Electronic Formulas Page

Ohms Law

Ohm’s law states that the voltage across a conductor is directly proportional to the current flowing through it, provided all physical conditions and temperatures remain constant.

To demonstrate this a demo circuit was set up.

The circuit is put together and given the values the expected result is

1.5v ÷ 10.09Ω = 0.148A or 148mA

A you can see the answer was not as expected but instead 104.43mA. The reason for this is that the circuit has additional resistance due to the multi-meters.

Luckily we are measuring these values

Looking a first measurement over the resistor

R₁ = V/I
= 1.05v ÷ 0.104A = 10.09Ω

Looking a second measurement over the multi-meter

Rₐ = V/I
= 0.21v ÷ 0.104A = 2.02Ω

Total Resistance calculated by adding together because we are in series

Rₜ = V/I
= 10.09Ω + 2.02Ω = 12.11Ω

Calculating the Total Amps

Amps = V/I
= 1.5 ÷ 12.11Ω = 0.123A or 123mA

Now lets calculate the Total Resistance using actual current meter reading (0.104A) and the (1.5) voltage.

R"ₜ" = V/I
     = 1.5v / 0.104A = 14.4Ω

Given we now know the actual resistance and the total resistance gives to the wiring resistance.

= 14.4Ω (calculated from the meter reading and the input voltage) - 12.11Ω (Calculated from total resistance) = 2.29Ω

Power Rating

Here is a practical example of powering an LED of 2.2v on a 12v power supply with 0.02A of current

Doing the maths is

V = 12v - 2.2v = 9.8v 
R = V/I = 9.8v/0.02 
  = 490Ω

Putting the resistor is series will then mean it works. However when showing this example the power rating was a factor. This can be calculated by

Resistance in Parallel

We measure resistance in parallel e.g.

With the following formula

Using the above demo for Ohms and two resistors and no accounting for wire and multi-meters gives

Recalculating adding these in gives

Voltage Divider

Here we have a voltage divider. The purpose of this is what the name suggests to divide voltage.

And here is the formula used to calculate. You need to understand three of the parameters.

And here is what it looks like in reality where the input is on the right and the output on the left. Note the resistors in parallel.