Use a relay to switch a separate circuit
Interpret coil-operated contacts and keep the control and switched circuits electrically distinct.
Section P4.3.1 · Circuit diagrams and circuit components
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Describe a current-carrying magnetising coil.
- Interpret open switch contacts.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a control coil need to produce its sustained magnetic action?
What you will learn
- Draw the main parts of a relay symbol.
- Explain a normally open relay control sequence.
Separate the coil and contacts
A relay is an electromagnet-operated switch. A relay diagram shows a control coil and separate switch contacts linked by a mechanical or magnetic-action indication, commonly a dashed line. That dashed action link is not a conducting wire. The control current magnetises the coil and moves the contact mechanism.
For a normally open relay, the contacts are open in the unenergised normal state and close when the coil is energised as specified. A normally closed relay has the opposite stated contact convention. Read the diagram rather than assuming every relay contact starts open.
Trace two circuits
The coil belongs to a control circuit, and the contacts belong to the switched load circuit. A small appropriate control input can operate a separate load supply through suitable contacts. The control current does not simply travel across the dashed link into the load.
In the ideal normally open example, opening the control path de-energises the coil, the contacts return open and the load path is interrupted. Real relays have ratings and switching transients; a schematic does not establish suitability for arbitrary voltages or loads. School paper examples use stated isolated low-voltage supplies and do not demonstrate practical competence.
Sources for this lesson
Pause and explain
A normally open relay coil loses its current. What is the ideal contact response?
Worked example
A normally open relay controls a separately supplied lamp. Describe what happens when its control switch closes.
Show the worked solution
- Current flows through the control coil.
- The magnetic action closes the normally open contacts.
- The separate lamp path becomes complete and can conduct.
Answer The coil energises, contacts close and the separate lamp circuit can conduct.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Draw a normally open relay using a control coil, separate contacts and a dashed action link.
Give me a hint
Show distinct control and switched terminal pairs.
Compare my reasoning
- Draw the coil with its control terminals.
- Draw an open contact pair with switched terminals.
- Join coil and contact mechanism by a dashed action indication, not a wire.
A coil and separate normally open contacts, coupled by a dashed action link.
Look for these in your work
- Keep circuits distinct.
- Identify the normal state.
Explain the sequence from closing the control switch to powering a separately supplied lamp.
Give me a hint
Follow coil current, contact motion and load path in order.
Compare my reasoning
- The control loop closes and coil current flows.
- The electromagnet action closes the contacts.
- The separate load loop now permits lamp current.
Control current → magnetic action → contact closure → separate lamp current.
Look for these in your work
- Explain the intermediate action.
- Do not join currents through the dashed line.
A learner draws a wire along the relay dashed link and omits the lamp return wire. Identify both errors.
Give me a hint
Separate coupling notation from the two conducting loops.
Compare my reasoning
- The dashed link denotes action and should not become a wire.
- The lamp needs its own complete source return.
- Restore two distinct complete circuits with the contact-controlled load path.
The action link is not conducting; the load return path is also missing.
Look for these in your work
- Correct both connections.
- Keep separate source paths.
Common mistakes
- The dashed action link is not a wire.
- Normal means the stated unenergised contact state.
- The two circuits need their own complete paths.
What can you explain now?
What links the relay coil to the separate contacts?
Compare with the explanation
Magnetic and mechanical action.
The action indication does not represent direct electrical conduction between circuits.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain relay coil action and contact switching while preserving two separate electrical circuits.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.