Switches and relays: NO/NC, poles and throws
A switch makes or breaks a circuit; a relay is an electrically operated switch. A normally open (NO) contact is open until operated; a normally closed (NC) one is the reverse. SPST is a plain on/off; DPDT switches two circuits at once. A relay's small coil current switches a separate circuit, so its value is isolation: a low-power signal safely switches a large or mains load. A DPDT relay can reverse a motor or latch on.
Diode: flyback protection and the LED resistor
A diode conducts one way (anode to cathode). (1) Flyback protection: switching off a coil induces a reverse back-emf spike that can destroy the transistor; a diode reverse-biased across the coil (cathode to the positive supply) blocks normal current but conducts the spike. (2) LED indicator: an LED needs a series current-limiting resistor, sized . A 7-segment display is seven LEDs (segments a to g).
NPN transistor as an electronic switch
An NPN transistor switch has three terminals: base, collector and emitter. A small base current turns it ON, letting a much larger collector-to-emitter current flow through the load; no base current means OFF. A base resistor limits the base current. It is tiny, has no moving parts and switches very fast, but handles only a small current — so to drive a motor or mains lamp it energises a relay for the heavy switching and isolation.
Drawn from real examiner reports.
Flyback diode drawn forward-biased
The flyback diode across a coil must be reverse-biased — cathode to the positive supply — so it blocks current while the coil is energised and conducts only at switch-off, when the back-emf briefly reverses the voltage. Drawn forward-biased it short-circuits the coil and stops the relay working. So cathode to the positive rail: off normally, on only for the spike.
The back-emf suppression diode must be reverse-biased across the coil, but some drew it forward-biased (w23 P42/P43 Q11d).
LED resistor: dividing the full supply
The series resistor for an LED is R = (Vsupply minus Vled) divided by I. The mark is lost when candidates divide the FULL supply by the current, forgetting that only the voltage REMAINING after the LED's fixed drop sits across the resistor. On 5 V, an LED dropping 2 V, at 20 mA: 5 minus 2 = 3 V, then 3 divided by 0.02 = 150 ohm. Deduct the LED voltage first, then divide.
Others lost a mark by not subtracting the LED voltage drop from the supply before dividing (s23 P42 Q12a(vi)).
NO and NC contacts swapped
Candidates lose marks by muddling normally open (NO) with normally closed (NC). A NO contact is open until operated; a NC contact is closed until operated — the opposite. It matters for design: a fail-safe emergency stop uses NC (a broken wire also stops the machine), while a momentary start uses NO. A swapped answer scores nothing.
Toggle vs push-to-make; SPST vs DPDT
The action and pole/throw words must be exact. A toggle switch latches — it locks in the position it is set and stays there. A push-to-make (PTM) switch conducts only while pressed, springing back when released (a doorbell). Get the pole/throw label right too: SPST is a plain on/off, DPDT switches two circuits at once. Guessed answers score nothing.
Switch/symbol identification needs the precise words — toggle locks in position, PTM is press-to-make, SPDT (s23 P42 Q8; w23 P42/P43 Q11b(iii)).
Milliamps not converted to amps
LED and coil currents are quoted in milliamps, but Ohm's law and the LED-resistor formula need amps. Convert first: 20 mA = 0.02 A, 10 mA = 0.01 A, 25 mA = 0.025 A. Substituting the milliamp number straight in makes the answer a thousand times too small — 3 V at 20 mA gives 150 ohm, not 0.15. Write every current in amps before dividing, and state the unit (ohm).
A transistor can switch any load on its own
A testable error: assuming a transistor switch handles any load by itself. A small-signal transistor switches only a small current and gives no isolation from a high-voltage load, so it cannot drive a large motor or mains device directly. It energises a relay, whose contacts switch the heavy load. Add a relay whenever the load is large, mains-powered or isolated.
Transistor-switch drawback is that it handles only a small current (s22 P42 Q12).
Lay the LED calculation out in stages
Set the LED-resistor calculation out in stages: write R = (Vsupply minus Vled) divided by I, subtract the LED voltage FIRST, convert mA to amps, divide, then state the unit (ohm). Method marks are given at each stage, so working scores even if the final number slips.
Match the switch to the load
A small logic or sensor output cannot drive a motor or mains lamp. Show the chain: signal to the transistor base via a resistor, transistor switches the relay coil, relay contacts switch the load. Transistor = fast low-power switching; relay = isolation and heavy current.
Use the exact switch and contact words
Symbol and identification questions reward the precise term, not a description. State NO or NC (not just open or closed), toggle or push-to-make, and the pole/throw label SPST, SPDT or DPDT. An exact word scores; a vague paraphrase usually does not.
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