Power Supplies and Consumer Electronics Applications
This domain links regulated power supplies to household appliance troubleshooting. The exam tests transformer action, linear versus switched-mode supplies, and a logical fault-finding sequence.
Core facts
- A step-down transformer reduces AC mains voltage; it works only on AC and provides isolation.
- A linear supply follows transformer → rectifier → filter → regulator. Common regulators include the 78xx fixed series and LM317 adjustable type.
- An SMPS switches at high frequency, is smaller and more efficient, and is common in TVs, chargers, and set-top boxes.
- Troubleshooting starts at the supply: plug, fuse, cord, switch, transformer, rectifier, regulator, then load.
Worked example
A transformer has 2300 primary turns and a 230 V primary. For a 12 V secondary, the turns ratio is 230/12 ≈ 19.17. Secondary turns = 2300/19.17 ≈ 120 turns. If the appliance is rated 60 W at 230 V, its normal current is 60/230 ≈ 0.26 A, so a 0.5 A fuse is a reasonable starting rating; a much larger fuse may fail to protect the circuit. A blown fuse should be replaced only after checking for a short or overload.
Traps
- Expecting a transformer to work on DC.
- Replacing a fuse with a higher rating without finding the fault.
- Assuming an SMPS output is isolated or safe to probe carelessly.
- Testing the load before confirming the supply rail.