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🌍 Overview

This is the design document for the Amperes Board. As 1 out of the 4 important boards within our system, we need to be able to track the current within the car so that it is in a safe and manageable level. We also want to think about how we can record the current to apply the result to a State of Charge (SoC) algorithm.

🪛  Design  Design options

There are only 2 ways to measure current. Either measure the voltage across a resistor in series with the load or we can measure the magnetic field of the wire. Invasive vs non-invasive

	Option 1 - Voltage Across a Resistor	Option 2 - Magnetic Field of Wire
Overview	Current Sense Amplifier or Isolated Modulators/ADC	Hall-Effect Current Sensing
Screenshot
Pros and cons	Tip Accuracy Tip Simplicity and cost Warning Affected by electric noise Warning Power loss and voltage drop	Tip Electric isolation and no power loss Tip Measures both AC and DC current Warning Affected by magnetic noise and temperature Warning Zero-drift problems

Decision 1

• Option 1 - Making the PCB though a shunt resistor

In the future, members can look into the second option for a hall-effect version

Ideas

Figure 1 is a common method to measure current using a shunt-resistor. There are 3 steps to read the current:

1. The differential voltage is fed into the Current Sense Amplifier and converted to a single-ended signal.

2. This single-ended signal is connected to an ADC, digitizing the signal.

3. The signal is sent to a microcontroller for processing.

⚡ High Side vs Low Side

For reading the current, there are 2 different configurations you can have your device hooked up to.

See Introduction to Current Sense Amplifiers to learn more

Figure 2 shows an example of the current sense amplifier connected to the shunt resistor in a high-side sending sensing configuration.

Current Sense Amplifier

Isolated Modulator

Design

Accuracy

Bigger shunt resistor allows for more accuracy

Power Dissipation

Advantages:

• Able to detect load short to ground

• Current is monitored directly from the source

Disadvantages:

• High voltage can limit the variety of devices

Image Added

Figure 3 shows an example of the current sense amplifier connected to the shunt resistor in a low-side sensing configuration.

Advantages:

• Wide range of available options

• Does not need an advanced sensor

Disadvantages:

• Difficult to detect load short to ground

Decision 2

• Both?!: use high and low-side sensing

We will get the advantages of both and no disadvantages except cost. Plus, built in fault monitoring.

🖼️ Chips to pick

There are different chips that you can choose to make your life easier. The current sense amplifier is great, but you could also use an isolated modulator (an integrated ADC) to track the voltage differential. Below is the selected shunt resistor given as a requirement.

Current Shunt Resistor: WSBM8518L5000JK

• Resistance: 500 µOhms

Requirements

Below is the specs that the sensor should be able to meet. There needs to be two different ones as there is some better for low-side sensing and some better for high-side sensing.

High-Side Pick - INA226

Image Added

• Electrical Specifications:

  • Supply Voltage: +3.3V

  • Common Mode Voltage Range: -2.7V to +120V

  • Shunt Voltage Range:

  • Power Consumption: 36 W

  • Accuracy:

    • Gain:

      • Gain error: ±0.1% (maximum)

      • Gain drift: ±5 ppm/°C (maximum)

    • Offset:

      • Offset voltage: ±12 µV (maximum)

      • Offset drift: ±0.2 µV/°C (maximum)

Link to TI: https://www.ti.com/product/INA226

✅ Follow up

🔤 Vocab

Current Sense Amplifier: Amplifies the small voltage drop across a shunt resistor to measure current accurately in circuits.

Isolated Modulator: Converts analog signals to digital while maintaining electrical isolation, often used in high-voltage or noisy environments.

Hall-Effect Sensor: Detects magnetic fields to measure position, speed, or current without direct electrical contact.

Zero-drift: the phenomenon where a sensor's output signal shifts away from its baseline (or zero) value when there is no actual current flowing through the sensor; affects Hall-Effect Sensor

💎 Resource files