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srinath

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

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

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

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

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.

Ian Finch

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

east electronics

....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

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

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.....

srinath

So as per the LED calculator wizard, with the 27v supply and 2 diodes in series setup, I can put a 10k resistor in it and limit the current to 2.5 milli amp.
Now will it light up worth anything ?

I guess I should try it, and if it fails to light, go to a 4.7K etc etc...
Thanks.
srinath.

KMossman

So as per the LED calculator wizard, with the 27v supply and 2 diodes in series setup, I can put a 10k resistor in it and limit the current to 2.5 milli amp.

Can not say until we know the LED specs: Vf, nominal current

KMossman

Vf varies from say 1.4 to 4V !!!

nomial current varies from 5mA to 50mA !!!

srinath

Its Vf = 3.4, If = 20 ma. To make it last forever I thought I should keep it low, like 2.5 ma.

I have a few other resistors, but the 10K will be my first try.

Cool.
Srinath.

KMossman

2 - ts Vf = 3.4, If = 20 ma.

3 - To make it last forever I thought I should keep it low, like 2.5 ma.

1 - can never be too sure. I must have 20 different LEDs on hand - all different specs

2 - is the If nominal? I assume so.

27v supply - [2 x 3.4] = 20V {close enough}

20V / 2.5mA = 8K or your 10K resistor will give 2mA

I would say the light would be very dim, you can safely use 5K or even 2K5

3 - forever is a long time. Do you want to use it as a night-light in your coffin?
at nominal current it ought to last 30,000 to 50,000 hours {you can do the arithmetic}

east electronics

normal leds if a few milliamps will not be able to replace bulbs that used to light the dial it can be replacement for stereo indication for example but this one works with DC any way .

Even if one uses high brightness leds to lit the faceplate this also will not work since leds focus -spot their lighting while bulbs flood all around

so what is the case here ?

KMossman

normal leds .... will not be able to replace bulbs that used to light the dial

Even if one uses high brightness leds to lit the faceplate this also will not work since leds focus -spot their lighting while bulbs flood all around

The problem with incandescent bulbs and FL lights is that the light emits from 'all directions. Some of it wasted, or must be re-directed - hence reflectors.

LEDS have rather narrow angled emission. If you have a broader angle you have to use either more LEDs or diffusers [either as part of the LED 'cap' itself, or a separate element]. This is done all the time.

One can also obtain broader coverage by using distance - the further the distance, the broader the 'foot-print'

srinath

I put LED's in the yamaha CR840 - 27v with 2 bulbs circuit, used a 2.2K 1/2 watt resistor.
The blue lighting is awesome. The old green meter lights look ancient in comparison.
Now this one has a sort of acrylic fitting that the light travels through, like a fiberoptic conduit, it works great. Also the lights were facing the acrylic, so LED's that fire in front work fine. The original bulbs had a silicone sleeve, I left that out, dont think I need it, but yellow will mimic the original better IMHO.
The Meter lights are next, and I think the pointer - which needs to be red, and since I dont have red, I have to get those ...
Of course the blue lighting doesn't get the metal good and bright so we can see the black lettering under it ... so its nearly unreadable ... but see that cos these LED's were blue ... not the fact that they are LED's. White or yellow will have fixed that, but NO .... I had to have blue ...

Then I put em in the MCS. That one uses an acrylic fiber optic conduit types too, but that was a cup shaped holder ... the side of the bulb was lighting the acrylic up. That ended up needing LED's installed facing forward, and I got all 4 slightly off from each other, so one is really bright, the other 3 ... not so, and of course blue lights and black letters ... same issue as before.

Either way ... get the right color and 1/2 the battle is won. Get them installed right, and maybe put a little tin foil or a bit of tracing paper and your LED will light the dial and not blind you.

I have ordered several other sets ... I also believe somewhere I got a cache of clear and bright very low Vf - like 1.5v LED's lying around. I remember playing with those and a button cell ~10 years ago. I have to find it ...

Cool.
Srinath.

KMossman

I put LED's in the yamaha CR840 - 27v with 2 bulbs circuit, used a 2.2K 1/2 watt resistor.
The blue lighting is awesome. The old green meter lights look ancient in comparison.
Now this one has a sort of acrylic fitting that the light travels through, like a fiberoptic conduit, it works great.

For the acrylic plate, what some people to do is;

1 - file off the top of the LED so it is flat, OR use a rectangular LED

2 - glue or mount the top of the LED to the side [or top] of the plate [the LED points into the plate]

Then it lights across the plate. Hope I made that clear.............

The effect is improved by using say 1 small LED every 1 or 1.5 cm along the top or side of the plate.

KMossman

Of course the blue lighting doesn't get the metal good and bright so we can see the black lettering under it ... so its nearly unreadable ... but NO .... I had to have blue ...

yep, got to pick a color that will 'bring out' the lettering or scale or whatever

Keep in mind, that another color or brightness will have a different Vf - you might have to use a different resistor.

Did you know there are 'flashing' LEDs? They change color randomly. Vf is 3 to 3.2V The effect can be cool, depending how and where they are used.
You can get slow and fast [color changing rates].

Also - just to mention it - do not put LEDs in // as one will likely hog the current and burn out.

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