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Burn out 120V lamp on 120V circuit?

A while back, in another Usenet ng, someone asked about using an ordinary 120V incandescent lamp to slow down a fan motor. A number of posters replied that they had successfully done this. The usual discussion of the merits of doing this ensued.

Then, one frequent poster replied that he had tried this with a small AC motor, and the 120V bulb, in series with the motor, burned out when the circuit was completed.

There was much discussion, with many saying that it was impossible for any two terminal passive device in series with a 120V incandescent bulb on a 120V circuit to cause that bulb to burn out.

Some even set up Spice simulations which were difficult because of the large variation in the bulb's resistance from cold to fully "on". The OP was asked to repeat the experiment, which he did several times, with the same results of the bulb burning out.

The final conclusion, perhaps not shared by all, was that it was possible for a passive device to act in this way. One poster even showed Spice results with an increase in line current due to motor inductance. It was never proved that this increase was enough to cause the bulb to fail.

I haven't tried the experiment myself, since I don't have a suitable small motor available, and with 120V incandescent bulbs on the endangered species list, I don't care to sacrifice even one! Some time ago, I did use a series 120V 100W bulb to slow down the compressor fan motor in my refrigerator, when the proper replacement was not available.

I have my own theory and can postulate a two terminal passive device capable of behaving in this way. (It doesn't even need pre-"charged" condensers.)

What say ye all?

--
Virg Wall, P.E.
Reply to
VWWall
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Having burned myself seriously as a kid, using an Erector Set motor winding to blow flashbulbs (?), I can assure you that a bouncing contact will do it ;-)

(?) The flashbulb (Press 25) melted over my finger :-( ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

                   Spice is like a sports car... 
     Performance only as good as the person behind the wheel.
Reply to
Jim Thompson

Not quite the same thing. I have had #25 flash bulbs go off in my pocket from EMF radiated by trolley lines. An ordinary incandescent bulb requires much more energy to cause its filament to burn out.

--
Virg Wall, P.E.

(Correct signature delimiter is "--  CR", i.e. "--(space)CR"...)
Reply to
VWWall

I don't know. Is charge conserved ?:-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

                   Spice is like a sports car... 
     Performance only as good as the person behind the wheel.
Reply to
Jim Thompson

Nowadays charge doesn't need to be conserved anymore, that's so yesterday. You just print new charge. And if that ain't enough you borrow some charge. If that still isn't enough you go into closed-door meetings with some bankers and leverage the leverage, a derivative scheme that can make 15 charges out of one real charge. Some folks on the hills are also quite good at that. Plus the bankers will make all the charge that had gone bad over the years look like new charge and repackage it for ya, with AAA rating and all the trimmings. Of course, the one real charge you walked in with will be required as a fee, so the proper charge bonus can be paid out later. Not to you, of course ...

--
SCNR, Joerg

http://www.analogconsultants.com/

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Reply to
Joerg

Flash bulbs aside, motors store significant energy in their magnetic fields. Motors are far from "two terminal passive devices".

It may take some sort of switching event (commutation for example) to get an inductive kick to blow a lightbulb, it may be possible.

--
Paul Hovnanian     mailto:Paul@Hovnanian.com
------------------------------------------------------------------
On a clear disk, you can seek forever.
Reply to
Paul Hovnanian P.E.

Ok, So you're thinking that the motor is creating some sort of inductive kick or something a bit like a slower version of inductive kick caused by the motor's flyweel effect and switching from a spinning motor to a spinning generator?

What kind of voltage do you suppose the spinning generator effect could add to the circuit?

Can't a 100W 120 Volt Incandescent light bulb take a momentary surge of double or triple the normal voltage?

Reply to
Greegor

Not if the filament is cold. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

                   Spice is like a sports car... 
     Performance only as good as the person behind the wheel.
Reply to
Jim Thompson

It depends on how you define "passive device". In this case I defined it as a "two terminal black box" containing no energy source. It's easy to dream up a "box" that takes input energy, stores it and sends it back in a way that can overvoltage the bulb to failure. It is hard to see how a common AC induction motor can do this.

You're on the right track! Those that tried a Spice simulation modeled the bulb's variable resistance well, but considered only the static inductance of the motor's windings.

Virg Wall, P.E.

Reply to
VWWall

JT > Not if the filament is cold.

But but... (grin)

By the time the motor gets spinning enough to generate the killer curent, isn't the filament warmed up? Naa....

How big of a plain old inductor would it take to cause this effect?

Reply to
Greegor

Translation: Obama really meant "charge" and not "change"..

Reply to
Robert Baer

I say no.

Reply to
Spehro Pefhany

As in "charge us with more taxes"? It sure seems like that. That process has already been started. As expected :-(

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
Reply to
Joerg

not they way they're made in china these days.

try an "rough service" bulb that actually still has has a supported filament and see what happens in the same application.

Reply to
Cydrome Leader

It sounds impossible, but motors are inductive things, and there lie dragons and other things that go bump in the night.

I once blew a 1/4A fuse that was in series with the primary of a Mouser

12.6V, 300 mA or so transformer; I was torture-testing the RS slide switch at the time, and presumably I switched it right at the peak of the inductive kick and the line voltage or some such.

But _every time_ he turns it on?

When he does just the motor without the bulb, does it run? What's it fused at? What's the rated current of the motor? What's the rating of the bulb?

Thanks, Rich

Reply to
Rich Grise

It was largely Bush's tax cuts (and his wars) that led to the huge deficit. The tax cuts are set to expire soon and if they are allowed to do so it will mean a reversal of the present trend and an additional tax burden mostly on the most wealthy. Not really an additional burden but elimination of a previously established burden that was removed for political reasons.

Paul

Reply to
Paul E. Schoen

What a bunch of crap.

What Demonicrat lies. The tax cuts benefited *everyone* who paid taxes.

Reply to
krw

Pleaes don't feed the trolls.

Reply to
AZ Nomad

Transformers (and other inductive devices) may exhibit high inrush current which can be as much as the applied voltage / resistance. This is especially true if you happen to connect it at a phase angle that corresponds to the last current that flowed, where remanent magnetism has the greatest effect. A 4 VA transformer may only draw 30 mA normally, but the primary winding resistance is probably 50 ohms or so, and could draw a surge of 3 to 4 amps long enough to blow a fast acting fuse.

A 120V 100W bulb will have an operating resistance of about 120 ohms, but a cold resistance of about 12 ohms. Thus it will normally draw a surge of about 10 amps until it reaches incandescence. A motor can act as a generator, and if it is a PSC type its capacitor may conceivably for a resonant circuit which might cause transients high enough to destroy the lamp.

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Paul

Reply to
Paul E. Schoen

Possible if the motor is large (high reactive current) and a suitable capacitor is placed across the lamp (thus resonating at 60Hz).

Tim

-- Deep Friar: a very philosophical monk. Website:

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Reply to
Tim Williams

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