Electrical theory 1.2d - how to calculate series-parallel circuits

In this section I'll explain how to calculate various electrical values for series - parallel circuits for DC. This will use the knowledge you gained from their individual topics. The complexity of the total circuit will be limited to lighting applications.

The electronic switch turns the 3 parallel light groups on and off together. SPS is a switching power supply with DC output of 24V.

Step 1

It's best to learn the process by starting at the end and work backwards to the total circuit. Then you can see the stages of reducing the circuit. This is like planning a route between points A to B. You don't start selecting streets at point A because you need overall direction to point B. Don't worry, it will become clear as we work through this.


First we need to know the voltage across the parallel circuits - call this Vp as shown. This is easy to calculate using series equation:

Vdc = Vswitch + Vp

re-arranging and putting in values :

Vp = Vdc - Vswitch = 24 - 1.1 = 23.9V

Step 2

We  briefly focus upon the parallel branches to recognise they all have the same voltage, 23.9V across them. This is an intermediate step. Note as we progress, we ignore the circuit parts we've already processed.

 Step 3

Because the 3 branches are in parallel and have the same voltage across each, we can focus on each branch and process them separately. This process is basically the same for any series circuit: group and process voltage source and voltage drops; total resistances; use Ohm's Law.

I'll show you how to calculate the hardest branch of mixed colour LEDs. For the remaining branches I'll state the current value flowing in each.

You remember for a series circuit, the voltage drops are added together.

So the voltage equation is:      23.9V = 2 + 3.3 + 1.9 + 3.4  + 2.1 + R* current

 

We know the resistor is 1K, so   23.9 = 12.7 + 1000 * current

Solving this                                current = (23.9 - 12.7)/ 1000 = 0.011A (or 11mA)
  

Red LED branch  current              = (23.9 - 5  * 1.9)/ 680 = 0.021A

Dark green LED branch current    = (23.9 - 5 * 3.3)/ 390 =  0.019A

Step 4

 The output of switching power supply is connected in series to electronic switch and parallel groups of LEDs/ resistors. So therefore, the TOTAL of 3 branch currents must equal the current from switching power supply.

To add all branch currents:   series SPS current = .011 + .021 + .019 = 0.051A


That's how to calculate electrical values for the light string's series parallel circuit shown in Step 1.

Summary: Just break the big circuit into series then parallel subcircuits then series branch circuits (individually) and process each according to their electrical laws.

END