Powering a Halogen Lamp from 50Hz without a driver? - Page 1
Forum section: electronics / projects

author: paul_g_787
author_rank: Frequent Contributor
timestamp: September 08, 2022, 01:44:54 pm
content: Just looking at an old Disco mini flower light that needs a new lamp.Interestingly I notice that the halogen lamp (GY6.35 12V 50W) Is driven from 12V 50Hz using a just a 240V to 12V transformer.Normally I have seen these lights use a halogen lamp driver unit of some kind. I always thought these supplies would use a higher frequency to drive the lamp?Is it right to drive such a lamp from 50Hz?Are there any cons to doing it this way?

author: david77
author_rank: Frequent Contributor
timestamp: September 08, 2022, 01:53:55 pm
content: That's fine. The filament inside the lamp does not care wether it gets AC DC or some kind of HF. It's (nearly) purely a resistive load.

author: RoGeorge
author_rank: Super Contributor
timestamp: September 08, 2022, 01:57:55 pm
content: Halogen light bulb are normal filament light bulbs. They are not gas-discharge lights in need of high voltage discharge circuits to start an electric arch.Halogen light bulbs can be powered by normal AC or DC low voltage. They only need particular care with the socket, because they go terribly hot. Same, do not handle them with bare hands, and be sure there are no grease fingerprints left on the glass before powering them. The fingerprints may crack/explode the glass under the very high working temperatures.

author: coppice
author_rank: Super Contributor
timestamp: September 08, 2022, 01:59:25 pm
content: Halogen lamps usually run on a lowish voltage, as they need to keep the filament small. The tungsten-halogen cycle only works with a small quartz envelope - small to keep the glass very hot, and quartz to tolerate that heat. The only ones I've seen with controllers more complex than a transformer are high power ones that need a soft start to mitigate and enormous inrush current.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 08, 2022, 03:54:33 pm
content: They traditionally were normal mains frequency transformers.The only reason those "drivers" you mention become commonplace was cost, size and weight savings. They are just very simple switch mode power supplies which output either DC or high frequency AC.There is nothing wrong using a traditional transformer if the size and weight is OK for you.

author: ajb
author_rank: Super Contributor
timestamp: September 08, 2022, 04:13:24 pm
content: Quote from: coppice on September 08, 2022, 01:59:25 pmHalogen lamps that run directly off of mains voltage are (or, uhh,wereanyway) pretty common, actually, particularly for high power applications, like flood lights and stage lighting equipment. These do have a compact quartz envelope, of course, both for the reasons you mention as well as wanting an optically compact light source for applications involving reflectors or other optics, which are common when you've got a lamp at a few hundred watts. But mains-voltage halogen lamps have been available in all sorts of formats from the little bi-pin types to standard A-bulb formats. These latter still have a small quartz envelope, but it's inside of a larger conventional glass one, so the ridiculously hot quartz isn't such an immediate fire hazard when used in a conventional household fixture. Example:https://www.1000bulbs.com/product/59035/Q-115070.html

author: Siwastaja
author_rank: Super Contributor
timestamp: September 08, 2022, 05:25:26 pm
content: Voltage choice and halogen vs. inert gas choice are completely orthogonal design choices, both halogen and "normal" bulbs are available in whatever voltages.But generally, all incandescent lamps greatly benefit from lower voltage. This is because higher voltage requires thinner and/or longer filament (simply by Ohm's law), which then means shorter lifetime, which is compensated by not running the filament as hot, which then results in poor efficiency and warmer color temperature.In other words, adjusting design voltage allows you to shift the whole lifetime vs. efficiency compromise curve. With lower voltage, you get a lot better efficiency, or a lot better lifetime, or a bit of both.This "lower voltage" is in relation to power. For a 25W lamp, 230V is super inefficient as can be evidenced from fridge bulbs and similar, they are not much more bright than 2W flashlight bulbs! OTOH, a 2kW stage lighting bulb is quite efficient (for a lightbulb, that is) even at 230V.THIS is the real reason for using 12V on those small halogen fixtures: if you compare to 115V or especially to 230V which is even worse, you get something likedoublethe efficiency,anddouble-triple the lifetime. Even with transformer loss, it's a significant power saving. For the same reason, the color temperature is more natural, something like 3000-3200K instead of 2700K of similar intensity 230V lights.Of course, making a crappy 10 lm/W light into 20 lm/W light by low-voltage design still does not compare well to modern-day 100 lm/W LED lights. But if you need 100% CRI and 3200K color temperature, 12V light bulbs have their niche use. You can finetune the color temperature by adjusting voltage.Projector lamps did take this efficiency and color temperature thing to the extreme with 3400K color temperature getting very close to the melting point of tungsten, with 50-hour bulb life as a tradeoff. But you can't afford generating much heat because cooling the bulb and shielding the film from excess heat is difficult. And obviously, these projectors use transformers because low-voltage bulb is a necessity.

