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Topic: Logic Mosfet as another Mosfet driver (Read 1 time) previous topic - next topic

jremington

The latest schematic has exactly the same problem (with Q3) as the very first schematic (with the 2N7000) in this thread.

For some reason, you aren't taking these lessons seriously.

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Seriously, just get a gate driver or get a logic level MOSFET.

LucidWolf

Doh.
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Q3 being an NPN type with 5V at the base will not allow the gate to even reach 5V
Yes sorry there was a typo R3 should have been connected to the 12v line. 

I must have messed it up when i reshuffled everything to fit and then sized resistors (current) based on the faulty diagram and didn't realize it.  So R3 and R4 should have higher resistance.  Also now that I better understand datasheet a 2N3904 wont work because 12v is bad.  Probably a similar transistor out there but its not important.  I think i get the general idea behind a driver, I wont be building it but buying it when i need it.

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For some reason, you aren't taking these lessons seriously.
I am using a logic level MOSFET with very low Qtot at 5V when it gets here in the mail. 

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In my LED lamp i am just going to dim to a certain amount and below that i will just switch off.  The IRLB8721PbFs will act reasonably well (Qtot = 8nC @5V) at the 20mA sourced by the Arduino for LED PWM ~300 ns switching.
I apologize the correct answer was given and I am following it.  I was just trying to get extra info.
I realize you get a lot of very basic questions and I must have exceeded my grace on this one.

If i had followed the online guide without the details I would have probably been fine but not know that at low PMW i would be overly heating the MOSFET etc.  So I still want to thank you all for your help.

Take care.

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