hi, can anyone please explain why value of R1 make such a big difference in the gate voltage? it's a switching circuit, when power B2 is on then use that as output and when it's not available than switch to batter power.
is the battery B2 is being shorted to the power (B1?
Look up "voltage divider", which is an extremely important concept in circuit theory.
What are the random diodes supposed to do? Are you just throwing parts into the circuit to see what happens?
If the B2 Source voltage is 15V, and the B1 battery voltage is 8V, you could reduce the battery changeover circuit to a couple of diodes.
I know the voltage divider concept, but didn't able to see it here. thanks
I trying to implement this part of the circuit, i basically combined them.
R1 and R2, in the circuit you first posted, form a voltage divider.
you could reduce the battery changeover circuit to a couple of diodes.
can you please explain? or give an example?
Yeah i figured them out after you mentioned voltage divider.
those diodes are preventing the battery not to short circuit to the power in, but i am not sure if they are working or not.
D3 is q1's didode in the left circuit
D2 is the vbus diode in the left circuit
D1 Zener is there to clamp the voltage on the gate to the max level it can take.
@jremington Which resistor is safe to remove to have proper gate voltage? R1 or R2?
Well, if we only knew which MOSFET you are using...
R2 is required to turn the P-channel MOSFET on.
R1 is required to turn the MOSFET off.
See more if you measure the effective gate-source voltage, not the gate-GND voltage.
The MOSFET can bypass its body diode only if the battery voltage is higher than its threshold + I/S voltage
@JCA34F this one ^^^^^
@DrDiettrich i am running the sim in the KiCAD, i don't see any way to measure the gate-source voltage.
using the mosfet mentioned above this comment. what should i look for in the data sheet? only found the Drain−Source Diode Forward Voltage which is 1.2v
The Gate Threshold Voltage and Forward Transconductance.
The threshold voltage tells when the MOSFET starts conducting, and the Transconductance tells how much additional voltage is required for a given current.
Sorry no.
I was commenting on the circuit diagram you originally posted in post #1. I did not know about the full circuit diagram that you only revealed in post #4
Here's a datasheeet link, I don't have time now to help, good luck.
https://www.onsemi.com/pdf/datasheet/si4435dy-d.pdf
In your circuit in post #1, you have two diodes and a resistor that aren't needed. I think these parts are all you need:
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You could measure the gate voltage and measure the source voltage, both WRT ground and then subtract.
a7
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