Ohms Law

Ohms Law

The key to intelligent troubleshooting of electrical circuits is a thorough understanding of Ohms Law. Ohms Law states that the current flowing in a circuit varies directly with the voltage and inversely with the resistance.
The pressure of one volt applied to one Ohm of resistance will cause one amp of current to flow. If the voltage increases, current will increase. If resistance increases, current will decrease.
Knowing any two of the three factors (volts, resistance or current) enables the third factor to be calculated using Ohms Law.

The mathematical expression is:

Volts = Resistance X Current

This formula is expressed in the Ohms Law Triangle.







To find a missing factor, insert the known factors in the appropriate position and perform the math. A horizontal line between two factors means to divide, a vertical line means multiply.

V = Volts
I = Current or Amps (A)
R = Resistance(Ohm)


NOTE: Voltage is sometimes expressed as U or E.


While complete understanding of Ohms Law is essential in the diagnosis of electrical problems, a practical understanding of how the three factors affect each other is equally useful.

Source voltage is not affected by current or resistance. It can only have three states
Too low - Current flow will be low.
Too high - Current flow will also be too high.
Correct voltage - Current flow will be dependent on the resistance.

Current Flow will be directly affected by either voltage or resistance.
High voltage or low resistance will cause an increase in current flow.
Low voltage or high resistance will cause a decrease in current flow.

Resistance is not affected by either voltage or current. Resistance like source voltage can have only three states.
Too low - current will be too high if the voltage is Ok.
Too high - current flow will be low if the voltage is Ok.
Correct resistance - current flow will be high or low, dependent on voltage.

Ohms' Law Series Circuits

Applying Ohms Law in a series circuit is easy. The current has only one path. Circuit resistance total is arrived at by adding the individual resistances. Amperage is calculated by dividing source voltage by the resistance total.

R1 + R2 = Rt
V / Rt = A







Key Features - Series Circuit
- Current through each load is the same.
- Total resistance equals the sum of the individual resistances.
- Voltage drop across each load will be different if the resistance is different.
- Total voltage drop equals source voltage.







Example:

If R1 = 2 Ohm
R2 = 2 Ohm
Voltage = 12.0 volts

What would the current be?

2 + 2 = 4 (Resistance Total)
R1 = 4
12/4 = 3 Amps (I)

Volts divided by Resistance = Amps
Expected voltage drop at each bulb?

A x R1 : A x R2
3 x 2 = 6 volts

We could expect a voltage drop of 6 volts across each bulb.

Ohms Law Parallel Circuit - 2 Branches

Working in a Parallel circuit requires a little more math. Each branch of the circuit has it's own path to the voltage source. Before amps are calculated total circuit resistance must be found.
In the diagram of the Parallel circuit both branches have source voltage available. The voltage drop across each branch will be the same regardless of individual branch resistance.







Key Features - Parallel Circuit
- Current flow through each branch can be different if the resistances are different.
- Total Resistance of the circuit is less than the resistance of the lowest branch.
- Voltage drop across each branch circuit is the same.
- Total current is the sum of the branches.







To find the total current draw (amps) in the circuit keep in mind that each branch of the circuit could have different resistance, so the current for each branch could be different. (Example 13/1)

Example: 13/1
If R1 = 3 Ohm
R2 = 6 Ohm
Voltage = 12.0 volts

What would the current be for the R1 branch? The R2 Branch?

12/3 = 4 amps = R1 branch current
12/6 = 2 amps = R2 branch current

What would the total circuit current be?

4 + 2 = 6 amps Total circuit current

To find the total resistance in the circuit independent of voltage and amperage use these formulas:

Parallel circuit with resistances that are the same on each branch.
Divide the resistance of one branch by the number of branches (Example 13/2).

Example: 13/2
If R1 = 2 Ohm
R2 = 2 Ohm

2 / 2 = 1 Ohm

Parallel circuit with resistances that are different on each branch (2 branches) (Example 13/3).

R1 x R2/R1 + R2

Example: 13/3
If R1 = 3 Ohm
R2 = 6 Ohm

3 x 6 / 3 + 6 = 18 / 9 = 2 Ohm

Ohms Law Parallel Circuit - more than 2 Branches












Calculating circuit resistance in a Parallel circuit with more than 2 branches adds even a little more math.

All the key features for a Parallel circuit still apply.

There are two methods to calculate the total circuit resistance in a Parallel circuit of more than 2 branches The formulas are:

Example: 14/1
If R1 = 3 Ohm
R2 = 3 Ohm
R3 = 6 Ohm
R4 = 4 Ohm

1 / 1/3 + 1/3 + 1/8 + 1/4 = 1 / 4/12 + 4/12 + 2/12 + 3/12 = 1 / 13/12

= 12/13 = .92 Ohm

Example: 14/2
If R1 = 3 Ohm
R2 = 3 Ohm
R3 = 6 Ohm
R4 = 4 Ohm

3 x 3 / 3 - 3 = 9 / 6 = 1.5 Then,

1.5 x 6 / 1.5 + 6 = 9 / 7.5 = 1.2 Then,

1.2 x 4 / 1.2 + 4 = 4.8 / 5.2 = .92 Ohm

Ohms Law in Series-Parallel circuit

When calculating resistance in a series-parallel circuit, always calculate the equivalent resistance in the parallel portion of the circuit. Then add this resistance (equivalent resistance) to the resistance of the series portion of the circuit.







Key Features - Series-Parallel Circuit
- Current in the series portion of the circuit is the same at any point of that portion.
- Total circuit resistance is the sum of the parallel branch equivalent resistance and the series portion resistance.
- Voltage applied to the parallel branch is source voltage minus any voltage drop across loads wired in series to the parallel branch in front of it in the circuit.







Example:

If R1= 4 Ohm
R2 = 6 Ohm
R3 = 2 Ohm

Calculate the equivalent resistance value of R1 and R2.
Remember the resistance of a parallel circuit is lower than the lowest resistance in that circuit. The resistance of this portion of the circuit must be lower than 4Q, the lowest resistance.

(4 Ohm X 6 Ohm) / (4 Ohm + 6 Ohm) = 24 / 10
2.4 Ohm Equivalent Resistance

Now follow the rules of a Series circuit.
The total circuit resistance is equal to the sum of the individual resistances.

2.4 Ohm + 2 Ohm = 4.4 Ohm

Total resistance for this series-parallel circuit is 4.4 Ohm

Alternate Formula for equivalent resistance:
Find the current draw of each parallel branch, add together to get the total current draw of the parallel portion, then using ohms law find the resistance of the parallel branch.

12 / R1 = 3. 12 / R2 = 2
3 + 2 = 5
12 / 5 = 2.4 Ohm