Problem with fan control, schematic not working


 
R

RStanislav

New member
Hello! I have project based on Arduino mega where i integrated part of the schematic of heatermeter - fan voltage control part, i'm, using pin 8 of arduino to output PWM, i have checked that it has ~62khz freq and at almost 100% duty over 3.3 volts on pin

photo_2022-04-29_18-15-13.jpg

But for some unknown reason to me on left pin of BS170 i read about 300mv at 90% duty, and at 0% - 4,46 volts, that looks inverted and also doesnt affect fan speed at all - its constant, if i short left pin of FQU11(that has connected to left pin of bs170) with 12 volt line it drops fan speed to zero as expected, i have checked everything like 40 times, i even tried to manually put 3.3v to BS170 gate pin (center), replaced bs170, replaced FQU11, checked all resistors.. no result and i dont understand why its not working, can someone help me please ? Only difference from BOM is that i used https://eu.mouser.com/ProductDetail/EPCOS-TDK/B82477G4224M?qs=/kiEPUfD/qmEI2BmzVrHdw== L1 coil.

This is part of schematic i'm talking about:
photo_2022-04-29_18-15-09.jpg
 
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Yes, it is directly connected to arduino pin8, i have measured signal from arduino with oscilloscope directly connected to gate pin of BS170Untitled-2.jpg
 
I have tried to remove Q3 and without it i still measure about 4.6 volts on drain pin of bs170 with no arduino connected, (so only Q1 and R14 are used in schematic), is it supposed to work like this ?
 
With no input connected to gate of Q1 and Q3 removed, Q1 should be off and drain pin 12v, but if it was left floating it could could pick up stray charge and turn on. Try tying Q3 gate to ground through a resistor and see if it turns off. Is this on a breadboard or a pwb?
 
Is the impedance between the Arduino the same as it is with the Atmega328. That could cause your issue. Is the diode installed correctly and has you checked it to make sure it is not open. What type of scope probe are you using. Is it ac isolated or direct dc coupled. If circuit is ok, you might want to wrap a .01uf cap around scope probe tip and the measure with free lead of cap. I have had issues with scope probes loading circuits down and the cap trick fixed that issue.
 
I have checked diode, its not open and installed correctly (with incorrect orientation it shorts 12v line), tried to tie q3 gate to ground via 22kohm resistor - almost no difference, about 4.5-4.6volts on gate pin of q3 with no arduino connected at all, what i don't understand - with with 3.3v applied to gate of Q1, on gate on Q3 there is almost zero volts ? Its supposed to be 12v right(so fan will be off) ? Can L1 on schematic cause all of this ? Its not BOM (220u, but different impedance) ?
 
I suspect the 3.3 vdc applied to gate of Q1 is turning on FET and that would explain why you see 0 vdc on gate of Q3. I suspect you should be reading across Q3 source diode drop voltage or or something around a volt dc.
 
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