Easy Brain Teaser

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steve66

steve66

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Illinois
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Engineer
This should be an easy one. You are running some conduit in an existing building. One conduit has to go through a concrete wall. So you plug in your trusty drill motor at the nearest receptacle, crank it up and bear down on the wall. To your initial supprise, the incandescent light in the room gets brighter.

You are a little supprised, since you are used to seeing the lights dim when your power hungry drill fires up.

Why did the light get brighter?

Just use basic electrical theory to solve this. Don't overthink it, and assume your eyes can detect subtle changes in light level.

(I realize that it would be pretty easy to nit pick the solution I'm thinking of, and you guys can probably come up with a 100 reasons that the light wouldn't really get brighter. But I am just looking for one basic way the light would get brighter.)
 
multiwire circuit, bad neutral connection, voltage to light went up when drill came on w/ more resistance on opposite leg.
 
Because you were using a hammer drill, the vibration on the wall shook the internals of the incandescent bulb causing damage. The increase in brightness insued. Most likely the buld had already burned out.
 
480sparky said:
MWBC. Lights draw less amps than drill, so voltage goes up.
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steve66 said:
and assume your eyes can detect subtle changes in light level
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yeh it wouldn't necessarily have to be a bad neutral connection if its only a subtle change; i'll go with your answer, but mine's still a possibility too. :)
 
hardworkingstiff said:
Right, the light circuit would wind up with less of a voltage drop. I would guess this is a 120/240-volt service.
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its not so much of a drop as it is an actual increase in voltage. with the drill running, its branch may have dropped to 100v or so, where as the voltage to the light could've gone up to 140v; it all depends on the resistance of the two items, the bulb and the drill.
 
Counter EMF from a lightly loaded drill motor on a circuit that otherwise had realized voltage drop at the lamp. The lightly loaded drill was also a 120V generator while it was running.

This is the same reason you can overload a phase converter if you start another 3-phase motor on the converter and let it idle or let it run lightly loaded.
 
brantmacga said:
its not so much of a drop as it is an actual increase in voltage. with the drill running, its branch may have dropped to 100v or so, where as the voltage to the light could've gone up to 140v; it all depends on the resistance of the two items, the bulb and the drill.
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Assuming the neutral and all other conductors are in good condition, the light is on A-leg and the outlet on B-leg of a MWBC, then it is very possible that the voltage drop on the A-leg would be less when B-leg draws current.

Example, A-leg lighting circuit draws 10-amps and B-leg is unloaded. A-leg gets the entire voltage drop. Now let's say that B-leg starts drawing 10-amps, the neutral amps go to zero (assuming a 120/240-volt service) and the voltage drop of 10-amps gets split between the 2 circuits effectively cutting the lighting circuit voltage drop in half.

I'm not ruling out there is a neutral problem, a bad neutral connection would do exactly what you are saying, but it does not have to be a bad neutral connection for the phenomenon to happen.

I'm not ruling out there is a neutral problem, a bad neutral connection would do exactly what you are saying, but it does not have to be a bad neutral connection for the phenomenum to happen.
 
You drilled through a cable in the wall and cut the white wire in half is the most likely scenario. Open the wall and fix it before you burn the place down.
 
kingpb said:
Because you were using a hammer drill, the vibration on the wall shook the internals of the incandescent bulb causing damage. The increase in brightness insued. Most likely the buld had already burned out.
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Agreed.:grin:

Electrical theory aside, vibration from the drill damages incandescent filament and it burns out.

Same as mentioned by 'ELECTRICALPERSON'. Turn off light, won't happen.
 
Speaking of lights, I have a burned out headlight in my car. I was driving last night, and two people flashed their lights at me as if to remind me to turn on the other headlight somehow. :-?

Carry on...
 
mdshunk said:
Speaking of lights, I have a burned out headlight in my car. I was driving last night, and two people flashed their lights at me as if to remind me to turn on the other headlight somehow. :-?

Carry on...
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There is less voltage drop with the one headlamp out. The working one is now bright enough that people are wanting you to dim it :wink:
 
mdshunk said:
Speaking of lights, I have a burned out headlight in my car. I was driving last night, and two people flashed their lights at me as if to remind me to turn on the other headlight somehow. :-?

Carry on...
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Don't fall for it, I hear it's part of a gang initiation...
 
brantmacga said:
its not so much of a drop as it is an actual increase in voltage. with the drill running, its branch may have dropped to 100v or so, where as the voltage to the light could've gone up to 140v; it all depends on the resistance of the two items, the bulb and the drill.
Click to expand...

Without the drill running and only having the light on the circuit wouldn't the whole 240V been dropped across the lights since going around a series circuit all voltages must sum to zero? Therefore putting another (higher) resistance on this series circuit would cause less of a voltage drop across the lighting resistance. Isn't the voltage on the lights then whatever voltage is dropped across the lighting resistance? I would think less voltage drop would mean less voltage read across lighting resistance? I'm having a hard time seeing this.
 
steve66 said:
. . . the incandescent light in the room gets brighter.
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What Steve didn't tell us is that the "light" was a floor lamp, that the bulb was a three-way, and that the apprentice working with Steve turned it up, to help Steve see better while he was drilling. :grin:
 
A lot of good answers. I think Charlie gets the award for the most creative answer - although there were a lot of other good creative answers.

A mulit-wire branch circuit (MWBC) was what I was thinking about. The currrent from the drill motor reduces the current from the light on the neutral wire, causing less voltage drop on the neutral wire. No loose neutral connection is require, although that would increase the change in brightness.


Steve
 
How about:

As soon as you pull the trigger on the drill, the power company switches it's high-voltage lines somewhere to draw power from another source, and that source allows for a slightly higher voltage where you are.

Or:

As you press down on the drill, you increase your blood pressure. Increased blood pressure causes your retinas to become slightly more sensitive. You simply percieve an increase in light output.

Or:

As you start drilling, the sun starts to become blocked by some clouds. The light level outside begins to drop, so the amount of light coming in the window is less. Your eyes adjust for this by opening your retinas, and, at first, you think the light output of the bulb has increased.
 
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