Hi everyone,

Can anyone tell me the formula to be used in order to measure PANEL VOLTAGE "P_V" across R28.

The schematic can be found in the attachment.

We have two ground here :
1st is PANEL Ground indicated by P- and 2nd is system ground which is marked as a general GND symbol.
Here Panel Gnd and system ground is separated by a MOSFET which is not shown in the schematic. However to measure Panel voltage there is NO requirement of MOSFET.

Basically i want to know as what conversion factor i should use to convert ADC reading to actual panel voltage (ADC is of 10 bit) which is NOT possible without the actual calculation.
Normally we use a resistor divider configuration for voltage measurement which is quite simple BUT what in case if BJT (PNP) is inserted in between.


Thank you

 

You will get something crazy like this:

Neglecting the base current the base voltage at Q6 (to P+, not ground) is approximately

Vb6 = Pv*R17/(R26+R17)

The the voltage across R18

Vr28 = Vb6 - Vbe6

The current thru R18 is approximately the current thru R28

Ir28 = Ir18 = Vr28 / R18

Thus the voltage P_V:

P_V = Ir28 * R28

I leave it as your exercise to substitute all these terms into the final to get all the dependencies, though I do first recommend a redesign.

 

You will get something crazy like this:

Neglecting the base current the base voltage at Q6 (to P+, not ground) is approximately

Vb6 = Pv*R17/(R26+R17)

The the voltage across R18

Vr28 = Vb6 - Vbe6

The current thru R18 is approximately the current thru R28

Ir28 = Ir18 = Vr28 / R18

Thus the voltage P_V:

P_V = Ir28 * R28

I leave it as your exercise to substitute all these terms into the final to get all the dependencies, though I do first recommend a redesign.

Hi,
Thank you for your support.
I got the solution and is matching with the practical result too. I used a different equation though but your explanation guided me to proceed in the other way.

My equation:
P(+VE) - Ie*R18 - 0.7 = Vb -----(eq 1)
Vb = R26/(R26 + R17) ------------(eq 2)
P_V = Ie*R28 ------------------------(eq 3)
Using eq 1 and eq 2, i got Ie, then i substitute in eq3 to get P_V

Thank you for your support..!!