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Power Outline
Introduction
to GEOCAM Power
Power Consumption Analysis
Components: Converter vs Regulator
Regulator: Calculations vs Measurements
Schematic and Circuit Implementation
From HASP to GEOCAM: Integrated
Power Connection
The external power supply on the HASP project provides 28Vdc to each project payload. The components used in GEOCAM that require power include the camera and servos. Converters were used to step-down the voltage from 28V to a given amount which varies depending on what voltage is needed for each component. (The camera requires an input voltage of 7.4Vdc while the motors require 5Vdc. The heaters were added later in series and connected directly to the 28V input, but were not used in the final flight design.)
Converters and voltage regulators will be soldered onto a proto-board to save space and weight. Constraints on our payload for the HASP project is that the payload cannot use more than 0.5A with a maximum power usage of 14W.
To ensure that GEOCAM did not use more power and current than allotted, a power analysis was devised. It included a standby and continuous picture mode for the camera and servo motors. Current consumption was the greatest concern with GEOCAM, especially while the camera was in a continuous picture mode. The power analysis was constructed before heaters were added, resulting in a much different current and power consumption.
One assumption made was that both servos consumed the same current, although they share the same components. In reality, the current reached roughly 0.2A. Reevaluation of current consumption was be made after initial integration and before the heaters were added. The estimated current after the addition of heaters was around 0.5A, but were not used in the final design for launch.
Components: Converter vs. Regulator
The Canon digital camera used 7.4Vdc input voltage from the 28Vdc provided by HASP. A voltage converter was first used to scale down the voltage from 28Vdc to 12Vdc, while a regulator then converted the 12Vdc to the 7.4Vdc used for the camera input voltage. The regulator allowed the voltage to be reduced to a wide range of possible values in order to obtain the correct input voltage.
A 28V-5V converter was used to bring power to the servos. The converter offered a simple solution to obtaining 5Vdc for the servos and components associated with it.
For information regarding components and parts used, see the following links:
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| Voltage Regulator | 28V-12V Converter | 28V-5V Converter |
Regulator: Calculations & Measurements
Using the formula provided in the datasheet of the LM317 Voltage Regulator, the relationship between the R1 and R2 was estimated using Iadj as 50uA. Once the components were set up using Schematic 1 (See Schematic & Circuit Implementation), the resistance of R1 was measured for the given output 7.4Vdc.
The resistance of the camera was measured to be around 4.35kOhms. It was then determined that in order to output a steady voltage, the resistance of the circuit must be significantly smaller. The theory behind this is if there are two resistances in parallel (circuit and camera) the lesser resistance will dominate the larger one.
Example: if R2 >> R1, then RParallel shows a similar resistance to R1 (Where R1 of the circuit are the two resistors of the circuit in parallel).
Therefore, the relationship of R2 (using two 47 Ohm resistors in series) produced an output voltage within reasonable range:
Schematic & Circuit Implementation
The PC Board and schematics are divided into two main parts based on the desired output voltage. Schematic 1 shows the layout for both the converter as well as the voltage regulator. Schematic 2 shows the layout for the 28V-5V Converter used to power the servo components.
Schematic 1: 28V-7.4V Design Layout
Schematic 2: 28V-5V Design Layout
Image 1 shows the layout of both parts of the power layout on the PC Board. Notice that the heat sink was added for testing purposes to ensure that the LM317 Voltage Regulator would not overheat. A fuse was also added to the 28V input to ensure that GEOCAM would be safe from any power surges or increased current intake from GEOCAM.
Image 1: PCB Design Layout
Due to space constraints, the final PC Board merged the servo chip and components with the power components. The change saved space and kept the electronic components together for heating purposes.
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| Image 2: PCB before merge | Image 3: PCB after merge |
From HASP to GEOCAM: Integrated Power Connection
The power connection from HASP to GEOCAM is dictated by a diagram with color-coded wires specified for different uses. There is only one battery for the HASP project, and to reduce the likelihood of strain caused by soldering multiple wires together, only one +28V and ground wire were attached to the finished product.