testing pin type bulbs

[Garth Liberman]

My lighting set up was only up for three weeks before one of my 20W pin type bulbs blew. I know that because the system came in place one run at a time (there are four total), premature bulb failure was most likely due to have only 47 watts running on a 200 watt transformer. Now that all runs are in place, the transformer has a more appropriate load of 127 watts total.

The question is how do you test a pin bulb to see if it is good without installing it in a known good lamp? I used an ohms meter which I uses a single 1.5 volt AA battery to power it. The good bulb showed continuity while the bad one showed none even though visually the filimant was unbroken. Will 1.5 volts somehow damage these expensive bulbs? Is there a better way to test halogen bulbs?

[sitelights]

Your multimeter with a continuity test mode is just what you need to check filament separation. When examining a failed lamp it is not always apparent that the filament is broken. You'd be surprised at what high magnification, such as a jeweler's loupe, reveals. The lower the wattage the thinner the filament; compare a 50w bi-pin to a 20w bi-pin.

Those with a keen interest in this topic would do well to procure a lamp manufacturer's catalog and peruse it closely; it is a bit like reading the white pages of a telephone book in that you must wade through a lot of irrelevant material to find the specific information you seek.

My well-thumbed Osram-Sylvania "Lamp & Ballast Product Catalog 2002" has 286 densely packed pages of small type but only about 3 pages list 12v halogen lamps of various types. All of the lamp manufacturers publish catalogs of their products and they are free for the asking. In addition to the O-S catalog I have a shelf of catalogs from Ushio, Philips, GE and others but almost any one will do. Some makers publish separate "minature lamp" catalogs also information packed.

The O-S catalog before me has sections dealing with energy costs, efficacy (the lumen or footcandle output per watt of energy), Color Rendering Indices or Index (CRI), lamp base types, filaments, bulb sizes and photometric data along with technical matters in which i have no interest: HID or fluorescents for example. There is a 3 page glossary of terms related to electric lighting that warrants close attention.

What to look for in a 20w bi-pin is a vertical ("burner") filament; this type has a slight mechanical advantage over the horizontal filament as it is less prone to sag when hot in a base up or base down position. Note that the filament type in a halogen lamp is designated "C" (gas filled) and "CC" (coiled coil) i.e the tungsten filament is coiled and then wound again into a coil.

I have been recommending xenon wedge base lamps in applications where 20w or less is needed but fixture makers have been slow to adopt this socket in place of a bi-pin socket. Bi-pin xenon lamps can be used as direct replacements for halogens again at 20w or less. Using the following individual key words on the site search engine will take you to more detailed discussions on this topic; (number of hits): xenon (13); halogen (25); wedge (9); bi-pin (31); socket (48); lamp life (22)...you get the idea.

Lamp life is a topic fraught with difficulties since the many factors: overvoltage, undervoltage, in-rush current, filament notching, mechanical shock, cycle (on/off) frequency and "dirty" power company current to name a few are essentially unknowable in combination and would take a Cray mainframe computer forever to analyze.

Lamp life has been the bane of my professional lighting career; we warrant lamp life for three years which is risky business indeed. We use top-of-the-line (read "expensive") lamps since our cost for a no-charge service call is more in the travel and labor than in parts. The homeowner is always on-site so the use of cheap, shorter lived lamps may actually be more cost effective for them.

Here I am on New Year's Eve pontificating on lamp life; maybe I should get a life too.