G4 capsule bulbs and 12-volt lighting systems

G4 is a tiny capsule with thin straight pins that usually runs on 12 V. Where it is used, how it works with a transformer, and what goes wrong when switching to LED.

A G4 bulb is small enough to lose in your palm: a capsule a couple of centimetres long with two thin wires coming out of the bottom. You find it in under-cabinet kitchen lights, display cases, some chandeliers and in low-voltage systems of the kind used in vehicles. To understand this bulb you need to understand the 12-volt system behind it more than the bulb itself.

How to recognise a G4 base

A G4 has two straight, thin pins about 4 mm apart. They are not looped, which immediately distinguishes it from a G9. The bulb pushes straight into the holder with no twist.

There is also the similar-looking GY6.35 base, whose pins are slightly thicker and further apart. Measuring the pin spacing or taking the old bulb to the shop avoids buying the wrong one.

Why 12 volts

Most G4 bulbs run on 12 V. Low voltage was chosen for several reasons: the bulb can be very small, exposed contacts are less hazardous, and installation inside furniture and display cases becomes simpler.

Household mains, however, is 220–240 V, so every G4 system has a device that steps the voltage down:

• Magnetic transformer: heavy, with an iron core, giving an alternating (AC) output.
• Electronic transformer: light, designed for halogen lamps, also AC, but with a minimum load requirement.
• LED driver: constant voltage, giving a direct current (DC) output.

Choosing a bulb without knowing which device you have is the root of most G4 problems.

What goes wrong when switching to LED

Flicker and blinking. Electronic transformers built for halogen lamps do not start reliably below a certain minimum load. Replace several halogen G4s with the same number of LED G4s and the total load drops sharply, leaving the transformer unstable. The cure is a constant-voltage driver designed for LEDs.

Polarity. Some LED G4s run on DC only and care about polarity: inserted the wrong way round, they do not light. Bulbs marked "12V AC/DC" work on either supply and in either orientation. If your existing transformer gives AC, choose AC/DC bulbs only.

Wrong voltage. 220 V G4 LEDs are also on sale. A 220 V bulb in a 12 V system simply stays dark; the reverse, a 12 V bulb connected straight to the mains, burns out instantly and is dangerous.

Size and heat. Some LED G4s are flat discs with diodes on a board, wider than a halogen capsule, and may not fit a fitting with a small glass cover. LEDs still give off some heat in an enclosed cabinet recess, so airflow should not be cut off completely.

Working out what system you have

• Find the power box behind the fitting or furniture and read its label: "Output 12V AC" or "12V DC".
• Note its rated power (W) and any "min. load" figure.
• Count how many bulbs are connected to that box.
• Write down the voltage and wattage printed on the old bulb.

With that information you can pick both the right bulb and, if needed, a new power supply.

Sizing a new LED driver is simple arithmetic: add up the wattage of all the LED bulbs that will run from it and choose a driver rated somewhat above that total. A driver running at its full rating with no margin runs hot. At the same time there is no need for a hugely oversized unit; it just takes up more space. If the furniture has several separate lit zones, giving each zone its own small driver makes faults easier to trace and later changes easier to make.

A short safety note

Even though the output is 12 V, the input side of the transformer is connected to the mains. Switch the system off before changing a bulb, and leave opening a power supply or wiring in a new one to an electrician. A transformer should not be buried in thermal insulation or tucked into an airless closed space either.

Alongside small low-voltage bulbs, every home also needs lamps that run directly off the mains for its main lighting. In the Zəfər İşığı catalogue those LED lamps can be found by base type.