What is the conversion relationship between current units?
The SI base unit for current is the ampere (A), with prefix units using a ratio of 1000: 1 A = 1000 mA (milliamperes) = 1,000,000 μA (microamperes). Adjacent units differ by a factor of 10³: 1 mA = 1000 μA, 1 μA = 0.001 mA, 1 mA = 0.001 A. That is, multiplying by 1000 when moving A → mA → μA to the right, dividing by 1000 when moving left; this is standard SI thousand-based scaling, consistent with other electrical units like voltage (mV/V/kV) and capacitance (pF/nF/μF/F).
How is the ampere (A) defined?
The ampere (symbol A) is one of the seven SI base units, named after French physicist André-Marie Ampère. After the 2019 SI redefinition, the ampere is derived from the elementary charge e (exactly 1.602176634×10⁻¹⁹ C): 1 ampere equals 1 coulomb of charge passing per second (1 A = 1 C/s), corresponding to approximately 6.2415×10¹⁸ electrons passing through a conductor cross-section per second. Previously the ampere was defined via the force between two parallel current-carrying wires; it is now defined using fundamental constants with greatly improved accuracy.
How do you calculate current in Ohm's Law? What is the relationship with voltage and resistance?
Ohm's Law is the most basic circuit formula: I = U/R (current = voltage/resistance), P = U·I = I²·R = U²/R (power-current relationship). Current I is in amperes (A), voltage U in volts (V), resistance R in ohms (Ω), power P in watts (W). For example, a 5V supply connected to a 1kΩ resistor gives current I = 5V / 1000Ω = 0.005 A = 5 mA; a 12V/10Ω load draws 1.2A with 14.4W power. This tool helps you quickly switch calculated current values between A/mA/μA.
What magnitude of working current do common appliances/devices have?
Common current magnitude reference: ultra-low power IoT device sleep current 0.1~10 μA; MCU standby tens of μA, working current few~tens of mA; typical LED indicator 5~20mA; USB 2.0 port max 500mA, USB 3.0 max 900mA, USB-C PD up to 3-5A; phone charging 1-3A; small appliances (fans, routers) 0.1~1A; laptop adapters 2-5A; electric kettles, rice cookers 6-10A (1-2kW at 220V); air conditioner starting current 10-20A; EV DC fast charging 100-350A (400-800V); lightning strikes can instantly reach tens of thousands to hundreds of thousands of amperes.
How much current is dangerous to the human body? What is the safe current?
Current hazards to the human body depend primarily on current magnitude and duration (IEC 60479 standard): no sensation below 1mA; tingling but self-releasable at 1-5mA; 10-20mA is the "let-go threshold" (muscle contraction prevents releasing the power source); above 50mA of power-frequency AC through the heart for 1 second can cause fatal ventricular fibrillation; above 100mA is almost always fatal. Human body resistance in dry conditions is 10-100kΩ, 220V contact current is about 2-22mA (dangerous); in wet conditions resistance drops to 1kΩ, 220V current can reach 220mA (fatal). Therefore household RCD leakage protectors typically have an operating threshold of 30mA with operating time <30ms.
Why is the microampere (μA) important in low-power design?
For battery-powered IoT/wearable devices, standby current directly determines battery life: assuming a 200mAh coin cell, device sleep current of 1μA gives theoretical battery life of about 200mAh/0.001mA = 200,000 hours ≈ 22 years; if sleep current is 100μA it's only 2000 hours ≈ 83 days. Therefore when designing smart watches, Bluetooth beacons, and wireless sensor nodes, the quiescent current (Iq) and shutdown current of MCUs, sensors, and LDO regulators must be optimized to μA levels. Hardware engineers often need to convert working currents in A/mA and sleep currents in μA to the same unit to calculate average current and battery life.
What are nA/pA current units? What if I need to convert them?
Nanoamperes (nA = 10⁻⁹ A) and picoamperes (pA = 10⁻¹² A) are smaller current units mainly used for: photodiode dark current (nA level), electrochemical analysis (pA level for voltammetry), semiconductor device leakage current testing, electrometer/picoammeter measurements. This tool currently covers the three most commonly used SI units μA/mA/A; if you need nA/pA you can manually convert using thousand-based ratios: 1 μA = 1000 nA, 1 nA = 1000 pA, meaning multiply μA value × 1000 = nA value.
Why do many people write uA instead of μA?
μ is the Greek letter Mu, which does not appear on standard English keyboards; in early SPICE circuit simulation, C programming, ASCII serial ports, and silkscreens it was inconvenient to input μ, so the electronics engineering industry universally used the Latin letter u as a substitute for μ (similar pronunciation), making uA = μA a conventional notation (e.g., 100uA = 100μA). The markings uA/mA/A are commonly seen on PCB silkscreens, multimeters, oscilloscopes, and power supply panels.
Why is current unit conversion 1000 instead of 1024?
Current is an SI physical quantity; all prefixes (m, μ, n, p) are strictly defined by powers of 10 (10⁻³, 10⁻⁶, 10⁻⁹, 10⁻¹²), with a conversion ratio of 1000. Only computer storage (bytes B) uses the binary factor 1024 (1KB=1024B); all physical quantity units strictly use 1000. Therefore conversions for electrical units like current, voltage, capacitance, resistance, frequency, and power all use a 1000 ratio.
How do I select the range on a multimeter's current setting?
Digital multimeter current ranges are generally divided into: μA range (0-200/2000μA weak current), mA range (0-20/200mA), A range (0-10/20A note some have dedicated high-current jacks). Range selection principle: first test on the high range (A range), if the reading is small then switch down to mA/μA ranges to avoid blowing the meter from overrange; after measuring always return the test leads to the voltage/resistance jacks to avoid short-circuiting the multimeter or circuit when accidentally measuring voltage next time. This tool helps you estimate which order of magnitude the measured current belongs to, so you can select the correct range.
What does the 'C' in battery charging current mean? How does it relate to mA?
C-rate is a current unit relative to battery capacity: 1C current = the current that fully charges/discharges the battery in 1 hour. For example, with a 2000mAh battery: 1C = 2000mA = 2A charging current; 0.5C = 1000mA, fully charges in about 2 hours; 2C = 4000mA = 4A fast charging charges in about 30 minutes. Standard lithium battery charging is typically 0.5C-1C; fast charging can reach 2C-5C (battery must support). When converting C-rate to mA multiply by the battery capacity (mAh); this tool helps you convert A to mA to view charging current.
Will entering my current value get uploaded? Is privacy secure?
No upload at all. The entire conversion logic uses pure front-end JavaScript running locally in your browser; the current values you enter generate no network requests, and the server cannot see any of your circuit parameters or test data. Input content is cleared immediately when you close the tab, with no cookies or localStorage used. Suitable for enterprise R&D, repair work, student exams, military electronics, and other data privacy-sensitive scenarios.
