proportional control over the voltage on the capacitor unless of course there is a known load on the capacitor matched to the input resistance in the filter.The datasheet also states that an input voltage of 12 V is considered high of course this is clamped anyway to 10 V by a Zener and that an input voltage of less than 1.25 V is considered low this kind of implies a totem pole drive does it not? The problem I have is that the data sheets from this company are notorious for being ambiguous and incorrect on such matters. So I do not really trust a diagram showing a highside switch in the customers controller.So how does low pass filtering a PWM into an analog voltage work? This is something often done straight out of a microcontroller pin which of course has a totem pole drive. So that leads me to believe I should be providing a totem pole output rather than a single highside transistor. I can provide either as I already have a dual transistor package in place with a resistor on each transistor to connected to the output so I can place or remove the resistors to get highside drive, low side drive or a totem pole drive.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 20, 2015, 10:51:58 pm
content: If you had an open collector (PNP or NPN, or FET drain as the case may be) output, you would need the complementary pull-up/down, yes.If the data is vague, don't support them, buy one that is well documented!Or if you must use it, qualify it by doing that potentiometer test -- read it with a voltmeter and RPM counter, and measure the curve. Then you will know unequivocally what gain it has. (You won't know how that gain and offset varies with temperature or supply, or how it varies across other parts by manufacturing variation. That requires more work...)Tim

author: Simon
author_rank: Unknown
timestamp: September 21, 2015, 06:37:13 pm
content: Unfortunately i don't have a sample, I am getting customers passed to me by the manufacturer so don't have much choice in which fan I design for. They are good fans but some information gets lost in translation and sometimes they just seem bad at writing datasheets, make a technical expert like me all the more valuable.I already have a totem pole output so will leave it as it is, I could remove a resistor to disable the high or the low side.