A level physics help

[Mjolinor]

Has anyone done it lately. I am trying to help my lad but the questions he has do not have answers and they are basically ambiguous crap so I can't tell what the answers should be.

This is a question:


" A red and a green LED are connected in parallel through a suitable series resistor to a variable voltage power supply.

Explain what will happen as the voltage across the LEDs is increased from 0v to over 2.1V"

In this circuit the "suitable resistor" is not defined.
What would actually happen with this circuit is that if you choose a resistor suitable for providing enough current for both LEDs then the LED with the lower forward volt drop will start to conduct, clamp the voltage across it and prevent the higher voltage LED from ever lighting, the lower voltage LED would allow too much current to flow and will fail possibly by going short and the fire starts. I can't believe that this is the answer they want but cannot find which answer they do want. I think this is from last years OCR A level paper but I am not sure.

Another one:



This requires you to make an assumption, will this be that both batteries are connected at one end to give a 12 volt battery or will it be that they are connected in parallel or are you supposed to say it is two 6 volt batteries?

What about the next bit, are you supposed to say "power supply" and leave it at that or are you supposed to say change the voltage and rectify (or rectify and change the voltage) or any one of the other 50 answers there are to this stupid thing.
Transformer and bridge rectifier, resistor and diode, switched mode power supply.

Exam questions with ambiguity are friggin useless unless you know the guy that is marking it.

[ITS A SCAM !]

why would there be a resistor anyway if there is a variable voltage PSU ? Is it to correct any overvoltage that may be produced if the voltage went over the max voltage allowed for the LED's.

[Mjolinor]

why would there be a resistor anyway if there is a variable voltage PSU ? Is it to correct any overvoltage that may be produced if the voltage went over the max voltage allowed for the LED's.

There has to be a resistor because an LED is not a linear device, if you just connect it to the power supply there is no current restriction and all hell will break loose, lots of smoke, bangs, flashes and fire.

[ITS A SCAM !]

Ok...so the resistor is working all the time in this circuit ?

[Mjolinor]

Ok...so the resistor is working all the time in this circuit ?

I don't understand that question.

[ITS A SCAM !]

Sorry, i should explain myself better...If the resistor is connected in series, then is it must be constantly working ?

[Mjolinor]

The resistor is connected in series with the parallel LEDs. It is passing current when there is a voltage across it which will happen when one of the LEDs gets forward biased. One also has to assume they are ideal LEDs not real ones.

[ITS A SCAM !]

Do you think too much is beig read into the question ? Wouldn't the LED's illuminate when the correct voltage is applied ? Are all A level questions a question within a question ? I had some like this when i did Electrical Engineering at college, and it ended up being a simple answer and to "think outside of the box" when reading them.
I would just say that the LED's would work, what other answers can be given ?

[Mjolinor]

Well that is what I need to know.

Do they expect you not to know what would happen or are you supposed to ignore what would happen. If you built this circuit then what would really happen is unknown as it very much depends on the voltage/current curve of the specific LEDs in question. If you assume ideal LEDs then the green one would never light and the red one would melt because the forward volt drop of red LEDs is lower than green LEDs.

What I really need is the specimen answer from the exam board but when I asked the teacher she obviously had no idea what the answer was. This is sadly what happens when teachers are provided with all the material they are allowed to teach, you end with someone with a degree in art, teaching physics.

[bonus2010]

The Badly Designed LED Circuit

The question refers to a very badly designed circuit, probably designed by Mr Bean.

To my knowledge, circuits involving LEDs will always have current limiting resistors placed in series to the LED. That?s because even with LEDs of the same type, they will each begin conducting at very slightly different minimum voltages applied across them. Placing an appropriately valued resistor in series with the LED, based on circuit maximum voltage, will always provide current protection for that LED.

The question has made it clear that we?re speaking about two different types of LED, each of which will have their own design characteristics, i.e the minimum voltage necessary to be applied across the diode before conduction will begin.

The important point with diode LEDs, is when there is insufficient voltage across the diode, it will present an open circuit, i.e a very high resistance.

