Fit a New Light Bulb

Last year we decided to add a room onto our house. In consequence, at one point we had to go down to the local DIY warehouse to pick out fixtures for the new room.

While there, we fell in love with a dining room lamp of, unusual design. It's hard to describe. Some visitors have said it reminds them of the insides of the Tardis; others are less complimentary. Apparently you either love it or you hate it.

The essence of the lamp is that a 250-watt halogen bulb is nestled in a metal reflector that looks like a long skinny pyramid with one of the flat sides missing. The missing side is on top, so the bulb shines up. It shines into a large curved frosted glass plate, which diffuses the light. This plate, plus the fact that the bulb is facing upwards, means that virtually no direct light escapes. The result is a beautiful even glow. The whole thing is held together by a network of gold-plated metal rods and suspended from the ceiling.

After it was installed but before we had moved the dining table under it, we discovered why it is important to have a table there -- a friend of ours was visiting and bumped his head on the lamp when he got up. No damage to his head or to the lamp, but the light went out. Oh well, time to replace the bulb.

So, wait for the thing to cool, off with the safety shield, reach fingers cautiously into the metal pyramid, and try to unscrew the bulb. It doesn't budge. Now, this isn't your ordinary light bulb ... it's a quartz cylinder about a
centimetre in diameter and four or five centimetres long mounted on a ceramic base. There's almost no room to get one's fingers into where the bulb is mounted, so one can't apply much torque. Can't apply enough, anyway, because it doesn't budge.

Well, maybe it's a bayonet socket? Try pushing in on it. It doesn't budge that way either. Finally take the whole lamp down from the ceiling. I'll spare you the details of that operation, except to say that I somehow managed to avoid destroying the whole lamp. Once I have it down, I can see into the socket enough to see that it is indeed threaded. It still won't budge.

Well, I know the answer to that problem -- a baby Mole-Grip! Fortunately I have one just the right size to chomp onto the ceramic base. Even more fortunately, I don't break the base. Finally, with much effort, the bulb turns and I manage to unscrew it. Just for fun, I check it with an ohmmeter. Nothing wrong with it!

Now I'm really getting frustrated. I use the ohmmeter to check continuity on the socket. That works fine too. I guess it just wasn't seated quite right and wasn't making contact. OK, let's clean off the bulb carefully and try to put it back.

Putting it back is easily as much of an effort as taking it out. That silly socket just doesn't fit right. But with a little help from the Mole-Grip, I get it back in. The ohmmeter now tells me it's making contact, so I reinstall the lamp, turn it
on, and everything works.


That was a year ago. The bulb burned out again last week, after making little sizzling noises for a few days. I had even more trouble getting it out of the socket than last time, but again the Mole-grip came in handy. When I took a close look at the bulb, I saw what had happened. The filament was fine, but apparently the socket was still not making real good contact. The result was arcing inside the socket that generated enough heat to melt the little solder droplet that held the ground lead to the threads (not the solder button on the tip; the other one). Well, I had a spare bulb so I tried installing it.

That light bulb just wouldn't find its way into the socket at all. I could thread it in a little way and then it wouldn't budge. What to do? Well I remembered I had a can of high-temperature silicone lubricant. I sprayed some of that on a facial tissue, wiped it on the threads, and tried reinstalling it. A great improvement -- it was now merely difficult instead of virtually impossible. I tightened it down and tried it out -- there was light! There were also the little sizzling sounds I had associated with the arcing before. Out with the Mole-grip to tighten it down some more. Still sizzles, and indeed the light was flickering a bit too. That's no good. Wait for it to cool, start to unscrew it, and the thing comes apart in my hand.

By that I mean that the quartz envelope broke right off the ceramic base. Gee, I didn't know I was that strong. I went down to the store where I bought the lamp and explained the whole thing to the salesman. He gave me two new bulbs and suggested I put petroleum jelly on the threads.

Back home, I tried the petroleum jelly trick. The second bulb came apart in my hands too. After thinking hard about it, I realized what had happened. There are only about two millimetres of clearance between the envelope and the closest part of the reflector. While trying to tighten things, I had bent the socket slightly away from its usual position; the first time my finger slipped the socket snapped back, cracking the envelope against the reflector.

