Unrestricted Current Flow?

I'm curious about something.

If a light circuit is turned on, the current flows thru the light bulbs and the circuit breaker does not trip (assuming of course that there are not too many lights).

However, a circuit breaker trips if there's a short (hot and neutral connected). This must mean that the light bulbs themselves limit current flow.

What is built into a light bulb (or any appliance for that matter) that limits the current flow? I know it's not the filament, because if that was the case the bulb would simply burn out due to excessive current.

 

It is in fact, the filament that "utilizes" the current flow. The amount of current that flows in any circuit is dependent on what work must be done by the current in the circuit.

In the case of a light bulb, the work is the amount of light that the bulb produces. The current flowing through the filament is converted into heat and light energy.

 

The filament of an incandescant bulb does have resistance. The filament does not burn out due to a vacuum inside the bulb. The metal filament glows (emits light) due to resistance. The filament does not burn out because there is no oxidation of that filament.

 

So the smaller the filament in an incandescent bulb, the greater the resistance? [ i = E/R ], correct?

What about a motor? What's the mechanism providing the resistance?

I realize these questions are more electrical theory than practical application which is the purpose of the DIY Forums, but it's interesting to understand.

 

Actually Joe, it is the filament. The filament is a predetermined resistance hence the wattage listed on the bulb. Since ohms (R) = volts squared ( (E) ²) divided by watts (P), a 120 volt 60 watt bulb has 240 ohms resistance. and amps (I) = watts (P)(60) / volts (120)(E), a 60 watt lamp draws .5 amps of current.

There are two basic types of systems that use but limit the current flow. Resistive, where a resistance in the circuit reduces the current flow to a controlable limit. The other being inductive. (actually termed indictive reactance)

A motor is an example of an inductive resistance to current flow. If you ohmed out a motor, you get a very low ohm value. If this was all the resistance the current would flow at a high rate and burn up the windings.

There is a 3rd type of resistance called capacitive reactance as well.

When a motor rotates, an electromagnetic field is what makes it do so and when a motor rotates, it produces counter EMF which restricts the current flow.

 

> This must mean that the light bulbs themselves
> limit current flow.

Exactly right. All appliances (including light bulbs) limit current flow.

> I know it's not the filament

It is the filament. The filament is a very thin wire which restricts the flow of electrons through it much like a very narrow pipe would restrict water flow. As a result of resisting the flow of electrons, the filament builds up heat and glows bright white.

> because if that was the case the bulb would simply
> burn out due to excessive current.

Light bulb manufacturers have developed filaments of the correct size and materials so they heat up enough to glow, but not enough to liquify or vaporize the metal. An improperly designed filament will burn up instantly.

 

Thanks NAP, That explains it perfectly.

 

"I'm curious about something.

If a light circuit is turned on, the current flows thru the light bulbs and the circuit breaker does not trip (assuming of course that there are not too many lights).

However, a circuit breaker trips if there's a short (hot and neutral connected). This must mean that the light bulbs themselves limit current flow.

circuit breakers trip on short circuit /to ground , to neutral, to phase to phase , and over current

What is built into a light bulb (or any appliance for that matter) that limits the current flow? I know it's not the filament, because if that was the case the bulb would simply burn out due to excessive current."

the terminolgy here is the confusing bit- The light bulb having a resistive value consumes power

 

"What about a motor? What is the "resistance"?

A D.C. motor is also a D.C. generator. At full speed, the motor generates a "back-voltage" that opposes the line-voltage, and restricts the line-current to the "full-load" current.

At Zero RPM's, the "back-voltage" = Zero, and the motor "appears" as a "dead short circuit" , so a resistance-type Starting controller is necessary to bring the motor to full-speed.

An A.C. induction motor has a "synchronous" speed, which is the speed of the rotating magnetic field of the stator winding in which the line-current flows. A 60-cycle 2-pole motor has a synchronous speed of 60 X 60 sec = 3600 RPMs.

The speed of the rotor is approx 94% of the synchronous speed---- for a 4-pole motor the synchronous speed in 1800, and the rotor "speed" is 1725 RPM's. The rotor "follows" the rotating magnetic field in the stator. the current induced in the rotor by the 4% frequency "slip" is the "full-load" current.

At Zero RPM's the rotor speed = Zero, and with a Zero difference between the stator/rotor speeds, a large starting current is "induced" in the Zero-speed rotor by the 60-cycle line frequency.

 

I have done work, for, with, and now over, electricians who are daym good at there jobs and cannot understand the concepts being discussed in this thread.

this is a good topic, but I am not sure that it belongs on a DIY thread.

 

Perhaps, but it sure throws some light on the topic. It also explains why you shouldn't put your finger in a light socket.

 

Personally, I find this thread very interesting and informative. With the web, You never know who may be reading/writing to this thread.

When you are too old to learn - you are too old.

 

We could talk for days on end about electricians we have worked with who somehow forgot every word of theory class and the code book they studied to get their license.