A Puzzle for Engineers

[GrayHare]

See if you can solve this actual happening that involves a lamp on a timer. A lamp attached to a electromechanical timer flips on at dusk as scheduled, except this time the LED bulb it controls flickers on/off roughly 10 times per second. It was working properly the prior night. The flickers are erratic, that is, there is no apparent temporal pattern to them and their intensity (brightness) varies. Other electrical devices on the same and other circuits are operating normally.

I let the flickering continue for about two minutes before finding a replacement bulb. The most convenient replacement bulb is a CFL type. I swap it into the lamp and find it will not light at all. I manually switch on a different lamp to prove its incandescent bulb is functional. I move that working bulb to the lamp that had the LED and it too will not illuminate. I reinstall the LED bulb and it resumes flickering as before.

What is causing the trouble? What would you check next? Hint: the lamp switch and timer are operating as designed. I eventually determined the issue, but had never seen this happen before. If no one figures it out within 24 hours, I can provide more hints. I'll respond to questions or guesses if there are any.

 

[omni550]

Light bulb socket?

 

[GrayHare]

Logical, but the light bulb socket is normal.

 

[PlayinwithFIRE]

seems related to the edison socket on the fixture side. somehow your bulbs are not making contact to
the socket base (ground connection i think) ?

 

[HFWR]

Fluted kneuter valve

 

[ls99]

A knackered ocket.

 

[Al18]

The lamp was plugged into a dimmer controlled outlet.

 

[GrayHare]

Yes, a dimmer could do that, but there was not one in this circuit.

 

[omni550]

Timer flaw?

 

[Masquernom]

Could be a bad contact on the timer.

 

[GrayHare]

that's getting closer, but the timer turned out to not be faulty

 

[jazz4cash]

Bad circuit in the LED bulb. CFL bulb is dead. You omitted many clues/troubleshooting steps and included other details that are not significant.

When I started reading I was sure it was gonna be hysteresis in a circuit that senses dusk but then you specified electromechanical timer.

 

[GrayHare]

Another reasonable idea, but all the bulbs turned out to be properly functional.

 

[TheWizard]

I'll guess a loose neutral wire feeding that socket.
You're not getting a solid 117 VAC or whatever your normal is...

 

[GrayHare]

Yes, there was a power problem, but why? Hint: the dog had it figured out long before I did.

 

[TheWizard]

An incandescent bulb would light at least dimly if there was any power going to that socket. This is a weird one.
Clearly the electromechanical timer is defective...

 

[GrayHare]

I was wondering that about the incandescent. It may have lit dimly without me realizing because the lamp shade hid the bulb. The electromechanical timer is not defective.

 

[Qs Laptop]

There is a bad ground somewhere in the circuit. Or there is some high current device plugged into another outlet, like a charger, that is on the same circuit as the lamp outlet.

I'm not sure how a dog would figure this out...

 

[Blue Poles]

A dog can hear higher frequencies than a human, but I am not sure if that has anything to do with this problem. In fact, a dog doesn't care if it's dark or not. He reads as well at night as he does in the day.

Edit: actually, I think it may have to do with an induced voltage on the power line that interferes with the florescent and LED lamps for some reason. Maybe a power factor issue degrading the power supply?

 

[omni550]

A loose neutral somewhere...the dog with 'moist' paws and nose might pick up on it.

 

[Fermion]

arc fault breaker gone haywire?

 

[tutan]

The lamp is bearly getting power. The dog caught mouse that has almost chewed through the lamp power cord. What happened to the cat?

 

[GrayHare]

A dog can hear higher frequencies than a human, but I am not sure if that has anything to do with this problem. In fact, a dog doesn't care if it's dark or not. He reads as well at night as he does in the day.

Edit: actually, I think it may have to do with an induced voltage on the power line that interferes with the florescent and LED lamps for some reason. Maybe a power factor issue degrading the power supply?

That is getting close to the answer. The dog's hearing is indeed significant in this puzzle. Let's see if anyone can fill in some specifics. Another hint: the time of year is a factor.

 

[38Chevy454]

Somewhere you are getting low or intermittent power to the light socket. The LED bulb uses little power and runs through a built in voltage reducer or capacitor, so it flickers. My guess is you have a bad switch on the lamp or a bad connection with poor contact or broken or chewed up wires.

 

[imjustawarrior]

I would put a volt meter on the light socket.

I thought an incandescent bulb would illuminate at a very low voltage. I don't know the voltage for the actual LED, but I think they are 5V or less. A very low voltage could turn on the LED but not other types of bulbs.

There could also be a current issue due to high resistance somewhere (bad contact, failing switch).

The more extreme would be that the switch in the timer is interacting with the voltage regulator in the LED bulb causing an apparent random flicker. But the random flicker sounds more like a mechanical issue.