By David Robinson
Updated Mar 24, 2022
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Connecting a 12‑volt light directly to a 24‑volt supply will instantly destroy the bulb. Light bulbs operate within a narrow voltage window, and exceeding that voltage can drastically shorten their lifespan or even melt the filament. Fortunately, by wiring two bulbs in series or adding an appropriately sized resistor, you can safely run 12‑volt bulbs from any 24‑volt source.
First, disconnect the power. Strip the outer insulation from the last quarter‑inch of each wire coming from the 24‑volt supply. If the wire is of the filament type, twist the exposed ends together to form tight bundles.
Using an electrician’s screwdriver, attach one end of a battery wire to the base of a bulb holder by wrapping it around the terminal and tightening it securely. Run a short length of wire from the opposite terminal of the same holder to the base of a second bulb holder. Finally, connect the remaining terminal of the second holder back to the wire that returns to the power supply.
Insert the bulbs into their holders. When the circuit is energized, each bulb will draw approximately 12 volts, leaving the other 12 volts for the second bulb. Together they will consume the full 24 volts, operating as if they were supplied by two separate 12‑volt sources.
Reduce the voltage to 12 volts by inserting a resistor into the circuit. Resistors drop voltage by converting part of the electrical energy into heat. To determine the resistor size, first find the current drawn by the bulb. Check the bulb’s wattage and divide it by the voltage difference (24 V – 12 V). For a 6‑watt bulb:
6 W ÷ (24 V – 12 V) = 0.5 A
The result is a current of 0.5 A.
Divide the voltage drop by the current to obtain the resistor’s resistance:
(24 V – 12 V) ÷ 0.5 A = 24 Ω
Thus, you need a 24‑ohm resistor.
Next, calculate the resistor’s power rating to avoid overheating. Power is resistance multiplied by the square of the current:
Power = 24 Ω × (0.5 A)² = 6 W
Use a resistor rated at least 6 W.
Place the 24‑ohm, 6‑W resistor in series with the wire leading to the bulb. When the supply is turned on, the resistor will drop the excess voltage, allowing the bulb to operate safely.
To run an LED from a 24‑volt battery, use a 1,600‑ohm resistor in series with a 3‑volt LED. If a bulb glows dimly, start with a larger resistor and then reduce the size as needed.
Most 24‑volt supplies are derived from lead‑acid automotive or marine batteries, which contain strong acid and should be handled with care. Avoid wearing metal jewelry or watches while working on these batteries, as touching metal across the terminals can produce a dangerous current capable of burning skin. Never touch a hot bulb after it has been energized; it can reach temperatures that cause severe burns.