Well, no problem -- just bend the socket a little away from the reflector. Now put the bulb back, with petroleum jelly on the threads, and tighten it down carefully. Turn the lamp on again, and everything works. Not even a flicker.

Of course, once the lamp warms up a bit, it starts smoking. This I attribute to the petroleum jelly. After a few hours the smoking stops. Then the crackles start up again. I turn off the lamp, let it cool, tighten the bulb a bit more, turn it on again. The crackles are still there but much fainter. Oh well, nothing to do but see what happens.


What happens is that the bulb burns out after about two days. It takes the Mole-Grip to get it out of the socket -- my fingers just aren't strong enough. In disgust and utter frustration, I cart the entire lamp back to the store.


The people at the store are very nice. They call the importer, who promises to send them a new socket for them to use to repair the lamp. I'd rather have them do it at this point. They also offer to lend me a fixture to use while waiting for the repair.

After much discussion, we pick out a simple one: three little reflectors on hinges, each of which can take a PAR20 bulb. For those of you who don't know what a PAR20 is, it's a miniature reflector flood, with the front about two inches in diameter. I take the fixture home, install it, put the bulbs in, turn it on, and NOTHING HAPPENS.

This is crazy. Could the breaker be blown? I go to the basement and check that. No problems there. What about the switch? Aha! There isn't one -- there's a high-tech solid state dimmer, one of those jobbies with a little row of LED's to tell you if it's turned on and make it easy to find in the dark. Evidently when the other lamp burned out, it somehow took the dimmer with it.

Well, I know how to prove that. Throw the breaker in the basement, remove the dimmer, substitute an old-fashioned switch. Back to the basement, turn the breaker back on, back upstairs, turn on the switch while holding my breath -- IT WORKS!

So I know the dimmer is bad. Fortunately I bought it at the same store. Run back there before they close, dimmer in hand. They give me a new dimmer, I go back home, turn off the breaker, remove the switch, install the dimmer, put everything back together, turn it on -- NOTHING!

This is completely crazy. I'm not going to believe I got a bad dimmer right out of the box immediately after another dimmer failed. Something else has to be wrong. But what?

There's one other possibility. These dimmers work by leaving a trickle current flowing through the lamp even when they're nominally off. That's how they can light their LED's. What if it isn't getting enough current? Look in the directions for the dimmer -- sure enough, they say that a minimum load of 60 watts is necessary for the dimmer to work. Well, I have three bulbs of 50 watts each; surely that's enough? And I know the bulbs work because I saw them work when the switch was there.

Just for fun, I remove the PAR20 bulbs and put conventional 60-watt bulbs there. The dimmer works! So 150 watts isn't enough, but 180 watts is! Well, I've seen incorrect documentation before; let's prove that that's the problem. Take out two of the 60-watt bulbs, try again with just the third. It still works! Hmm... what if I put two PAR20 bulbs in the now-empty sockets? IT STILL WORKS!!!

So apparently the presence of a conventional bulb is necessary to make the dimmer work. That makes no sense at all. Wait a minute, the resistance of a light bulb changes greatly with temperature. What if these PAR20 bulbs have an unusually high resistance when cold? Then the dimmer might not be getting enough current.

Well, out with the ohmmeter and check one of the PAR20s. Infinite resistance! That's impossible!! Try it again and it's about a thousand ohms! I guess the contact was lousy. Try it a few more times, and it's still a thousand ohms. What happened the first time? Did I do anything differently?

Yes I did -- I reversed the contacts. That can't make any difference, or can it? Yes, it does! These silly light bulbs act like rectifiers at low voltage. Well, what if the dimmer requires current in a particular direction to work? That would cause precisely this problem.

If that's true, how can I prove it? I know -- I'll swap the wires on the dimmer. It's possible to connect it either way in any event. So I swap the wires and put it all back together. This time, the red LED lights on the dimmer, but faintly, and it still doesn't do anything. Put one conventional bulb back in, and it works fine.

So that's clearly the problem -- these silly light bulbs act like diodes at low voltages. I wonder if there's some kind of soft turn-on circuit for them; that might account for it.


At this point, then I have a working temporary fixture, albeit without the little reflector floods. It gives light, anyway. I'm not going to touch anything until the other lamp comes back from the repair shop.

Then maybe they'll be a Part II.