volt.txt

Voltmeter measuring from 0- 5Vdc

Theory behind calculation of Voltage divider circuit.
All we want is input voltage to ADC should not increase 5V and 
Our maximum I/p Voltage to voltmeter is 15V only. So we design
the voltage divider circuit as follows.

where Vmax is Maximum i/p voltage to voltmeter.
Vip is i/p voltage to ADC.
R1 and R2 are resistance of voltage divider circuit.

    Vmax
    ---
     |
     \
     / R1
     \
     /
     |___ I/p (Vip)
     |
     \
     / R2
     \
     /
     |
     |
     V
    GND

Vmax = 15V

V = R2*Vmax/(R1+R2)

5/15 = R2/R1+R2

3 = R1/R2+1

2 = R1/R2

R2 = R1/2

Lets take R1 as 200K
and R2 will be 100K

Maximum current: Imax = (Vmax-Vipmax)/R1 (approx)

Vmax = 15V
Vipmax = 5V
R1 = 200K

Imax = (15-5)/200 = 10/200 = 0.02mA