When Mr Bean was designing his circuit?. he?s looked at the lowest maximum current of the two LEDs. If we assume that the resistance of LED1=LED2, then the current passing through each leg of the parallel circuit would be half that of the total circuit current. So he?s chose a value of R to take this into account, based on half total current. What a ~~~~en idiot!!!

As the voltage is gradually increased across the parallel LEDs, one of the LEDs is going to conduct before the other. The other is going to present an effective open circuit. Thus, all the total current will pass through the single conducting LED leg of the circuit. The total current will be twice the maximum specified current of the LED.? thus blowing it?s ass out the water with smoke and fire!!!

[Mjolinor]

The Badly Designed LED Circuit

The question refers to a very badly designed circuit, probably designed by Mr Bean.

To my knowledge, circuits involving LEDs will always have current limiting resistors placed in series to the LED. That?s because even with LEDs of the same type, they will each begin conducting at very slightly different minimum voltages applied across them. Placing an appropriately valued resistor in series with the LED, based on circuit maximum voltage, will always provide current protection for that LED.

The question has made it clear that we?re speaking about two different types of LED, each of which will have their own design characteristics, i.e the minimum voltage necessary to be applied across the diode before conduction will begin.

The important point with diode LEDs, is when there is insufficient voltage across the diode, it will present an open circuit, i.e a very high resistance.

When Mr Bean was designing his circuit?. he?s looked at the lowest maximum current of the two LEDs. If we assume that the resistance of LED1=LED2, then the current passing through each leg of the parallel circuit would be half that of the total circuit current. So he?s chose a value of R to take this into account, based on half total current. What a ~~~~en idiot!!!

As the voltage is gradually increased across the parallel LEDs, one of the LEDs is going to conduct before the other. The other is going to present an effective open circuit. Thus, all the total current will pass through the single conducting LED leg of the circuit. The total current will be twice the maximum specified current of the LED.? thus blowing it?s ass out the water with smoke and fire!!!

Totally agree but I don't think "blowing it's ass out of the water" is a valid response suitable for a pass at A level so one has to wonder, as I am doing, what the expected answer is. This is what I would like to know but his teacher can't tell me and the exam board have not responded to my request for an answer either.

All in all it makes it damn difficult to help my lad understand this not to easy subject when I can't answer the questions he gets set.

[bonus2010]

Do agree... that "blowing ass" is not the expression for an 'A' grade paper. ... I'm a bit old in the tooth now. All I'll say is.. I've designed many electronics circuits... and that's in the real world.


For example, take a LED brake light for a car... open it up, and you'll see a bank of serial resistors connected to LEDs. That's how it's done.........

I think the question is directed towards the LED design charactistics.... and if you're lad can desicribe this... then no one can argue.

I agree... it's very disappointing that high school teachers can't answer your question correctly... I experienced this also.. in my younger days.. but went to study at degree level.

The fact is, the question is vague... a circuit diagram should be shown... and the operating paramaters of both red and green LEDs should be listed...

When designing circuits you look at the possibilities for component failure .... low current & high current. I(total)/2 approx will equall that of least one LED. The resistor connnected as described... will offer no protection unless diodes are balanced (even then not a good design)

[Lainie]

~~~~ me thats complicated and i got an o level in physics - mind you that was a few years ago noo

[Itsme]

Don't quote me on this as it has been nearly 40 years since i did physics but perhaps the question is designed to see if the student has an understanding of what might happen and not what will actually happen, if that makes sense.

If your lad has to explain why connecting LEDs in parallel is not a good idea due to the reasons already suggested above (unbalanced load, high value current limiting resistors etc.) then this site may be useful.

http://www.horrorseek.com/home/halloween/wolfstone/Lighting/litlpo_PoweringLEDs.html#WiringMultipleLEDsInParallel

Also which exam board is doing the A level, I have just done a quick search on google for "A level past papers" and there are various available - some with marking schemes.

[bonus2010]

@Itsme

Excellent post... in my fummbled way that's exactly what I was trying to describe... again from an aging memory now...

Brilliant link...

My "blowing ass" expressing could be changed to " running the risk of operating a component outwith design limitations or just not functioning properly..

Surely on DK we've got it answered now...?