Power Sources Electronics Unit – Lecture 5

1. Power SourcesElectronics Unit – Lecture 5 Bench power supply Photovoltaic cells, i.e., solar panel Thermoelectric generator Battery Power Budget Electronics 5

2. Bench Power Supply Adjustable, regulated output voltage Often includes a current limiting feature Great for designing and prototype testing Not for flight Electronics 5

3. 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 Electronics 5

4. Photovoltaic panel Potential flight power source “Free” electricity while Sun shines Size and weight concerns Expense Electronics 5

5. Photovoltaic panel Panels 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 Electronics 5

6. 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 Electronics 5

7. Thermoelectric Generator Uses 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 ☺ Electronics 5

8. 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 Electronics 5

9. Battery Types Primary 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/discharges lead-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) Electronics 5

10. Battery Characteristics Terminal voltage - depends on specific chemistry Capacity - rated in ampere-hours, or milliampere-hours 3600*(ampere-hours)*(average terminal voltage) = energy capacity in joules Physical size and weight - energy density in joule/gram Discharge characteristics - especially at low temperature Electronics 5

11. Battery Characteristics 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 Electronics 5

12. Battery Characteristics 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 Electronics 5

13. Battery Characteristics Discharge characteristics - the discharge curve Use this typical lithium battery as an example Electronics 5

14. Battery Characteristics Notice the degradation of capacity at low temperatures, especially at higher load currents. And lithiums are about the best! Electronics 5

15. 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 Electronics 5

16. Power Budget Example The GPS radio telemetry package: Transmitter requires a minimum supply voltage of 9 V GPS receiver requires from 3 V to 6 V Flight 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. Electronics 5

17. Power Budget Example 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 Electronics 5

18. Power Budget Example The GPS radio telemetry package: 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 Electronics 5

19. Power Budget Example The GPS radio telemetry package: 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. Electronics 5

20. Power Budget Example 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) Electronics 5

21. Activity Perform a cold environment battery test Compare alkaline, NiMH, and lithium cells Use HOBO to collect data Electronics 5