I'm trying to understand the automatic power source selector of the Arduino boards but can't get my head wrapped around it. I've rearranged the circuit and have come up with the schematic below. I understand that IC7B is a opamp used as a comparator to switch the FET (T1) either on or off depending whether USB power is used or external power is used.
The thing is, the positive input for the comparator is the external source applied through a voltage divider and the negative input for the comparator is the 3V3 output from the 3V3 linear voltage regulator as I understand it. This makes sense since if your using USB power, then there will be 3.3V on the negative leg of the comparator and nothing on the positive leg, and hence the FET will be switched on. But the thing is, the 3V3 regulator is situated after the FET, so when the system is first powered up, how does the opamp get the 3V3 input to switch on the FET when connected to USB power only?
The internal diode inside T1 becomes forward biased and supplies the needed voltage to [IC6] to get the 3.3 regulator working.
IC7B comparator keeps the fet on all the time unless Vin power is plugged in ,
When VIN is plugged in , T1 turns OFF and allows the 5V from IC1 (NCP1117STSDT39) 5V regulator to pass to supply the 5V for the board (and IC6 , 3.3v Regulator)
The 3.3V regulator IC6 (LP2985-33DBVR) ( see attached datasheet) is powered from the 5V from one of the two above sources. Only one of the above is allowed to supply 5V due to the way it is designed (the explanation above). When nothing is plugged into the VIN, there is no voltage on the gate of T1
P-channel FET, (see attached datasheet), consequently it is ON, and the USBVCC passes to IC6 3.3V regulator. When voltage is applied to VIN, the comparator turns off T1, blocking USBVCC1. For understanding operation of Comparator see this:
Vo=0, if V+<V-
FET IS ON for 0V.
@OP,
At some point , maybe now , maybe later , maybe never, you might wonder "if the +5V from USBVCC1 can pass freely through the internal diode, then how can the comparator block the voltage when power is plugged into VIN ?
The answer is that when power is plugged into VIN there is 5V coming from IC1 5v regulator on the CATHODE of T1 diode. If the diode has 5V on it's CATHODE and you apply 5V to it's ANODE , WHAT HAPPENS ? Answer, NOTHING , because the diode has the same voltage on both terminals and no current flows through the diode, thereby blocking USBVCC1 by not conducting. (thanks Larry)
All I was stating was the process starts with current flowing trough the internal T1 diode then going to the 3.3 volt regulator.
I believe that was what the OP was asking about.
Thanks for all the info. I went through the FET datasheet and I wasn't aware of the internal diode.
@ Raschemmel
You seem to be a few steps ahead of me:
At some point , maybe now , maybe later , maybe never, you might wonder "if the +5V from USBVCC1 can pass freely through the internal diode, then how can the comparator block the voltage when power is plugged into VIN ?
The answer is that when power is plugged into VIN there is 5V coming from IC1 5v regulator on the CATHODE of T1 diode. If the diode has 5V on it's CATHODE and you apply 5V to it's ANODE , WHAT HAPPENS ? Answer, NOTHING , because the diode has the same voltage on both terminals and no current flows through the diode, thereby blocking USBVCC1 by not conducting. (thanks Larry)
Thanks for that explanation as well, it is much appreciated. Thanks to both of you gentlemen I now know a bit more about FETs and am very thankful for it.
Thanks for that explanation as well, it is much appreciated. Thanks to both of you gentlemen I now know a bit more about FETs and am very thankful for it.
Most of the really usefull applicaitions for fets (or IGBTs for that matter) is not in knowing how to use ONE but knowing how to use MANY. An RC ESC (Electronic Speed Control) that can deliver more than 100A @34V, is a stack of pcbs with dozens of little tiny fets on it each with a very low internal resistance. Larger parallel fet (or IGBT) circuits can deliver hundreds of kw of power.
It's probably worth your while to get a handful of fets of different sizes off ebay and just play around with them, switching lights, relays, motors, piezo alarms, laser diodes, or anything you can think of. Remember to do the math first XD.
