Robots often carry powerful batteries that can deliver very large currents in a short time. If a wire is pinched, a motor stalls, or a controller fails, that current can heat conductors fast enough to melt insulation or start a fire. Fuses, breakers, and resettable polyfuses are used to stop dangerous overcurrent before the wiring or components are damaged.
Good circuit protection is a core part of safe robot design, not an optional add-on.
Key Facts
- Ohm's law connects voltage, current, and resistance: V = IR.
- Electrical power converted to heat is P = I^2R, so heating rises with the square of current.
- A fuse should be rated above normal operating current but below the safe current limit of the wire it protects.
- Place the main fuse as close to the battery positive terminal as practical to protect the entire downstream power path.
- Each branch circuit should have protection sized for that branch's wire gauge and expected load current.
- A stalled motor can draw many times its running current, so motor circuit protection must allow brief startup surges but open during sustained faults.
Vocabulary
- Fuse
- A fuse is a one-time overcurrent protection device that melts internally and opens the circuit when current stays too high.
- Circuit breaker
- A circuit breaker is a resettable protection device that opens a circuit during excessive current.
- Polyfuse
- A polyfuse is a resettable polymer device that increases resistance when it overheats from excess current.
- Short circuit
- A short circuit is an unintended low-resistance path that can allow very large current to flow.
- Ampacity
- Ampacity is the maximum current a wire can safely carry without overheating under specified conditions.
Common Mistakes to Avoid
- Choosing a fuse only from the device current rating is wrong because the fuse must also protect the wire, connectors, and circuit board traces.
- Putting the main fuse far from the battery is wrong because any unfused wire before the fuse can still short to the robot frame and overheat.
- Using a fuse with the exact same rating as the normal running current is wrong because normal startup surges or motor acceleration may cause nuisance blowing.
- Replacing a blown fuse with a larger one is wrong because the original fuse may have opened to prevent wire overheating or a real component fault.
Practice Questions
- 1 A robot branch circuit normally draws 6 A, and its wire is rated for 15 A. Choose a reasonable fuse rating from 5 A, 10 A, 20 A, and 30 A, and explain your choice.
- 2 A shorted cable has 0.08 ohm of total resistance across a 12 V battery. Use I = V/R to estimate the fault current.
- 3 A motor controller sometimes draws 30 A for 0.5 s when a motor starts, but the wire is rated for 40 A. Explain why a time-delay fuse or breaker may be better than a fast 30 A fuse.