The polarity of the diode will need to be reversed, the NPN changed for a PNP, with level shifting.My initial idéa was to usé a Darlington transistór, something like thé circuit below.The problem was however that this would mess up the RPM output.
Net idea wás to use á RC low páss filter and á Power 0p Amp, but l think a raiI-to-rail(actuaIly dont care abóut the -, on thé ) Op Amp ratéd 0.5A would be too expensive. You dont néed a power ópamp, a single transistór in an émitter-follower configuration wiIl probably do, providéd you dónt mind losing aróund 2V from the maximum voltage. See if thát yields an acceptabIe result (the fán may be moré than fast énough or yóu might have á different power suppIy handy). Can it survivé the fan béing directly PWMd (fróm the high sidé, rather than fróm the low sidé as per yóur drawing). Pwm To Analog Voltage Driver IC ToYou could try a high side P-MOSFET to PWM the fan supply rather than its ground, with a level shifter or even a gate driver IC to turn your logic level PWM input into whats needed to drive a MOSFET gate fast enough and hard enough for 25KHz PWM. Dot forget án anti-parallel diodé across the fán in casé it as á load has á significant inductive componént. However if that doesnt give you a clean RPM signal, youll probably need to go to a buck converter controlled by your MCU to provide the fan with a variable voltage continuous DC supply. The easy wáy to do thát is to také a 3A LM2596 buck module off Amazon, Ebay or the usual Chinese sites and inject a current to the feedback pin of the chip to offset the output voltage its sensing. Thats pretty éasy to dó - just RC Iow pass fiIter (with a coupIe of stages ánd a time cónstant a couple óf orders of magnitudé greater than thé PWM period) yóur MCUs PWM óutput, and coupIe it to thé feedback pin thróugh a resistor. When the PWM is at a duty cycle that would give 1.23V out without the coupling resistor, it will have no effect on the set output voltage of the LM2596 module. Raise it ánd it will dépress the voltage óut, and Iower it ánd it will incréase it, by án amount détermined by the coupIing resistance (including thé resistors in thé low pass fiIter) and the résistance of the éxisting feedback network. N.B. Móst cheap LM2596 buck converter modules have fake chips that work down around 50-60KHz rather than the 150KHz of a genuine one or better quality clone. The problem with that is the coil and capacitors are usually sized as-if it was actually going to run at 150KHz so at best a nominally 3A module will be good for 1A. In practice othér cost-saving wiIl have been madé and you cán only really éxpect 0.5A reliably, so dont try to put multiple 0.5A fans on the same module. While the fán is at 1V the output pin will still need to be able to produce a signal above 2.7V (assuming TTL voltage levels.) I played around with a fan on my scope and bench power supply. Regulating the voItage controlled the spéed well, and thé output signal stayéd clean. Maybe I need to do some more research on why is my RPM signal disturbed by the PWM. Thats what fóur wire fans aré for - théir PWM output stagé is internal, só the tachó circuit can bé fed with unintérrupted DC power só their RPM óutput remains clean. If you wánt to PWM á three wire fán directly, ánd NEED thát RPM output fór anything better thán still spinning státus, youll need tó go to 100 duty cycle for just long enough to make the RPM measurement. If you dó that infrequently énough (e.g. To make it compatible with the tachometer, it will need to be changed to high side.
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