Open / Short Circuit Values.

Basically i'm not "stuck" just a little confused as to what they're asking.

  1. Construct the above Series LED Circuit on a physical breadboard with your calculated resistance value and ensure the LED lights up. Make sure you connect the anode (the longer lead) towards the positive side of the power supply.
    `
  2. Measure all circuit values and record in Table 2.

table 2, I already completed the table, it's attached as an image.

  1. Troubleshooting Exercise:

PREDICTIONS

(i) Assume your LED became ‘short circuited’ (ie. it’s resistance became zero). Determine values for the following and explain…
circuit current voltage across the LED
(*Insert your answers in Table 3)

(ii) Now assume your LED became ‘open circuit’ (ie. it’s resistance became infinite). Determine values for the following and explain … circuit current
voltage across the LED

"Assume your LED became ‘short circuited’ (ie. it’s resistance became zero). Determine values for the following and explain…
circuit current voltage across the LED "

What do they mean by that? how's an LED become "short circuited" did someone twist the LED? and short it? or are they talking about shorting the circuit elsewhere?

What do they mean by that? how's an LED become "short circuited" did someone twist the LED? and short it? or are they talking about shorting the circuit elsewhere?

It is called a thought experiment, it is meant to test your understanding of what is going on. It does not mean that this did happen it is asking you what would be the consequences if it did happen.

Any component can fail, they can fail either open circuit ( like a fuse ) or fail short circuit, you have to know what will happen under each of the two circumstances.

Grumpy_Mike:

What do they mean by that? how's an LED become "short circuited" did someone twist the LED? and short it? or are they talking about shorting the circuit elsewhere?

It is called a thought experiment, it is meant to test your understanding of what is going on. It does not mean that this did happen it is asking you what would be the consequences if it did happen.

Any component can fail, they can fail either open circuit ( like a fuse ) or fail short circuit, you have to know what will happen under each of the two circumstances.

I still don't understand what it means.

If i put a piece of wire between the anode and the cathode? is that a short? (because even though the LED would go out, the rest of the circuit would be fine) or
do they mean a short at the +/- terminals ?

If i put a piece of wire between the anode and the cathode? is that a short?

Yes.

(because even though the LED would go out, the rest of the circuit would be fine)

No it would change the current in the resistor and the voltage across the LED.
The voltage on the LED would now be zero. The resistor current would increase. Do you know the supply voltage? If not the resistor is getting another 2.2V ( or what ever is measured ) across it. That will cause 2.2 / R to flow this is in addition to the current already flowing through it shown in the table.

ohhh right, I think i'm with it now... it literally does mean a short of the LED then, not a short of the circuit.

thanks :slight_smile:

cjdelphi:
What do they mean by that? how's an LED become "short circuited" did someone twist the LED? and short it?

We Earthlings have a thing called "imagination".

Here's a link to the Wikipedia page so you can read about it: Imagination - Wikipedia

PS: Good luck with your mission.

cjdelphi:
ohhh right, I think i'm with it now... it literally does mean a short of the LED then, not a short of the circuit.

thanks :slight_smile:

Yes short circuit means a short on the component. Often that can mean a short of the power supply but not always.

my imagination does not stretch as far as a "short of an led" twisting one until the legs meet.... maybe.

as for a short across only the LED... not going to happen!

go ahead drop a some metal! i bet you can't short out ONLY the led :slight_smile:

cjdelphi:
my imagination does not stretch as far as a "short of an led" twisting one until the legs meet.... maybe.

as for a short across only the LED... not going to happen!

go ahead drop a some metal! i bet you can't short out ONLY the led :slight_smile:

I manufacture LED modules. Just occasionally I get a PCB with a badly etched footprint, where one of the surface mount LEDs is shorted out by a whisker of copper. The post-build testing procedure picks this up by there being an increase in current flow through that one part of the circuit.

You see, it happens, and it's important to know what the effect of it is. It also helps you to understand electronics and the related formulae.

cjdelphi:
my imagination does not stretch as far as a "short of an led" twisting one until the legs meet.... maybe.

as for a short across only the LED... not going to happen!

So... why do LED driver chips have built-in LED short circuit detection?

eg: http://www.ti.com/lit/ds/symlink/tlc5916.pdf

cjdelphi:
my imagination does not stretch as far as a "short of an led" twisting one until the legs meet.... maybe.

Well try it does happen. An LED is a just lump of silicon with a bit of impurities at each end. It is very easy to imagine this going short when the charge carriers get depleted.

as for a short across only the LED... not going to happen!

Yes it does happen to parts when they fail. Excess heat melts the chip and destroys the junction and they in effect go short.

Not going to happen consisting of a 300 ohm resistor and a 5mm LED :slight_smile:

As for copper and pcb tracks, I can see that being an issue though.... but i'd love to see a 5mm LED get shorted just by dropping something onto the track of the circuit... I can't see it happening, but yeah in the real world sure (just not with this circuit)

You are totally missing the point:-

  1. An LED can fail short circuit without anything hitting it, it can just fail. All components can just fail, you have to accept that because it is true. The fact that you can't see it yet is down to the fact that you are still learning and have little experience and you don't know how components fail.

  2. When you do safety assessments on equipment you have to consider each component in turn being open circuit and closed circuit. The safety of the equipment must not be jeopardised by any one component fault. So this is both possible and necessary to be able to look at this.

  3. This is an exercise, if you can't see the practical possibility of component failure then at least try to imagine what would happen if invisible pixies from the fourth dimension slipped into your LED and wired it up wrong when you weren't looking. Which would not help you anyway as they are invisible.

hmmmmm

nope i still disagree, yes it can fail and short it happens but in the context of this exercise i can't see it happening furthermore i could not care less how or why either...

now for a real world example, the recv light fails on your router, it becomes open or closed, what would happen? i presume a little extra current will flow in a closed circuit will the router die?

if this was not an led, then i'd be concerned or if it lead to burning the building down thrn i agree with you... but so much fuss over an led