author: coppice
author_rank: Super Contributor
timestamp: September 08, 2022, 06:30:12 pm
content: Quote from: Siwastaja on September 08, 2022, 05:25:26 pmIf you make a very small high voltage filament for a halogen bulb, the filament has to be quite thin to get the required resistance in the limited space, however much you coil that filament. That's not good for the lifetime of the bulb, even though the filament continuously regenerates in a halogen bulb. It just can't regenerate consistently enough to last forever. The 240V halogen bulbs in E23 and bayonet form, which became common just before LEDs too over, didn't have nearly the lifetime that a 12V bulb of similar output has.

author: james_s
author_rank: Super Contributor
timestamp: September 08, 2022, 06:41:24 pm
content: Not only that but a longer/thinner filament has considerably higher losses from convection, this is the reason that 120V incandescent lamps are considerably more efficient than 240V lamps, and 12V lamps are considerably more efficient than 120V lamps. Enough so to overcome the additional losses of having a transformer.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: September 09, 2022, 10:06:09 am
content: On this flower light it is simply a 12V 50W halogen GY6.35mm lamp run from a 240V to 12V transformer.There is also a small PCB with it's own capacitive dropper supply powered from the 240V AC, this board has a microphone and a motor driver and just rotates the mirror to and fro using the mic.It is a very simple unit really.

author: james_s
author_rank: Super Contributor
timestamp: September 09, 2022, 05:10:10 pm
content: That was the standard way of doing it for many years. Those little pin spot lamps that were common in discos and other such venues have a 6V lamp with a small transformer in the housing. When MR16 lamps first became popular for retail display lighting those used a mains frequency iron transformer in the fixture.

author: themadhippy
author_rank: Super Contributor
timestamp: September 09, 2022, 05:20:14 pm
content: Quote.Or a load of em in series to run straight off the mains ,no tranny required,posher models often put a neon in parallel with each lamp so you could find the blown one easily.It also meant the slip rings and brushes were carrying a lower current

author: vk6zgo
author_rank: Super Contributor
timestamp: September 12, 2022, 12:23:37 am
content: Quote from: coppice on September 08, 2022, 06:30:12 pmThey did, however "run rings around" that invention of the devil---CFLs!Even the early LED bulbs were crappy, & would gracefully fade to a noticeably lower light output over less than a year.Really cheap ones still do!In terms of lighting effectiveness, little matches big old clunky Fluoro tubes------except maybe LED "pretend ones"!

author: james_s
author_rank: Super Contributor
timestamp: September 12, 2022, 02:31:17 am
content: Quote from: vk6zgo on September 12, 2022, 12:23:37 amNot even close, the most efficient halogen lamps for general illumination are somewhere around 22 lm/W. Those curly compact fluorescent lamps were typically around 55 lm/W. Yes they had issues but efficiency was miles ahead of any incandescent lamp, even after fading a bit. Typically the brightness would degrade rapidly through about the first 100 hours and then settle to a gradual decline. End of life was considered 70% of initial output.

author: coppice
author_rank: Super Contributor
timestamp: September 12, 2022, 04:01:57 pm
content: Quote from: vk6zgo on September 12, 2022, 12:23:37 amThere was a huge range of CFL quality. Many had weird spectra, often a spooky looking greenish colour. Many had flicker much worse than most full sized florescent lamps. Many had a short life, especially when used in a lamp shade or luminaire that restricted air flow. However, there were also the Matsushita (National, Panasonic, or whatever name they used in your area) which had none of these issues. Great colour. An electronic ballast eliminating any flicker. A long long life, with only very gradual fading, and little blackening.Quote from: vk6zgo on September 12, 2022, 12:23:37 amThe fading of some makes of early LED bulb could be pretty rapid. Some were not that bright, even when new. Some had funky colours, too. They aren't very reliable now, but its a while since I've seen the severe fading I saw a few years ago.