I took a look at one the other day. The plastic case was clear so it was easy to see inside.
It only had a few parts: case, LED, battery, and a chunk of plastic tubing. The tubing was around one leg of the LED. The LED legs straddled the battery. The tubing kept the leg on that side of the battery from making contact with the battery. The length of the tubing was about half the dia of the battery. When you squeezed the case you bent the end of the leg in the tubing down so it made contact with the battery.
But _I said_ that if the supply voltage is too high. Then it _will_ cause reverse current which _will_ destroy the led. But I agree that supply voltage should be pretty high to do that. But in some cases, it can be.
My point was just to say that reverse polarity _can_ cause harm, just in the case where the supply voltage is higher than the reverse breakdown voltage.
This gibberish sounds very much like physics according to George Lucas! I think that you must be trying to quote Ohm's law - but doing so in a way that makes no sense whatsoever.
"Simoc" wrote in news: snipped-for-privacy@b28g2000cwb.googlegroups.com:
Good. So you didn't make David Lee's error.
You also said "But notice that if the LED is accidentally connected in reverse polarity, there will be no current..."
THAT is the focus of my point, and you said it correctly yourself. :) While there IS no reverse surrent, it promes that the reversed EMF isn't enough to produce one. IF it it is truly high enough to cause damage, then there WILL be a current, and as I said, it is that current which causes the damage.
I don't care where you think that you remember being taught it - it's still meaningless technobabble!
A volt is the unit of electric potential - which is the energy of a unit charge. The EMF of a battery, which despite the name does not have the units of force, is a potential difference and is the amount of energy required to move a unit of charge between the two points at which the potential was measured . It is the open circuit voltage of the battery and is indeed measured in volts - however this does not in any way imply that every quantity that is measured in units of volts is an EMF - just the same as the statement that cabbages are green vegetables does not imply that all green vegetables are cabbages!
It is the electric field, measured in volts per metre, that represents a force - the force acting upon a unit charge. I fail to understand how you envisage this force being "moved about" by "volts".
Current is a flux - the flux of charge moving under the accelerating influence of the electric field, limited by the scattering processes that give rise to electrical resistance. It would be totally incorrect to say that the current represents "the force".
All you really needed to quote was I = V/R. However reverse conduction in an LED is a breakdown process and irreversible damage can be caused at reverse biases of only a few volts so that a limiting resistor suitable for normal forward bias operation cannot be assumed to protect against inadvertent connection of the device back-to-front. Generally if it is liable to see significant reverse voltages (such as when operated from AC) you would expect to find another diode (with a higher breakdown voltage) in series with the limiting resistor.
David Lee (and since you quote City and Guilds at me - PhD CPhys MInstP)
"David Lee" wrote in news: snipped-for-privacy@eclipse.net.uk:
Lazy speech, probably, I didn't say it clearly.
Once that current flows, energy IS being moved from one place to another. I was taught that the voltage moves a charge, as current, the voltage being like pressure. And like a high pressure vessel with a pinhole in it, if that leak is of high enough resistance that no significant current is pushed through it, it's safe to say that under those conditions, that pressure (voltage) is not enough to damage something.
Maybe the confusion might also come from an assumption that I challenged the possibility of reverse biasing an LED causing damage to it. I never stated this, I only said that if in the reverse condition, no current is seen to flow, then damage won't occur. I always put a signal diode in inverse parallel with any LED that might see a reversed supply polarity, because although at low vlotages you might be providing enough for forward drive, yet NOT enough for reverse breakdown, I'd rather play safe. signal diodes are cheap. .
"David Lee" wrote in news: snipped-for-privacy@eclipse.net.uk:
Well, ok, but this is why I use that inverse parallel diode. :) A lapse in thought and word at one point can happen to most of us. Fortunately I don't do it all the time, or I might have lost a lot more LED's than I have, i.e. very few...
You have got that right! Which is why the statement was total gibberish - in science clarity of thought is paramount!
The analogy of a pinhole under pressure actually represents exactly the opposite of the point that you are trying to make. The pinhole only causes resistance to flow, so when there is no flow then there is no resistance. For an example: in an area of high water pressure there is a tendency for a hosepipe fitting to blow of a tap and this can be reduced by closing the tap somewhat to reduce the flow of water. However if you turn off the water at the nozzle end then the pressure in the hosepipe will always rise to mains pressure, even if the tap is only open a crack, and bang goes the hose fitting! This failure mode does not require any significant current of water - just an increase in the volume of water in the pipe by a few millilitres over a time period that can be as long as you like, just sufficient to push the pipe out of the fitting.
In the case of an electronic device, a series resistor will indeed limit the potential dropped across the device, and hence the current, and prevent damage due to thermal effects whilst operating in forward bias. However it will do nothing to limit the voltage drop that can appear in reverse bias whilst no significant current is flowing. In many breakdown processes it is the electric field that is important, with irreversible damage caused by very small currents, which is why you can't rely upon the same series resistance that was effective whilst operating in forward bias.
These torches tend to rely on the internal resistance of the button cells to limit the current. They over-run the LEDs dramatically to produce higher light levels.
Flickering and then going dim or dying completely is a pretty common failure mode of the nasty quality Chinese Gallium Nitride LEDs. It appears to be failure of the film on the substrate.
Now I suppose I should get on my flameproof kimono to protect me from the "I love Chinese crap" brigade who think that everything that comes out of China is technically perfect. :)
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