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srinath 7th February 2013 10:28 PM

LEDs in vintage receivers
 
Hi,
I am trying to understand how LED's work.
I have looked into the LED resistor calculator and I know what resistor I need in the circuit.

However my question is a little fundamental ...

The LED measures something like 1200 ohm.

So why does it measure 1200 ohm in both directions ? Shouldn't it measure open in one direction ? its a diode after all.

Then the thing measures 1200 ohm, put across say a 3.4v source - its correct forward voltage, it should work just fine right ? However this is what I get from the calculator -
The wizard recommends a 1/8W or greater 1 ohm resistor.

What does a piddly 1 ohm do to a 1200ohm circuit ?

Thanks.
Srinath.

SY 7th February 2013 10:34 PM

They're diodes, which means that ohmmeter resistances aren't particularly significant.

LEDs emit light when forward biased. The voltage drop depends on the color- red LEDs, for example, drop 1.6-1.7V when they're on. 5mA is a typical current and safe 99% of the time. So you can calculate the series resistor from Ohm's Law by subtracting the diode's forward drop from the source voltage (that gives you the voltage you need to drop across the resistor), set I = 5mA, then calculate the resistor from R = V/I.

srinath 8th February 2013 12:01 AM

OK so now I got more questions - What does drop 1.6-1.7v mean ? As in a 3.4 v dc source will lose 1.7v ?

My led's are 20ma - does that mean something ? These are 3mm dia, If= 20ma Vf - 3.4 - 3.6 V.
They are supposedly blue. So I should follow that calculator pretty close huh ?

Thanks.
Srinath.

Andrew Eckhardt 8th February 2013 12:23 AM

The forward drop means you will have that much voltage across the diode before it goes into forward conduction. 3.4 to 3.6 volts forward drop is typical for blue diodes, but it also means that if you only have a 3.4 volt supply you will only be able to force microamps to 1 mA through the diode, with no dropping resistor added. The lower and higher voltages might be measured at a minimum versus maximum current respectively.

If you have a DC source it's not an issue, but blue diodes have about a 5 volt reverse breakdown voltage, so be careful not to connect the diode backwards with the power on, if you have, say, a 12 volt or more supply, especially not without a current limiting resistor in series.

Normally you subtract the expected forward drop from the power supply voltage when calculating the resistance needed for a specific current. So for instance 5V - 3.5 = 1.5 / 100 ohms = 15mA. For high life you probably shouldn't stuff more than 10mA through a diode rated 20mA max. As was already implied, 5mA is usually enough for indicator lights.

srinath 8th February 2013 12:43 AM

Oooo that is what forward voltage means.
So let me rephrase it. Below voltage that it will not light.
Above that - it will light, but unless you soak up the excess with a resistor that keeps the current down below the forward current (like way below the forward current), it will fry the diode.
If its equal to that, since the diode resistance is 1k ohm or so, you wont get enough current through the diode to light it up worth a darn.

Cool.
Srinath.

Andrew Eckhardt 8th February 2013 02:38 AM

I attempted another reply but decided to check and sure enough this does a better job:

Light-emitting diode - Wikipedia, the free encyclopedia

Ian Finch 8th February 2013 09:48 AM

Wow! All those fun facts, pics and references and not a jot of maths... :scratch:

east electronics 8th February 2013 10:07 AM

....of course one have to ask why the Japanese choose to run their bulbs at AC and not DC

after that we may be able to discuss how and if its possible to replace them with LED

kind regards
sakis

srinath 8th February 2013 12:33 PM

My LED says AC or DC.

Also I measure DC in the yamaha cr840 I am working on. But they have a 27v dc line running 2 14.4 v bulbs in series.

Cool.
Srinath.

KMossman 8th February 2013 12:53 PM

Quote:

Originally Posted by srinath (Post 3360999)
My LED says AC or DC.

Also I measure DC in the yamaha cr840 I am working on. But they have a 27v dc line running 2 14.4 v bulbs in series.

Cool.
Srinath.

LEDs are diodes. and like tunnel, zener, etc they have unique properties.

So, sure they can be used in AC circuits. On the website Instructables there was a guy who build a 220VAC LED lamp - runs off VAC directly - no rectification at all.

As long as you are careful and aware of their limitations/constraints.....


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