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### Topic: LEDs in series and forward voltages. (Read 10116 times)previous topic - next topic

#### QSuff

#15
##### Apr 01, 2009, 09:24 pm

If Vs is below, or equal to Vf, you DO NOT need a resistor! Contrary to what most seem to think, there IS internal resistance in the LED after Vf, about 17ohms in our everyday 25mA devices.

Another fact about the LED junction is that it does not demonstrate the characteristics of a Si junction, so there is little chance of 'thermal runaway' or 'current hogging' so many are saying like it's the gospel.

#### minime72706

#16
##### Apr 02, 2009, 12:48 am
So if you're right, I'm not an idiot?

#### QSuff

#17
##### Apr 02, 2009, 01:37 am

I'm just awfully tired of hearing so-called experts tell everyone that the resistor is such a life saver. I mean it's such a simple thing to prove, or disprove:

Take a power-supply with an accurate V and A meter and hook it directly to a little LED, with big heavy wires so external R is minimized. Crank it until you see  20mA, now push the V another 10% higher, the current goes up, probably to around 40mA - which coincidentally shows that there IS internal resistance involved.

We'll get a warmer LED, for sure, but its not the armageddon they all claim it to be! No flames, no explosion, and no avalanche current-sucking catastrophe either!

As most experimenters who worked with multi-watt LEDs know, heat is the #1 enemy of the LED, and as long as the heat can be dissipated, things will be, er, cool.

Besides, we're all talking Green the same breath we say LED, but burning up the power savings in resistors? What are we thinking?

#### Grumpy_Mike

#18
##### Apr 02, 2009, 10:54 am
Sadly the internet is full of idiots like you two.

I taught Physics in a University for 21 years and I now design electronic circuits so I do know a thing or two about solid state design and designing reliable circuits.
It is all too easy to throw something together that works but not every time.

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Besides, we're all talking Green the same breath we say LED, but burning up the power savings in resistors

Never herd anything so stupid on this forum. Pity you missed your vocation, four years ago you could have been President.

#### Otacon2k

#19
##### Apr 02, 2009, 01:06 pm

By the way, as you talked about constant-current sources I remembered there was an easy way to build one (I know they're not that expensive, but I like to build stuff myself...).
The wise old man in my electronics store told me about it, when I was buying an LM317T (adjustable Voltage regulator) and some TL431 (adjustable shunt regulator), but I don't remember which one was the one capable of doing so...
Diving through the datasheets of both of them I keep reading things like "constant current sink"  and "current regulator" with their application diagrams, but unfortunately my capability of understanding datasheets is still very poor... which btw for me (as a german) is like reading Shakespeare in original when I was in school: spend a year reading it over and over again and you start to realize

Well, i would be very pleased to have someone of you point me in the right direction

#### Grumpy_Mike

#20
##### Apr 02, 2009, 01:23 pm
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Well, i would be very pleased to have someone of you point me in the right direction

try this:-
http://en.wikipedia.org/wiki/Current_source

#21
^^^

Ha-ha, oh wow.

#### QSuff

#22
##### Apr 02, 2009, 03:44 pm
Oh wow, indeed.

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I remembered there was an easy way to build one (I know they're not that expensive, but I like to build stuff myself...)

The one I use is with the LM317L (TO92 size) and a resistor:

A value of 68R will give you 18mA, 100R for 12mA. Good from 5mA to 100mA.

#### big_mark_h

#23
##### Apr 03, 2009, 10:29 am
Hmmm.... some interesting ideas, but I think I'll re-design my LED arrays to have 3 in series with a resistor to limit the current. Many thanks for everyones help with this.

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