Which is the practical rule to substitute two or three 1N4148 series diodes for one red or green LED?
What is the main benefit?
If I have four 1N4148 diodes in series, can I substitute with two LEDs? Red or green?
Well, the main benefit of using a LED is that you get some nice coloured light which you do not get from a 1N4148 (at least at voltages under 100 volts or so).
I guess this is a school exercise and you have been asked about forward voltages and maybe reverse break down voltages also .
Find a data sheet for a red led, a green led and a 1N4148 and compare the forward voltages (that is the voltage drop across the diode when, for example, 10 mA is flowing through it with a suitable resistor in series with it to limit the current to this value.
LEDs generally make lousy diodes/rectifiers because they have higher forward voltage, lower reverse voltage, and lower current ratings.
LEDs are better (stiffer) voltage regulators than low-voltage zener diodes though, and are often used in their place for things like current sources.
Let’s look at your proposal
3@ 1N4148
2.1v fwd voltage (3 * .7)
75v Cont reverse voltage
150 mA avg current (about 250 max DC)
1 5mm RED LED
2.1v fwd voltage
25 mA max current
Reverse zener voltage 5v (max may be 15-70v but not reliable from chip to chip)
So as the others have said yes. It can be done and when it’s forward biased you will get light and if you’re possibly using them for low current gating purposes you can see them work and it is only 1 component vs 3 in series.
There are a lot of disadvantages also and much higher failure rate depending on situation. LEDs does not like reverse voltage.
If you are wanting several diodes in series, then you must be wanting a voltage drop that's more than about 0.7V, which means you are looking for a shunt voltage regulator or reference of some sort.
Several ordinary diodes in series have the worse temperature sensitivity as this multiplies with the number
of diodes. Zener diodes have relatively good temperature stability, but low voltage types are very soft
kneed as they rely on tunneling. LEDs are simply better at the job of regulating voltage in this voltage
range (1.1 to 3 V), even though the voltage spread may be quite large (and not documented). A single
LED has about the same voltage tempco as a single silicon diode, so you avoid the multiplying tempco
effect.
But yes, LEDs will not tolerate much reverse voltage, and typically only handle small currents (compared
to zeners).
For real performance a specialized voltage reference chip is the way to go though, but for rough and ready
applications like setting a current source, an LED is a good match.
So what is the application?