Author Topic: Boolean algebra, A AND 1 = 0 WTF ?  (Read 8906 times)

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Offline vk6zgo

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Re: Boolean algebra, A AND 1 = 0 WTF ?
« Reply #50 on: May 20, 2014, 03:38:40 am »
Ian, well done but your missing the point entirely. Take that LED off and get another one from a different source that looks the same and wee what happens. Tell me honestly would you propose that solution for a design that was going to be made by the 1000's ? what would happen when the exact model of your LED was not in stock of a different batch came in from the manufacturer, can you stake whatever you like on it still working ? what happens with temperature variation, if we are going to have the advanced version of the discussion which the course material was certainly not trying to have lets put all the factors on the table not just a one off experiment where you fine tuned it for that one single individual LED.

Yes of course, that's why we use a series resistor. But aren't your words above exactly what your course material was saying, which you rejected as wrong back in post #8?

I'm not trying to tell you that you can make a design that runs LEDs off a constant voltage supply. That would be silly. I am trying to tell you that the explanation given in your course material, that variations between LED samples and other factors mean that you need a dropper resistor to control for those variations, is quite a reasonable explanation. It's the same explanation you have given above.

Specifically, my experiment was designed to show that an LED without a dropper resistor can in fact control the current flow and it will not necessarily turn into a short circuit and blow up as you suggested. There is quite a range of voltages I can apply to that LED above and below 2.0 V without it blowing up.

I think I can see what you & the original text are getting at:-

Because of variations in LED characteristics,it is unwise to do something which may seem intuitive,which is to use six "2.0v" LEDs in series between a 12 volt line & Common.

Parallel "2.0v" LEDs across a 2.0volt regulated supply is not a good idea,either.

Even having several LEDs in parallel,with one series resistor is problematic.( I have seen it done,though).

Although the intention may have been good,the obvious implication in the original text is that the series resistor needs to be "selected on test" to obtain either the correct voltage across the LED,or correct brightness

The contrary is in fact true,& mass produced equipment uses a fixed value of series resistor with many different batches of LEDs from different suppliers & manufacturers,without any problems.

As I pointed out earlier,any difference in resultant brightness is not evident to the human eye.

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Offline Simon

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Re: Boolean algebra, A AND 1 = 0 WTF ?
« Reply #51 on: May 20, 2014, 05:37:57 am »
The V/I curve of the LED is very steep so it is hard to control the current, the resistor takes some of the slope out and makes it more linear so that minor variations will have virtually no effect, there I daid in one line what my course babbled a whole paragraph about and still got wrong.
 


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