G4, G6.35 and G9 Bulbs: Differences, Product Links and Why Old Lutron Dimmers Still Use Halogen

Small capsule lamps are still common in kitchen fittings, cabinet lights, chandeliers, display lighting and older commercial control systems. The names can be confusing — G4, G6.35/GY6.35 and G9 all look like small two-pin bulbs, but they are not interchangeable. The pin spacing, voltage and control gear matter.

At APM Plumbing & Electrical in Acton, customers still ask for halogen capsules because many older fittings, transformers and dimming systems were designed around the steady electrical load of halogen lamps.

Quick shop: G4, G6.35/GY6.35 and G9 bulbs

If you already know the lamp type, use these direct APM product links. Always match the old lamp's base, voltage and wattage before ordering.

G4 LED bulbs

G4 and G6.35/GY6.35 halogen bulbs

G9 LED bulbs

G9 halogen bulbs

Transformers, drivers and dimmer modules

For low-voltage G4 and G6.35/GY6.35 lamps, the transformer/driver matters. Old halogen transformers often need a minimum load, so LED replacements can flicker or fail to start if the transformer is not suitable. Use a proper LED driver for LED lamps, or keep halogen lamps on a compatible halogen transformer.

Browse more: search G4, G6.35 and G9 bulbs or browse all light bulbs.

What does G4, G6.35 and G9 mean?

The letter G means the lamp has a two-pin base. The number normally refers to the distance between the pins in millimetres.

If the pins do not line up easily, do not force the lamp. A G4 will not safely replace a G6.35, and a low-voltage G4 or G6.35 lamp must not be fitted into a mains-voltage G9 holder.

G4 capsule bulbs

G4 lamps are very small two-pin capsules with 4mm spacing. They are usually used on 12V systems with a transformer. You will often find them in under-cabinet lighting, small bathroom fittings, display cabinets and older decorative lights.

Because G4 halogen lamps were low wattage but still gave a warm, bright light, they were popular before LED became standard. LED replacements are now common, but compatibility depends on the transformer and dimmer.

Shop G4 options:

• Integral LED G4 1.1W 2700K 100lm non-dimmable
• Integral LED G4 1.1W 4000K 100lm non-dimmable
• Integral LED G4 1.6W 2700K 200lm non-dimmable

G6.35 / GY6.35 halogen bulbs

When customers say “G6.2”, they usually mean G6.35 or GY6.35. These lamps have pins spaced 6.35mm apart, normally used on 12V low-voltage lighting. Compared with G4, G6.35 lamps are often used for higher-output capsule applications.

They are still useful when the fitting was designed for a halogen capsule and the customer needs reliable dimming, instant full brightness and the same warm colour temperature as the original lamp.

Shop GY6.35 halogen:

• Philips Halogen GY6.35 12V 50W Capsule Bulb — 2700K Warm White, dimmable

G9 halogen bulbs

G9 lamps use 9mm loop pins and are normally mains-voltage lamps. They are common in chandeliers, wall lights, bathroom lights and decorative fittings where a compact but bright capsule is needed.

G9 halogen lamps are often easy to dim on traditional dimmers because the lamp behaves like a resistive load. LED G9 lamps use much less power, but some older dimmers and lighting controllers do not see enough load to dim smoothly.

Shop G9 halogen and LED options:

• Halogen G9 240V 20W
• Eveready G9 Halogen Light Bulb 25W Clear
• Meridian G9 240V Energy Saving Lamp
• Integral LED G9 2.7W 2700K 300lm dimmable
• Knightsbridge 230V 4W G9 LED dimmable lamp 4000K

Why are halogen bulbs still used with old Lutron and other lighting controllers?

Many older dimmers and lighting control systems — including older Lutron systems, leading-edge dimmers, magnetic transformer circuits and some scene-setting controllers — were originally designed around incandescent or halogen lamps.

Halogen lamps are simple resistive loads. That means they draw a steady amount of power and usually dim smoothly from high to low when matched with the correct dimmer or transformer. This is why some older lighting systems still behave better with halogen than with LED.

Common problems when changing halogen to LED

• Minimum load: old dimmers may need a minimum wattage. A 50W halogen may work perfectly, but a 4W LED may be too low.
• Transformer compatibility: 12V G4 and G6.35 circuits may have electronic or magnetic transformers. Some LED capsules need an LED-compatible driver.
• Flicker or shimmer: LEDs can flicker on older leading-edge dimmers or control modules.
• Poor dimming range: the light may jump on, drop out, buzz, or refuse to dim below a certain level.
• Mixed lamps: mixing halogen and LED on the same circuit can sometimes help load, but it is not always a proper long-term fix.

For old Lutron, Grafik Eye style systems, older scene plates, or other lighting control systems, electricians often test one fitting first before replacing every lamp. Sometimes the best answer is a compatible dimmable LED and a suitable trailing-edge dimmer. Other times, keeping halogen in place is the simplest reliable option until the control gear is upgraded.

Dimmer and controller options

If you are moving from halogen to LED, the dimmer matters as much as the bulb. Check the minimum and maximum load, whether the dimmer is leading-edge or trailing-edge, and whether the LED lamp is actually dimmable.

Shop dimmers and lighting controls:

• Knightsbridge 1G 2-way 10–200W intelligent dimmer, brushed chrome
• Knightsbridge 4G 2-way 60–400W leading-edge dimmer, polished chrome
• Browse dimmers at APM

Quick buying checklist

• Check the pin spacing: G4 = 4mm, G6.35 = 6.35mm, G9 = 9mm.
• Check voltage: G4/G6.35 are usually 12V; G9 is usually 230V.
• Check wattage and heat limits of the fitting.
• Check whether the lamp must be dimmable.
• For old Lutron or other controllers, check minimum load and transformer compatibility before changing to LED.

Need help matching a capsule lamp? Bring the old bulb into APM Plumbing & Electrical, 24 Western Avenue, Acton, London W3 7TZ, or browse our light bulbs online. We can help identify the correct fitting and suggest a suitable halogen or LED replacement.

Visit us at APM Electricals, 24 Western Avenue, Acton, London, W3 7TZ or call 020 8896 0800 for trade and retail electrical and plumbing supplies.