Electronics Tutorials: Power in electronic circuits

Power is a basic measure that lets you know how much energy your effect pedal (or other electronic circuit) is consuming. This is very important when using a power source or designing a pedalboard. In this post we’ll look at how to analyse basic electric circuits to know how much power they consume.

If you want to dig deeper into electric power, we’ll look at different electronic circuits and how you can find out the power consumed by the different elements. To keep things relatively simple the circuits will only be formed by power sources and resistors, the only passive element that consumes power. If you struggle with anything, with some basic resistor concepts and Ohm’s Law you should have all the tools you need to go through these examples!

1 - Simple circuit

Consider this simple circuit: a resistor and a DC 9V generator.

basic concepts power circuit example
Power in electronic circuits – example 1

To know the power consumed by the resistor we need to know the voltage across it and the current flowing through it. The voltage is directly VCC = 9V, as the resistor is connected directly to the voltage generator.

Now we can get the current flowing through the resistor if we use Ohm’s Law (I = V/R). That’s all we need to calculate the power!

electric power consumed circuit 1
Eq. 1: Electric power consumed (example 1)

On the other hand, the power source is generating energy as it is “throwing” the current into the circuit. The voltage across the source is VCC = 9V, and the current is the same that flows through the resistor (I=V/R). The amount of power generated is:

electric power generated circuit 1
Eq. 2: Electric power generated (example 1)

To know if a device generates or consumes power you just have to ask yourself this question: if you changed every other device by a piece of wire, would the current still flow?

basic concepts generated consumed power schematic
Power consumed or generated?

Battery: if we replaced the resistor by a wire the current would still flow. The battery or DC power adapter generates power (left).

Resistor: if we replaced the battery by a wire we would have no current as the resistor is a passive device unable to generate energy: it only consumes the energy generated by the source (right).

As there’s only two elements in the circuit, the power consumed by one is the same as the power generated by the other.

2 - Shared current (devices in series)

Now let’s look at a second schematic:

basic concepts series resistors power consumed
Power in electronic circuits – series resistors

The current through all the devices is the same as they are in series. We can calculate it using Ohm’s Law and the [equivalent series resistor]:

electric power consumed series resistor circuit 2
Eq. 3: Power in series resistors circuit

To calculate the voltage in R1 and R2 you just have to notice how they form a voltage divider.

electric power voltage divider series resistors circuit
Eq. 4: Series resistors voltage drop

To be sure that there are no errors always check that VCC = V1 + V2. Now the power consumed by each resistor is given by:

electric power consumed series resistors circuit 2
Eq. 5: Power consumed in series resistors circuit

As for the power source,

electric power generated circuit 2
Eq. 6: Power generated in series resistor circuit

In this example we have two resistors, so the total generated power (9mW) is divided between R1 (1mW) and R2 (8mW).

3 - Shared voltage (devices in parallel)

In this case the resistors are placed in parallel. We’ll start by calculating currents I1 and I2, as we know the voltage across both resistors (VCC). By using Kirchoff’s current law we can also calculate I3 as I1+I2.

electric power current parallel resistor circuit 3
Eq. 7: Current through parallel resistors

Now we find the power in the resistors and source:

electric power consumed circuit 3
Eq. 8: Power consumed in parallel resistor circuit

As in the previous examples, the total power generated (Wdc) is the sum of all the power consumed (W1 and W2).

4 - Mixed parallel & series resistors

This is the hardest case! Now we have parallel and series resistors so we’ll have to take extra steps. First of all, you need to understand the relations between currents and voltages:

  • The voltage across R2 and R3 is the same (V2)
  • The current through the power source and R1 is the same (I1)

Now you can use Kirchoff’s Law to establish the relations:

electric power parallel series resistor relations circuit 4
Eq. 9: Current & voltage relations

To calculate V2 and I3, it would be easier if we could reduce the circuit to something simpler. To do that, Req can be calculated as the parallel of R2 and R3:

electric power parallel series equivalent resistor circuit 4
Eq. 10: R2 and R3 parallel equivalent

With Req, you can find I1 as the total current through R2 and R3 (Req) like we did in example 2, and V2 with a voltage divider. Once we have V2, V1 is calculated with Kirchoff’s voltage law (VCC = V1 + V2):

electric power total current circuit 4
Eq. 11: Total current through Req
electric power voltage divider circuit 4
Eq. 12: Voltages across Req and R1

As now we have the voltage across R2 and R3, we can find the current through them using Ohm’s Law:

electric power branch current circuit 4
Eq. 13: Currents through R2 & R3

To be sure that there’s no mistakes, always check that I1 = I2 + I3!

With all the voltage and current values, finding the power consumed by each element is easy:

electric power total circuit 4
Eq. 14: Power consumed & generated

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