Power Supply System – SS1 Computer Hardware & GSM Repair Lesson Note (Third Term)

Power supply devices are the stabiliser, UPS and inverter, which protect computers and phones from bad electricity and keep them running when the power fails.

Behavioural objectives

By the end of the lesson, students should be able to:

1. State the three main problems with the power supply: surges, brownouts and blackouts.
2. Explain what a stabiliser does and what it cannot do.
3. Explain what a UPS is and why it beeps when the power fails.
4. Describe how an inverter works and how it differs from a UPS.
5. State the safety rules for power systems.

Why do we need protection for power?

Electricity from the wall (NEPA/PHCN) or a generator is not always steady, and bad power can damage computers, phones and repair tools.

There are three main power problems: surges, brownouts and blackouts.

1. Surges (high voltage): the voltage suddenly rises too high and can damage the circuits inside a computer.
2. Brownouts (low voltage): the light is “dim”, so the computer does not get enough power. This can stress the motor in a hard drive.
3. Blackouts: the power goes off completely. The computer shuts down at once, so files can be corrupted and unsaved work is lost.

What is a stabiliser?

A stabiliser is a device that watches the incoming voltage and keeps it at a steady level, usually about 220V.

• Low voltage: if the power from the wall is too low, the stabiliser boosts it.
• High voltage: if the power is too high, the stabiliser trims it down.
• The “click” sound: a click-click sound is the internal relay switching to adjust the voltage.
• Rating: stabilisers are usually rated in VA or kVA. Check the rating plate before connecting devices.
• Limitation: a stabiliser cannot keep a computer on when the power goes out. It only protects the device while power is present.

What is a UPS?

A UPS (Uninterruptible Power Supply) is a device that gives a computer battery power for a short time when the mains power fails.

• Battery: a small rechargeable battery stores energy while the mains is on.
• Charger: keeps the battery full while the power is available.
• Inverter circuit: changes the battery’s DC into AC for the computer.
• Voltage control: many UPS units also correct high and low voltage, like a simple stabiliser.

What happens when the power fails?

1. The mains power goes off.
2. The UPS switches to its battery almost at once, so the computer does not shut down.
3. The UPS starts to beep as a warning that it is running on battery.
4. You save your work and shut the computer down properly.

• The beep: it tells you to save your work and shut down now.
• Time limit: most small UPS units give only about 10 to 15 minutes of power.

What is an inverter?

An inverter is a device that changes DC power from large batteries into AC power so equipment can work for hours without mains power.

• Battery bank: one or more big, heavy batteries, like car batteries but larger, store the energy.
• Conversion: batteries store DC (Direct Current), but computers need AC (Alternating Current). The inverter changes DC to AC.
• Charging: the batteries charge from the mains, a generator or solar panels.
• Solar connection: many Nigerian inverters are linked to solar panels. The batteries charge by day and supply power at night without fuel.
• Running time: it depends on the battery size and the total load connected.

What is the difference between a UPS and an inverter?

A UPS has a small battery and gives minutes of power. An inverter has large batteries and gives hours of power.

• UPS: built for a quick “save and shut down” after a blackout.
• Inverter: built to keep devices working through a long power cut.

Which device should you choose?

A technician should advise customers on what to buy based on how long they need power and what they want to run.

What are the safety rules for power systems?

1. Do not overload: a giant fridge on a small UPS meant for a computer will trip it or drain its battery fast.
2. Ventilation: these machines get hot, so never cover them with cloth or keep them in a tight cupboard.
3. Battery care: do not let inverter batteries drain completely. Recharge them early so they last longer.
4. Dry place: keep power equipment away from water and wet floors.

Class activity

Look at the labels on the back of the computers in the lab. Find the “wattage” (for example, 250W). Now look at the stabiliser. Can it handle that amount of power?

Evaluation

1. Name the three main problems with the power supply and state one effect of each.
2. Why can a stabiliser not keep a computer on during a blackout?
3. Why does a UPS beep when the power fails, and what should you do?
4. Explain why an inverter changes DC to AC.
5. State two safety rules for power systems.

Assignment

A customer wants to keep a computer working for several hours after the power goes out. Write a short note advising which device to buy. Give two reasons, and state one safety rule the customer must follow.