12. Power Flashcards
What voltage/how many cells does logic work happily from?
3v - 2 cells
Why do you need a power supply unit PSU?
because your mc system probably works at 3V or 5V DC but mains is 20V at 50Hz in Europe and 110V at 50Hz in N. America.
What does a PSU do?
Converts mains AC to a low voltage DC power.
What is the problem with plug in the wall supplies?
The may be very poorly regulated
If the plug in the wall supplies are very poorly regulated what does this mean?
This means that the output voltage depends strongly on the load.
When must you therefore provide if using a plug in the wall supply?
A regulator to ensure your system sees a steady voltage of 3 or 5V.
What are the two main varieties of PSUs?
- Linear
2. Switching
What are linear PSUs, adv and disadv?
The classical approach, using transformers, rectifiers and smoothing capacitors. Electrically quiet but heavy, bulky and not very efficient.
What are switching PSUs, adv and disadv?
Uses power electronics to get a much smaller, lighter and more efficient product. However, they produce a lot of electrical noise.
Applications of switching PSUs?
Universal in computers, increasingly common in products such as battery chargers.
Can you draw the diagrams for 5V switching PSU and 5V linear PSU to indicate how much electrical noise they produce?
Yes or no
What are the features of mains supplies?
- Cost - virtually free when compared with batteries
- Available readily in buildings, but limited portability, even indoors
- Reliable (generally) - don’t run out like batteries but power cuts are not unknown so backup supply may be needed.
- High voltage may present potential danger
- May be problems if equipment is used abroad
- Noisy - transient pulses produced when heavy loads switch on and off and the PSU must filter these out. In more extreme cases e.g. lightning strikes, power surges may occur.
Features of battery supplies?
- Finite lifetime - run down and need to be replaced
- Portable - but may be heavy
- Polarity - usually needs protection against reverse polarity (if the user can insert the batteries the wrong way around)
- Expensive (very)
- Little noise - no transient or switching noise at all from the battery itself (but may get noise within the system)
- Characteristics change throughout life - depending on type of battery
- Bad for the environment - especially if not rechargeable or recycled
What are the two classes of batteries and examples of each?
- Primary: zinc chloride, alkaline (manganese), lead (many different types), silver oxide, zinc air
- Secondary (rechargeable): Nickel Cadmium (NiCad), Nickel metal hybrid (NiMH), lead acid, lithium ion
Two things to choose a battery depending on?
- What capacity is needed
2. How portable the product needs to be
What feature should the battery always have?
It needs to be easy to replace, so exotic batteries are a nuisance even if they have great electrical properties.
Can you draw the table showing different features/characteristics of different types of batteries?
Yes or no
Features of alkaline cells?
You can buy them anywhere. At large current drains their capacity is much bigger than cheaper zinc chloride cells but not so much at low currents. Nominally 1.5V but with strongly sloping discharge characteristic.