🔧Circuit Design Applications
Power Supply Design
USB Power Bank
Capacity: 10,000 mAh (10 Ah)
Output: 5V @ 2.4A max
Power: P = V × I = 5V × 2.4A = 12W
In mA: 2,400 mA output current
Runtime: ~4 hours at max load
LED Driver Circuit
LED Forward Current: 350 mA
Supply Voltage: 12V
LED Forward Voltage: 3.2V
Required Resistor: R = (12V - 3.2V) / 0.35A = 25.1Ω
Use 27Ω resistor (nearest standard)
Motor Control Systems
Servo Motor Sizing
Rated Current: 8.5A continuous
Peak Current: 25.5A (3× rated)
Supply Voltage: 48V DC
Controller Rating: Must handle ≥30A
Choose 40A motor controller for safety margin
3-Phase Induction Motor
Motor Rating: 15 HP (11.2 kW)
Line Voltage: 480V
Full Load Current: 22A per phase
Starting Current: 6-8× FLA = 132-176A
Use soft starter to limit inrush current
📊Current Measurement Techniques
Measurement Methods
Shunt Resistors
Principle: V = I × R
Example: 100A shunt, 75mV drop
Resistance: R = 0.075V / 100A = 0.75mΩ
Scaling: 1mV = 1.33A
High accuracy, low cost
Current Transformers
Ratio: 1000:5 (200:1)
Primary: 1000A AC
Secondary: 5A AC
Burden: 5VA at 5A
Isolated, safe for high currents
Hall Effect Sensors
Range: ±50A to ±2000A
Output: 2.5V ± 625mV/A
At 20A: 2.5V + (20 × 0.625mV) = 2.5125V
Resolution: ~40mA
DC/AC, non-invasive
Digital Multimeter Ranges
| Range | Resolution | Accuracy | Best Use |
|---|---|---|---|
| 200 µA | 0.1 µA | ±0.5% | Microcontroller sleep current |
| 2 mA | 1 µA | ±0.5% | LED circuits |
| 20 mA | 10 µA | ±0.8% | Logic circuits, sensors |
| 200 mA | 0.1 mA | ±1.2% | Small motors, relays |
| 10 A | 1 mA | ±2.0% | Power tools, appliances |
⚠️Electrical Safety Guidelines
Current Safety Thresholds
Human Body Effects
1 mA: Barely perceptible
5 mA: Maximum safe current
10-20 mA: Muscular control lost
50 mA: Ventricular fibrillation possible
100-200 mA: Certain ventricular fibrillation
Always use GFCI protection (≤5mA trip)
Circuit Protection
15A Breaker: Household outlets
20A Breaker: Kitchen, bathroom circuits
30A Breaker: Electric dryer, water heater
50A Breaker: Electric range
100-200A: Main panel service
Size conductors for 125% of load current
Arc Flash Hazards
Low Voltage (<1kV)
Available Fault Current: 10-50 kA
Arc Flash Boundary: 4-10 feet
PPE Category: 1-2
Requires flame-resistant clothing
Medium Voltage (1-35kV)
Available Fault Current: 5-25 kA
Arc Flash Boundary: 10-50 feet
PPE Category: 3-4
Requires arc-rated suit
High Voltage (>35kV)
Available Fault Current: 1-40 kA
Arc Flash Boundary: 50+ feet
PPE Category: 4+
Remote operation required
💡Current Conversion Tips
Quick Conversions
Ampere Hour Conversions
1 Ah = 1,000 mAh
100 mAh = 0.1 Ah
Battery life: Capacity (mAh) ÷ Load (mA) = Hours
Power Calculations
DC Power: P = V × I
AC Power: P = V × I × cos(φ)
3-Phase Power: P = √3 × VL × IL × cos(φ)
Design Guidelines
Wire Sizing
18 AWG: Up to 7A (16A max)
14 AWG: Up to 15A (20A max)
12 AWG: Up to 20A (25A max)
10 AWG: Up to 30A (40A max)
Derating Factors
Temperature: -2%/°C above 30°C
Bundling: 0.8× for 4-6 conductors
Continuous load: Use 125% of rated current