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Substituting LEDs for Diodes

Thread starter #1
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?
 

MikeMl

Well-Known Member
Most Helpful Member
#2
The forward voltage drop (Vf while conducting) of LEDs is much higher (1.6 to 3.3V) than Si diodes (0.6 to 0.7V).

The Vf of LED depends on the their color (doping). You need to look up their data sheet, or measure them if the they come from an unknown source, like FleaBay.

The maximum current rating of a LED is much less than any diode.

The maximum reverse voltage rating of any LED is a tiny fraction of that of a diode.
 

gophert

Active Member
#3
The maximum current rating of a LED is much less than any diode.
The current carrying spec of of modern high output LEDs are much higher than an the 100mA rating of a 1N4148.
350mA is typical for a 1W GaN-on-sapphire LED and 1amp for 3 watt LEDs. Recently, 5 and 10watt LEDs have become available but those are typically multiple diodes in parallel or series on a common substrate. Like you said, however, LEDs have limited reverse (blocking) voltage capabilities (5V is a typical absolute maximum Vr).
 

MikeMl

Well-Known Member
Most Helpful Member
#5
Thread starter #6
I would presume the main benefit is you get a free indicator to show things are working.
From what I have read it's not that, apparently. There seem to be noise reasons or distortion, that makes LEDs behave better than those diodes in audio.

I just simulated replacing two diodes with a red LED on an amp, in LTSpice, and distortion went up, not down.
 

audioguru

Well-Known Member
Most Helpful Member
#7
I just simulated replacing two diodes with a red LED on an amp, in LTSpice, and distortion went up, not down.
Two 0.65V diodes in series are 1.3V but a red LED is about 2V, a huge difference. You would need to re-design the circuit to use a red LED.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
#8
From what I have read it's not that, apparently. There seem to be noise reasons or distortion, that makes LEDs behave better than those diodes in audio.

I just simulated replacing two diodes with a red LED on an amp, in LTSpice, and distortion went up, not down.
It rather depends on what the diodes were doing in the circuit?, you can't just randomly replace diodes with LED's and expect things to work correctly.
 

unclejed613

Well-Known Member
#9
i've seen LEDs used as the reference in current sources for audio power amplifiers. not only do they work well, but also provide a visual indication of whether the current source is functioning normally.
 

MikeMl

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Most Helpful Member
#10
i've seen LEDs used as the reference in current sources for audio power amplifiers. not only do they work well, but also provide a visual indication of whether the current source is functioning normally.
RadioSHack used to sell a several-Amp regulated-DC power supply where the reference was a Red Led...
 

Nigel Goodwin

Super Moderator
Most Helpful Member
#11
i've seen LEDs used as the reference in current sources for audio power amplifiers. not only do they work well, but also provide a visual indication of whether the current source is functioning normally.
As have I, but the circuit was designed to use an LED (of that specific colour), it wasn't randomly changed from a diode to an LED with no consideration.

A famous TV chassis (the Thorn TX10) used an LED on the CRT base as a reference.

The problem isn't using an LED in this way, it's trying to replace a diode with it.
 

schmitt trigger

Well-Known Member
#12
Infrared LEDs actually have a lower Vf. About 1.5 volts if I recall from memory.
But check the datasheet! I could be wrong.
 

tomizett

Active Member
#13
From what I have read it's not that, apparently. There seem to be noise reasons or distortion, that makes LEDs behave better than those diodes in audio.
This really does all depend upon the context of the circuit. We'd have to know what the intended function of the diodes was in order for anyone to make an informed comment on this.
Personally, I can't think of any "normal" audio circuit where diodes appear in the signal path.
Perhaps you would share the simulation in #6? If the diodes you changed where just setting the bias (as others have mentioned here) then you've likely just moved the bias away from it's optimal point - so the distortion has gone up.
 

unclejed613

Well-Known Member
#14
As have I, but the circuit was designed to use an LED (of that specific colour), it wasn't randomly changed from a diode to an LED with no consideration.
yes, the LED is chosen for a particular forward drop, and to maintain that particular voltage drop, i wouldn't be surprised to find the amplifier manufacturer doing sorting/binning of the LEDs to insure the forward drop is within tolerance (or selecting a trimming resistor during board test to keep the output current within spec if the LEDs have variations in Vf).
 

Nigel Goodwin

Super Moderator
Most Helpful Member
#15
yes, the LED is chosen for a particular forward drop, and to maintain that particular voltage drop, i wouldn't be surprised to find the amplifier manufacturer doing sorting/binning of the LEDs to insure the forward drop is within tolerance (or selecting a trimming resistor during board test to keep the output current within spec if the LEDs have variations in Vf).
I don't think it usually matters that much, the sort of application that uses LED's in this way is commonly a constant current source, and it doesn't really matter much exactly what the value is, as long as it's constant. So simply using an LED of the correct colour is normally all that's needed.
 

unclejed613

Well-Known Member
#16
one of the amplifiers i've worked on using LEDs in the current sources was a Monster MPA-3250 amplifier. the amplifiers used a symmetrical design that was basically two identical but opposite polarity amplifiers in tandem. in other words, there were two diff amps, one using PNP transistors, and the other using NPN transistors. everything on the (+) side of the amp is duplicated with opposite polarity devices on the (-) side. the mirrored signal paths merge at the VAS (voltage amplifier stage). the VAS loads are current sources, and if you are building (or simulating in LTSpice) this is where things get interesting (and somewhat unpredictable) because there are two opposite polarity voltage amplifiers, and two opposite polarity current sources working directly against each other. if you look at just the two current sources in opposition, it can be an oscillator, especially when the voltage amplifiers, and their compensation caps are added. instead of a smooth "see-saw" action, it's more like a tug of war. in LTSpice, it can cause the kind of errors you see when you use an op amp and get the +/- inputs mixed up (all those wonderfully ambiguous "gm step too small" errors). this is one reason there aren't a lot of amplifiers made with a totally symmetrical input/VAS stage. i would need to look at the schematic for the Monster amp again to be sure, but i think the way they stabilized this beast was by providing a nested feedback loop from the VAS stage to the input stage, or by making the VAS current sources "mushy" (by adding some series resistance to the current source output, making them "nearly constant current").
 

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