Capsules bulbs are available in both mains voltage (230v) and low voltage varieties and have a wide range of uses from main room lights to torches and under cabinet lighting. Historically all these bulbs were halogen, but now there are a range of more efficient LED options to choose from.
The halogen capsule bulbs should not be touched directly as natural skin oils will shorten the life of the bulb. Always use something to handle these lamps, such as the protective packaging they are shipped in. This is not an issue for the LED versions.
Halogen capsules are compatible with dimmer switches, but in the case of the low voltage types (G4 and GY6.35) you should check that the transformer can be dimmed too. Some LED capsule bulbs can be dimmed with a compatible LED dimmer, but some can not. Non-dimmable LED bulbs must not be used with a dimmer switch.
G9 bulbs run at mains voltage (230v). These G9 bulbs have two connectors at the base of the bulb. These connectors are often either two wire loops, some manufacturers use a solid, flat, spade type connector instead.
Most G9 bulb manufacturers embed the electrical connectors directly into the glass base of the bulb, but some add a ceramic base too.
G9 bulbs are most often used in household decorative lighting, where they are most often found inside a glass light shade. Their small size and high light output means that they can be used in main room lights.
G4 capsule bulbs are low voltage voltage bulbs. e.g. 6v, 12v, 24v, 28v. G4 bulbs run at a low wattage, with the maximum being 20w, the most efficient LED G4 bulbs use just 1w of power. The pins of a G4 bulb are 4mm apart.
The 12 volt G4 bulbs are very common in household lighting, often used in decorative light fittings which have multiple bulbs. They are also used in some small utility lights such as under cabinet lighting.
The other voltages are less common and are used in a variety of applications from torches, to bicycle lamps to commercial vehicles and boats.