Power Sources Electronics Unit – Lecture 5 Bench power supply Photovoltaic cells, i.e., solar panel Thermoelectric generator Battery Power Budget Prepared by Jim Giammanco <giamman@rouge.phys.lsu.edu> LSU 11/05/2013 Electronics 5

Bench Power Supply Adjustable, regulated output voltage Often includes a current limiting feature Great for designing and prototype testing Not for flight LSU 11/05/2013 Electronics 5

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 LSU 11/05/2013 Electronics 5

Photovoltaic panel Potential flight power source “Free” electricity while Sun shines Size and weight concerns Expense LSU 11/05/2013 Electronics 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 LSU 11/05/2013 Electronics 5

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 LSU 11/05/2013 Electronics 5

ATIC LSU 11/05/2013 Electronics 5

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 ☺ LSU 11/05/2013 Electronics 5

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 LSU 11/05/2013 Electronics 5

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) LSU 11/05/2013 Electronics 5

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 LSU 11/05/2013 Electronics 5

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 LSU 11/05/2013 Electronics 5

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 LSU 11/05/2013 Electronics 5

Battery Characteristics 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) LSU 11/05/2013 Electronics 5

Battery Characteristics Notice the degradation of capacity at low temperatures, especially at higher load currents. And lithiums are about the best! LSU 11/05/2013 Electronics 5

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 LSU 11/05/2013 Electronics 5

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. LSU 11/05/2013 Electronics 5

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 LSU 11/05/2013 Electronics 5

Power Budget Example The GPS radio telemetry package: GPS: 140 mA x 1 = 140 mA Flight Computer: 0 mA x 1 = 50 mA Radio Standby : 80 mA x 0.93 = 75 mA Radio Transmit: 1050 mA x 0.07 = 74 mA Add them up….. about 340 mA If the beacon must operate for at least 8 hours… 340 mA x 8 hours = 2720 mA-hr required from the battery pack LSU 11/05/2013 Electronics 5

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. LSU 11/05/2013 Electronics 5

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) LSU 11/05/2013 Electronics 5

Voltage Regulator IC Fixed Voltage outputs available (3.3, 5, 9, 12, 15, etc.) Vout < Vin - Voh Positive or Negative models available (Check pinout!) Variable Voltage models – set with resistor values. LM317 Low Dropout models have lower overhead voltage Ratings: Voltage In/Out, Max Current LSU 11/05/2013 Electronics 5

DC/DC Converter Step-Up or Step-Down Voltage (or both) Uni-Polar, Bi-polar, or multiple Vout Specifications: Voltage In (Min and Max) Voltage(s) Out Maximum Current, Maximum Power Out Efficiency: Pout = Pin*Efficiency Mechanical (and Thermal) Packaging, connections Size and Weight LSU 11/05/2013 Electronics 5

Power System Example LSU 11/05/2013 Electronics 5

Regulators DC/DC Converters LSU 11/05/2013 Electronics 5

Regulators DC-DC Converters 50mA 50 mA I total= 50mA +75mA +50mA =175mA 75mA 75 mA 480mW/0.80= 600mW 20 mA I=P/V so I=600mW/12V 24V*20mA = 480mW I = 50 mA if efficiency = 80% LSU 11/05/2013 Electronics 5

HOBO® Data Logger Combines sensors, transducers, signal conditioning, A/D conversion, storage, and readout into a compact, battery powered unit. LSU 06/04/2007 Electronics 6

DIY Data Logger Sketch a System Diagram for a data logger that logs a timestamp, temperature and battery voltages of two different packs (to compare alkaline and lithium battery performance.) LSU 06/04/2007 Electronics 6