Challenge Circuit 2

Relay Circuit Currents

Solution and Practical Results

In this post, I’ll show one of the ways of solving Challenge Circuit 2: Relay Circuit and connect the circuit to get practical results. As you can imagine, the real results are rarely the same as the theoretical; we will compare how far off they are from each other. 

Here is the circuit from Challenge Circuit 2

Challenge Circuit 1 details

Relay Circuit

The values that were given are:

Resistance of the relay coil from datasheet: 70 ohms
Incandescent lamp rating: 24V 25W

The challenge was to find the currents through the control and power circuits.

Theory Solution

Here is one of the ways of solving the circuit above. I have broken it down into steps that I thought were the easiest for students to follow.

Challenge Circuits-3

Circuit Connection and Measurement

As you would expect, the theory solutions can be different to the practical results, but calculations are a good place to start to understand a circuit. 

Here is a video of our circuit connection for this challenge circuit and the measurements.

Comparison: Theory VS Practical

Let’s compare the difference between the calculations and the measurement for this circuit

Value Theory result Practical result
Current through A1 (control circuit)
171.43 mA
178.3 mA
Current through A2 (power circuit)
1.04 A
1.09 A

Why is there a difference in the theory and practical values? 

There can be many reasons for this difference. Here are some of them that could be affecting the results in this circuit

  • Relay coil resistance
  • Cable and connector resistance
  • Power supply accuracy
  • Meter calibration

Let’s see a couple of reasons why we see the difference in results in this circuit

Resistor Theoretical Value Practical Value
Relay coil resistance
70 ohms
68.7 ohms
Power supply voltage
24 V
24.2 V

The relay coil resistance in the datasheet is 70 ohms but when measured, the value was 68.7 ohms. This lower resistance causes a slightly higher current in the control circuit.

In the power circuit, the power supply was supposed to be 24V, but it was slightly higher than that. This increase in voltage causes a slightly higher current through the lamp.

There could be more reasons for these discrepancies, but these give us a good idea.


If you used a different method of solving this circuit or have other reasons for the difference in the theory vs practical results, please put it in the comments so our readers and students can benefit from your input.

Thanks for reading.

Picture of Husnen Rupani

Husnen Rupani

I help electrical training organisations increase learner engagement by designing innovative training equipment. I have a saying "Electricity - you cannot see, you cannot hear it, but by the time you feel it, it may be too late." My main aim is to turn this black magic that we call electricity into something that people can understand.