LSU 06/04/2007Electronics 51 Power Sources Electronics Unit – Lecture 5 Bench power supply...

LSU 06/04/2007 Electronics 5 1

Power SourcesElectronics Unit – Lecture 5

Bench power supply

Photovoltaic cells, i.e., solar panel

Thermoelectric generator

Battery

Power Budget


Bench Power Supply

Adjustable, regulated output voltage

Often includes a current limiting feature

Great for designing and prototype testing

Not for flight


Bench Power Supply

Important specifications:

Voltage range - should be 0 V to ~12 to 15 V

Load current - should be a few hundred mA

Regulation - hold output with a few tenths of V

Current limiting - very desirable if adjustable

can protect circuits from accidental damage


Photovoltaic panel

Potential flight power source

“Free” electricity while Sun shines

Size and weight concerns

Expense


Photovoltaic panelPanels made up of an array of individual cells

Each cell produces about 0.5 volt potential

Cell current depends on surface area and illumination

In full sun expect perhaps 10 milliwatts per cm2

For 6 V at 100 mA (600 mW) 12 cells would be needed, each having surface area about 5 cm2

Must face the Sun - and payload is probably rotating

Multiple panels needed - unless auto pointing


Photovoltaic panel

Cells are delicate and subject to breakage

Voltage output may vary - electronic regulation needed

Backup battery needed for cloudy intervals

Not the best choice for short duration student flights


Thermoelectric GeneratorUses the Seebeck Effect to convert a temperature difference directly into an electric current

Needs a heat source - deep space missions use the heat of radioactively decaying plutonium

Probably not a good choice for a student project ☺


Battery

Strictly, a battery is a combination of discrete cells

Best choice for short duration flights

Inexpensive

Reasonably lightweight

Variety of voltages and energy capacities available


Battery TypesPrimary batteries - one time use

carbon-zinc (old fashioned flashlight batteries)

alkaline (most common in consumer products)

silver-mercury (used in hearing aids and older cameras)

lithium - lots of energy for small weight

Rechargeable batteries - multiple charges/dischargeslead-acid (car batteries, or “gel cells”)

nickel-cadmium, Ni-Cd (older style rechargeable chemistry)

nickel-metal hydride, NiMH (popular now in consumer products)

lithium ion, (high end uses - laptop computers, digital cameras)


Battery Characteristics

Terminal voltage - depends on specific chemistry

Capacity - rated in ampere-hours, or milliampere-hours3600*(ampere-hours)*(average terminal voltage) = energy capacity in joules

Physical size and weight - energy density in joule/gram

Discharge characteristics - especially at low temperature



Terminal voltage - depends on specific chemistry

carbon-zinc about 1.5 V per cell

alkaline - about 1.5 V per cell

lead acid - about 2.0 V per cell

Ni-Cd and NiMH - about 1.2 V per cell

lithium - about 1.5 V per cell

but often made as double-cells for 3 V



Capacity - rated in A-hr or mA-hr for small cells

Usually specified at the “ten-hour discharge rate”

Example - an Energizer AA 1.5 V lithium cell rated 2900 mA-hr

should deliver 290 mA for 10 hr before voltage falls below 1V

But, will not last proportionally as long at higher currents



Discharge characteristics - the discharge curve

Use this typical lithium battery as an example

Nominal Voltage 3.0V

Rated Capacity 5Ah to 2.0V at 20°C (68 F)

Average Weight 55g (1.94oz)

Volume 26.5cm3 (1.60in.3)

Operating Temp. Range -20°C to 60°C (-4 F to 140 F)



Notice the degradation of capacity at low temperatures, especially at higher load currents.

And lithiums are about the best!


Calculating a Power Budget

Given: minimum permissible voltage

maximum load current

average off-peak load current

load current versus time data (duty cycle)

mission duration

minimum expected temperature


Power Budget Example

The GPS radio telemetry package:Transmitter requires a minimum supply voltage of 9 VGPS receiver requires from 3 V to 6 VFlight computer requires 5 V

If a single battery pack powers all three units, its terminal voltage cannot fall below 9 V.

Voltage regulators will be used to reduce the voltage for the other units.



The GPS radio telemetry package:

GPS receiver draws 140 mA, continuously (100% duty)

Flight computer draws 50 mA, continuously (100% duty)

Transmitter draws 80 mA when in standby mode (93% duty)

Transmitter draws 1050 mA when transmitting (7% duty)(since transmitter sends a 2 second data burst every 30 seconds)

Peak current = 1320 mA, minimum current = 270 mA




140 mA x 1 = 140 mA

50 mA x 1 = 50 mA

80 mA x 0.93 = 75 mA

1050 mA x 0.03 = 32 mA Add them up….. about 300 mA

If the beacon must operate for at least 8 hours…

300 mA x 8 hours = 2400 mA-hr required from the battery pack




A sufficient number of cells must be wired in series so that, even when delivering 1320 mA, the composite terminal voltage remains above 9 V, even at the minimum expected temperature.

Consulting the battery curves previously viewed, cell voltage can drop to about 2.2 V at -20 celsius when delivering 600 mA or more. Therefore, four cells will be needed. Allowing a reserve, cells should have a capacity of about 3000 mA-hr.



Mission Duration Considerations

A science package must operate through pre-launch preparation and for the full duration of flight. If data is in non-volatile storage, it need not operate after touchdown. (~ 4 hours)

A tracking beacon must operate through pre-launch, the entire flight interval, and be able to continue well after touchdown to assure recovery. (8 hours or more)


Activity

Perform a cold environment battery test

Compare alkaline, NiMH, and lithium cells

Use HOBO to collect data

LSU 06/04/2007Electronics 51 Power Sources Electronics Unit – Lecture 5 Bench power supply...

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