MYP Design Power Sources and Energy Efficiency Notes

Voltage Says If It Works, Capacity Says How Long

1. Every power source carries two numbers that decide whether it suits your product: its voltage in volts and its capacity in milliamp hours.
2. Voltage has to match the circuit, because 3 V will not start a 5 V board and 12 V will destroy it.
3. Capacity in mAh says how much charge is stored, so 2000 mAh means 2000 mA for an hour, or 200 mA for ten hours.
4. An alkaline cell gives 1.5 V whatever its size, so an AA and an AAA differ only in how long they last, roughly 2500 mAh against 1000 mAh.
5. Cells in series add their voltages, so four AA cells in a holder give 6 V while the capacity stays at one cell's worth.
6. Cells in parallel add capacity at the same voltage, but two cells at different states of charge will discharge into each other, so it is rarely worth doing in a school project.

Picking a Battery Chemistry That Fits the Job

1. Rechargeable NiMH cells give 1.2 V rather than 1.5 V, so four of them make 4.8 V where four alkalines make 6 V, and a circuit designed right at the edge stops working.
2. A PP3 block looks tidy but holds around 500 mAh, so a project drawing 200 mA flattens it in about two and a half hours.
3. Lithium polymer packs the most energy into the least mass, but it needs a protection board and a matched charger, and a punctured or over discharged cell can catch fire.
4. A coin cell cannot deliver much current at once, so a CR2032 runs a clock for a year and stalls the instant you ask it to turn a motor.
5. Match the chemistry to the job, then check the holder physically fits the space you left for it in your CAD model.

Working Out How Long It Will Actually Last

1. The sum is capacity in mAh divided by the current the circuit draws in mA, and the answer comes out in hours.
2. Add up the current of every component that is on at the same time, taken from datasheets rather than guessed.
3. Worked example: a board at 20 mA, two LEDs at 20 mA each and a buzzer at 30 mA add up to 90 mA.
   1. On a 2000 mAh NiMH pack that is 2000 divided by 90, which is about 22 hours.
4. Real batteries do worse than the sum, so multiply by about 0.7 to allow for the voltage sagging, cold weather and losses inside the cell, leaving roughly 15 hours.
5. Use an average for anything that runs intermittently, so a 300 mA motor that is on for 6 minutes in every hour counts as 30 mA, not 300 mA.

Mains and Low Voltage: Where the Power Comes From

1. UK mains is 230 V alternating current at 50 Hz, and no school project should ever be wired straight to it.
2. A plug top mains adaptor brings that down to a safe low voltage direct current supply, normally 5 V, 9 V or 12 V.
3. Check three things printed on any adaptor: the output voltage, the maximum current in amps, and the polarity symbol showing whether the centre pin is positive.
4. A 12 V 1 A adaptor runs a 12 V 400 mA circuit perfectly well, because a supply rated above what you draw simply runs cooler.
5. USB is the handiest low voltage source in school, giving 5 V at 500 mA from an old computer port and up to 3 A from a modern charger.
6. Mains suits anything that stays in one place, such as a greenhouse controller, while batteries suit anything a user carries, wears or wheels around.

Solar Cells Give You Small, Slow, Free Power

1. A solar cell turns light into electricity, and a single silicon cell produces about 0.5 V no matter how big it is.
2. Area controls the current instead, so doubling the cell size roughly doubles the milliamps at that same half volt.
3. Cells are wired in series to reach a useful voltage, which is why a 6 V panel has twelve cells sitting in a line under the glass.
4. Panel ratings assume bright direct sun, and the same panel on a grey November afternoon may give a fifth of its rated current.
5. Solar nearly always needs a rechargeable cell beside it, so the panel tops the battery up all day and the battery runs the product after dark.
6. A garden light is the classic layout: a small panel, one NiMH cell and an LED that only switches on once the panel stops producing.

Cutting the Power a Product Wastes

1. Standby drain is the current a product uses while doing nothing useful, and it is what empties a battery sitting in a drawer.
2. A microcontroller left spinning round a loop pulls about 20 mA all day, which is 480 mAh in 24 hours and a flat pack of AA cells inside a week.
3. A hard switch that physically breaks the battery connection removes standby drain entirely and costs around 60 pence.
4. Choose components on their current draw, since an OLED display uses a fraction of what a backlit LCD needs and a red LED is visible at a lower current than a white one.
5. Flash an indicator LED in short pulses rather than leaving it on, because it reads as almost as bright while using a small share of the charge.
6. Let a sensor or a switch wake the controller when something happens, instead of asking it to check hundreds of times a second all night.
7. Low power design is a sustainability decision as well, because a product that runs a year on one set of cells sends far fewer batteries to landfill.

Mains Voltage Rules You Do Not Bend

1. 230 V can kill, and the current that stops a heart is far smaller than the current a desk lamp draws every evening.
2. Never open, rewire or modify a mains adaptor, and treat the low voltage lead coming out of it as the only part of the design you touch.
3. Any mains switching goes through a ready made relay module in a closed enclosure, and a teacher checks it before it is ever plugged in.
4. Keep mains wiring inside an insulated case with a cable grip, so pulling the lead can never drag a conductor into the open.
5. Test every new circuit on a current limited bench supply first, because that turns a short circuit into a beep rather than smoke.
6. Most designs that seem to need mains can use a shop bought mains appliance switched by a smart plug, leaving everything you build safely on low voltage.