Electronics > Beginners

2 diodes in series: why ?

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Nounours18200:
HI,

I have read in this topic: https://www.eevblog.com/forum/chat/vd1212-diodes-replacement/

that a good replacement for a VD1212 diode is to use 2x1N4148 in series.

I apologize for my lack of knowledge, but why a single 1N4148 is not enough ? I do not understand why to connect 2 in serie, because a diode pass or does not pass the current ?

Thank you for your explanation...

tautech:
The Vf required is the clue to why two in series are required.  ;)

bd139:
Exactly that.

That's the only characteristic required most most cases for these. They are usually used to give two semiconductor junction drops in amp biasing circuits which is conveniently the same as the B-E drops on the PA transistors added together (or near enough).

You can also replace them with a BJT and a couple of resistors (or a trimmer) for a precise drop but it's hardly worth the effort in a reasonably well designed circuit. Look up "Vbe multiplier".

ledtester:

--- Quote from: Nounours18200 on September 30, 2019, 09:12:39 am ---
I apologize for my lack of knowledge, but why a single 1N4148 is not enough ? I do not understand why to connect 2 in serie, because a diode pass or does not pass the current ?


--- End quote ---

Diodes can have different V-I curves. THe forward voltage (the point in the V-I curve where the diode starts conducting) for the 1N4148 is around 0.65 V whereas the forward voltage for the VD1212 is around 1.3 V. Putting two 1N4148 diodes in series will approximate the V-I curve of a VD1212.

The same thing is going on with LEDs -- red, yellow, green, blue LEDs all have different voltage drops ranging from around 2 V to 3.5 V.

Update: Here's are some V-I curves for LEDs. Note that the curves start bending at different voltages.



001:

--- Quote from: ledtester on September 30, 2019, 09:59:43 am ---

The same thing is going on with LEDs -- red, yellow, green, blue LEDs all have different voltage drops ranging from around 2 V to 3.5 V.



--- End quote ---

Is it common constant voltages for LEDs? Can I use LED as a zener diode with low capasitance?

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