In a **circuit diagram,** we can see a number of **Resistors** connected in **series** or **parallel** between the positive and negative rails. These resistors are controlling the voltage and current to the particular sections of the circuit. When we design a circuit, we should have an idea about the voltage and current requirements for each section of the circuit. Then only we can select the proper values of resistors. If you know this, you can easily answer the questions in Project Viva regarding the selection of resistors.

First understand the Series and Parallel resistors. Series resistors means two or more resistors connected in series. This mainly appears as potential dividers in comparators, transistor amplifiers, oscillators etc. Parallel resistors mean two or more resistors connected parallel between the positive and negative rails.

**Voltage across the Series resistor chain**

See image. Two resistors **R1** and **R2** are connected in series across the** 12 volt **supply. So the total resistance is

**R1 + R2 = 10,000 Ohms ( 10K) + 1000 Ohms ( 1K) = 11,000 Ohms**

The relationship between Current, Resistance and Voltage as per Ohms law is Current **I = V / R , IR = V, V = IR**

** Where I is the current in Amperes, V is the input voltage and R is the resistance in Ohms.**

Now use basic algebra to change Ohms law to solve for voltage instead of Current.

**I = V/R , IR = VR / R , IR = V, V= IR**

**So current I through R1 + R2 is = V/R = 12 V / 11, 000 Ohms**

= **0.001,090,91 Amps**

**Voltage Drop across R1 and R2**

We know **Current** and** Resistance**.

Current is **0.001,090,91 Amps**

Resistance is **11, 000 Ohms**

So Voltage **V = I x R**

**Voltage drop across R1= V1 = 0.001,090,91 Amps x 1,000 Ohms ( 10K) = 10. 9091 V**

** Voltage drop across R2 = V2 = = 0.001,090,91 Amps x 1000 Ohms ( 1K) = 1.09091**

** So total voltage drop V1+V2 across R1 and R2 is**

**10.9091 + 1.09091 **= **12 Volts.** That is equal to the input voltage

**That means, 10 K resistor allows 1,09091 volts and 1K resistor allows 10.9091 volts to pass.**

See below images. The theory above explained exactly matches with the real situation.

** Voltage across Parallel Resistors**

See image. Here two resistors ( 10K and 1 K ) are connected in parallel across a 12 volt power supply.

So total voltage is **V1 + V2**

The total voltage through the resistor remains same as the entire circuit irrespective of the value of the resistors in parallel.

**Total current through the resistors**

Total current will be the sum total of the current running through each parallel path.

**I total = I1 + I2**

Here the resistors are 10,000 Ohms (10K) and 1000 Ohms (1K) and input voltage is 12 volts.

So **1 / R total**

**That is 1/10,000 + 1/ 1000 = 0.0001 + 0.001 = 0.0011**

So R total is** 1 / 0.0011 = 909.090,909 Ohms**

**Now Voltage across a Resistor**

**V = I x R**

**Input voltage (V)** – **12 Volts**

**Current (I)** = **V / R** = **12 V / 909.090,909 Ohms** = 0.0132 Amps

So Voltage (V) across the resistors is **I x R**

That is **0.0132 A x 909.090,909 Ohms = 12 V**