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| # | Title | User Story | Importance | Notes |
|---|---|---|---|---|
| 1 | Work with Nomura MPPTs | We're using 10 Nomura MPPTs, 5 in series for 2 strings total. They don't have any public way of interfacing with them, but we'd like to collect data on our array. | Must Have | Most likely using a master board for current sensing and I2C slaves for each MPPT |
| 2 | Record voltage and temperature per-module | We'd like to know the voltage and temperature of each module so we can tell how each module is performing and any potential hotspots. | Must Have | |
| 3 | Record current per-string | We'd like to know the current provided by each string so we can calculate the overall power of the solar array. | Must Have | |
| 4 | Isolated | We want to make sure that our ADCs are isolated from the rest of the boards and ADCs. | Must Have | |
| 5 | Low-cost/Simple | We'd like to make these boards as simple as possible to reduce development time. Ideally we shouldn't need to write firmware for the slave boards. | Must Have |
Meeting Notes - September 30, 2017
Interface Borad Isolation
The one of the requirements of designing the interface board is how to isolate the high voltage environment (i.e. MPPT) from the low voltage environment (i.e. microcontrollers). The main challenge is the isolation of analog to digital converter, which measures the high voltage analog signal and converts it to low voltage binary digital signal. There are two options of isolating the ADC:
- Analog Isolation
- Interfere the the analog measurement
- Isolation amplifier requires power supply on both the input-side (MPPT) and output-side (Interface broad)
- Digital Isolation
- Digital signal is immune to the measurement interference
- The supply voltage of the ADC must be isolated
- Optocouplers can be used to isolate the signal
Voltage Sensing
Use a voltage divider to map the high voltage (0-30v) to a low voltage (0-5v) for the ADC
Current Sensor
There are two ways to sense the current: direct and indirect
- Direct
- Place a shunt resistor with known resistance in the circuit
- Measure the voltage of the shunt resistor and calculate the current based on Ohm's law
- Typically amplifier is used to amplify and an ADC is used to measure to the voltage
- There are current shunt monitor chips that provide I2C communication
- Require isolation
- Indirect
- Use Hall effect sensor to measure the current
- Does not require additional isolation
- Example: ACS722LLCTR-40AU-T
Cost
The cost of the interface board should be cheap, because we will build 12 interface boards.
Polarity
MPPTs might output a slight amount of negative voltage, thus, we nee to use a diode to ensure the unipolarity or use ICs that can operate in bipolarity.
Potential ADC Chip
ADS7924
- 12-bit, 4 Channels with I2C
- Link: http://www.ti.com/lit/ds/symlink/ads7924.pdf
Power Supply
ADCs, current sensing monitors, temperature sensors, isolation amplifiers and other ICs need a small amount of current to operate. Due to the isolation requirement, supplying power to the ICs is tricky. There are few options of supplying the power to the ICs.
- Supply power from the high-voltage side (i.e. MPPT)
- Use ICs with wide supply voltage range
- Use voltage regulator?
- Supply power from the low-voltage side (i.e. boards)
- Use a DC-DC converter to create an isolation barrier
- DC-DC converters are expensive
- Supply power from the high-voltage and low-voltage sides
- Only needed if we plan to use isolation amplifier
- It is really complicated
- Use AA battery to power the interface board
Questions
Below is a list of questions to be addressed as a result of this requirements document:
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