Electrical Troubleshooting 101 Pop Quiz

[AJ1G]

So there I was, trying to sort out the various loads and circuits  of the power panel at our summer cottage in Charlestown RI.   I was looking at the circuit for the kitchen overhead light.  I was pretty certain I knew which circuit it was on, but when I looked into the circuit with an ohmmeter  (with the circuit disconnected from its supply breaker of course) looking for the resistance of the bulb to come and go as its switch was placed in on and then off, I was always seeing an open circuit.  Thinking that maybe by some weird coincidence, the bulb had failed, I checked the fixture.  The installed bulb was in fact OK, but I did have to replace it with another one before I could and did  confirm the circuit with the ohmmeter.  What do you think the problem was with the original bulb?

[K6JEK]

Was not an incandescent bulb

[W2PFY]

The lead blob soldered on the bottom is flat and not making contact. Da New bulb probably has a nice round tip on it.

[KL7OF]

switch is bad?

[steve_qix]

There are a number of possibilities    By the nature of the question, I'm going to assume an incandescent bulb.  Obviously, a CFL, LED or other non-incandescent technology bulb will behave rather differently and may in fact even present an open circuit.  If this is not the case, some other possibilities are:

1) Corrosion between the bulb and the fixture either in the screw base or the center

2) A hairline fracture in the filament which may, when "bumped" close (I've seen this quite a bit with incandescent bulbs)

3) The soldered connection at the screw portion of the bulb's base was poor, cold, open or intermittent

4) The solder at the center conductor of the bulb's base was poor, cold, open or intermittent

5) The bulb was not screwed all the way into the fixture for some reason

6) The bulb was incompatible with the fixture, such as a 3 way bulb, etc.

7) .... and there are a whole lot more!

[W7TFO]

It was just you.

73DG

[KF1Z]

Since he said  "...The installed bulb was in fact OK, but I did have to replace it with another one before I could and did  confirm the circuit with the ohmmeter."""


So, the bulb was OK, but had to be replaced before he could check it with an ohmmeter,..

I have to assume it was a compact flourescent, or LED bulb etc.

[AJ1G]

K6JEK and KF1Z - go to the head of the class!  And I'll put on the dunce cap.....I had forgotten that when I put in the fixture ( an inverted frosted glass dome, with a flat pan type base that takes two bulbs) a few years back I had put in two CFL bulbs in lieu of conventional incandescent bulbs.  I normally don't like them , but used them in this application to cut down on the heat generated, because the ceiling is some sort of fiberboard tile, like acoustic tiles without the holes.  i haven't looked up what the circuit of a CFL bulb looks like,but apparently there is some sort of capacitor in series with either the hot/back lead or neutral/white lead that of course has a very high DC impedance.  A major DOOOH Homer Simpson moment for me! .

[flintstone mop]

yup CFL's will get you every time.
I was troubleshooting a 3-way switch problem in my garage, which I changed over to (equiv 300W bulbs---8 bulbs) CFL's. Open ckt !!!. Stuck an incandescent in one socket so I could start thinking.
BTW the biggest energy savings seemed to be the change to CFL's. Going to LED's is hard to shake in the "is it worth it category!"
In a few instances changing to LED lighting is worth it. The newer brighter than H@&& outdoor security lights are worth the extra cost!
Fred

[WA3VJB]

Chris I was going to suggest that there was high enough resistance in a dirty bulb bottom to prevent your ohmmeter from registering, but not to block AC voltage.

I also would caution against using CFL in an enclosed ceiling fixture. The damn things fail easily enough from their own heat and startup surges, let alone when it's contained by an outer housing. 

I've had evidence of melting where the glass tube enters the plastic housing over the ballast above the socket. The damage was so pronounced that the glass had started to shift.  I was using them in an enclosed fixture in the laundry room.

[flintstone mop]

Chris I was going to suggest that there was high enough resistance in a dirty bulb bottom to prevent your ohmmeter from registering, but not to block AC voltage.

I also would caution against using CFL in an enclosed ceiling fixture. The damn things fail easily enough from their own heat and startup surges, let alone when it's contained by an outer housing. 

I've had evidence of melting where the glass tube enters the plastic housing over the ballast above the socket. The damage was so pronounced that the glass had started to shift.  I was using them in an enclosed fixture in the laundry room.

I have thought/worried the same thing, Paul. We have 8 R-30 CFL's in our kitchen in enclosed fixtures. The normal heat of the ceiling, then cooking. There has never been an incident of one of those bulbs loosing their mind in that heat. And they are lasting a lot longer. Getting almost 4 yrs with long hours of daily use.
fred

[W2PFY]

Wow, just experienced something last night that really surprised me about left turn signal on an SUV. The guy has his left blinker on and it broke the squelch on my two meter rig. I wonder if it was the leds or the associated circuitry?

[flintstone mop]

Wow, just experienced something last night that really surprised me about left turn signal on an SUV. The guy has his left blinker on and it broke the squelch on my two meter rig. I wonder if it was the leds or the associated circuitry?

Oh No!! Mobile RFI.