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Old 13th January 2004, 02:16 PM   #11
crown300 is offline crown300
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High Efficiency Red.
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Old 13th January 2004, 02:46 PM   #12
gwolf is offline gwolf  Austria
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For protecting mosfets (zener as a shunt across gate and source) I have used with good success a reverse biased base-emitter junction of generic BC550 or BC560. Capacitance is low and the zener voltage is quite the same across different devices.


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Old 13th January 2004, 02:48 PM   #13
rbroer is offline rbroer  Netherlands
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Originally posted by johnferrier

From it, one can see that the red LED has a steeper If v Vf curve. This is desirable because the voltage drop is less susceptible to changes in current.
My measurements are similar.
I once measured the red, green and yellow ones I had.
Biased them from 1.0mA to 2.0mA, with steps of 0.1mA and put the voltages accross them in a spreadsheet.

Red was best, then green and yellow had a considerably less steep curve.

Note, this is with my LEDs, YMMV of course.
Rudolf Broertjes
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Old 13th January 2004, 03:18 PM   #14
Christer is offline Christer  Sweden
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Out of curiosity I checked the datasheets of some LEDs from
Everlight, which is what the local supplier here sells. I basically
just checked a couple of 3mm LEDs all having their typical
values rated at 20mA. Their mileage does seem to vary indeed.

All diodes in the range from yellow to yellow green had an
almost linear characteristic, while the red ones had a more
normal exponential type curve and the blue ones were rather
inverse exponential. I checked the voltage variation
between 10mA and 30mA for these and there were no big
differences, but yellow, sunset orange and yellow green seemed
the to have the smallest voltage variation, ie. the steepest
curves in this range.
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Old 13th January 2004, 03:37 PM   #15
fscarpa58 is offline fscarpa58  Italy
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Do you think there are any possible drawbacks in putting
one LED at the catode of a triode, say a 6DJ8 as
common catode stage, instead of the paralleled
cap and res ?

I saw this somewhere.

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Old 13th January 2004, 04:13 PM   #16
Fred Dieckmann is offline Fred Dieckmann  United States
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Lightbulb She LED me on........

"I would like to find actual noise data for LEDs."

This was discussed on the forum. Try site search engine.

I believe John Curl did say that there was a drop in noise as the bias current exceeded about 2 mA. The comparisons of dynamic impedance between colors is interesting but it would seem that the size of the LED would be more important. For any more investigations into this the impedance of the LED in the 1 mA to 5 mA region would be of the most interest, since that is the range in which they will likely be used for voltage references (for high supply rejection and low noise). The rejection of noise from the supply may be more important the voltage noise generated by the device itself, since the noise from the supply appearing across the LED will often be the greater voltage. This is definitely the case for resistor bias of the LED and CCS bias is highly recommended. It seems that LED size would be more important with large and possibly high brightness LED having the lowest impedance.
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Old 13th January 2004, 04:53 PM   #17
sam9 is offline sam9  United States
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When using as diode of any kind in a CC source it's pretty common to bypass with a 47uF cap primarily (I think) to shunt AC from the rails to ground. This leaves me wondering whether as a secondary effect it also serves as a snubber for the diode. If that's the case the concern with the diode as a noise source would be reduced.

This is idle speculation. I'm not asserting this as something I know.
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Old 13th January 2004, 05:17 PM   #18
johnferrier is offline johnferrier  United States
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Default Re: She LED me on........

Originally posted by Fred Dieckmann
"I would like to find actual noise data for LEDs."

This was discussed on the forum. Try site search engine.
Hi Fred,

Yes, I was aware of those discussions. There was no data. The most conclusive thing I read was that MAT03 datasheet shows a super low noise amplifier that uses an LED for a voltage reference. So it was implied that if it was good enough for PMI/Analog Devices, then it must be low noise. (Additionally, yes, Walt Jung uses an LED in his low noise regulator as well.)

Former thread that mentions LED noise:

In the past, I've google search for "LED" and "noise". I found no LED datasheet that indicated a noise spec.

Though it sounds as if others are interested too, it's just a matter of personal curio.

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Old 10th August 2009, 10:16 PM   #19
juanlara is offline juanlara  Spain
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in the web
they helps you with the models, and also can implement the model for the led you request.

I hope this helps.
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