Voltage drop
Voltage drop is the phenomenon where current passing through a part such as a resistor consumes and loses voltage by V=IR; in a series circuit the sum of each part's drops must equal the supply voltage, a direct consequence of KVL. With a 12V battery and two 6Ω resistors in series, 1A flows and each resistor drops 6V, so the total 12V matches the supply. When large motor currents flow, as in a kickboard BLDC, drops across wiring, MOSFETs, shunts, and battery internal resistance turn into weakened output and heat, making this the starting point for voltage-divider, internal-resistance, and power-loss calculations.
The concept in one line, with a water analogy
- Voltage drop is the amount of voltage consumed, equal to V=IR, as current passes through a part.
- By the water analogy it is like water pressure dropping through a narrow pipe: the battery pushes voltage, and it falls as current passes the resistor (the narrow pipe).
- Ohm's law (V=IR) gives each part's drop, and KVL guarantees that their sum equals the supply voltage.
Working it with numbers — 12V with two 6Ω in series
- Total resistance R = 6 + 6 = 12Ω, and total current I = V/R = 12V/12Ω = 1A.
- Each resistor's drop is V_drop = I × R = 1A × 6Ω = 6V; following the circuit, 12V falls to 6V after the first resistor and to 0V after the second.
- The drops sum to 6V + 6V = 12V, exactly the supply voltage, as guaranteed by KVL.
- voltage drop at that part [V]
- current through the part [A] — same everywhere in series
- resistance of that part [Ω]
Kickboard/STM32 context — where drops occur
- Wiring has small resistance, but large current makes its drop grow, lowering motor voltage and weakening output.
- A MOSFET's Rds(on) turns current × Rds(on) into heat that lowers efficiency, while battery internal resistance causes voltage sag where terminal voltage falls as load grows.
- A shunt resistor uses an intentional drop to measure current, and ultimately the answer to 'why does the motor weaken under large current' is voltage drop.
A voltage drop is not waste but each part taking its share by V=IR, and KVL guarantees their sum equals the supply voltage. Heat is I²×R (power loss), so at large currents even a small drop produces large heat — the basis for choosing wire gauge and MOSFETs.