As per title, I was wondering about the effects, if any, of the aging of the LED references. I have a vintage amp that has bipolar BJT current sources with LED voltage references (see positive and negative rails schematic snippets below). These are your garden variety 5mm cheapo red LEDs. They are not run very hard (3.6mA).
I live in an imaginary world where licorns thrive and silicon junctions are eternal, but I guess they must have a life expectancy. The amp is currently open on the bench and I note that the LEDs are quite dim - much dimmer than a new red LED passing 3 mA; some have almost an unoticeable glow. Furthermore they are not all equally lit between halves of the bipolar supply on each current source section. Note that the transistors including the power finals and all other diodes don't seem to suffer aging...
Information on these old amps is scarce but once I stumbled upon an article on upgrading a member of the same series and vintage of amps, which has basically the same circuit but with scaled-down components and power (I have an exemplar of this model as well). The author, which apparently had a good deal of experience in restorations and upgrades, wrote that he systematically replaced current source LED references.
So I guess I should too, or was he over-zealous?
I live in an imaginary world where licorns thrive and silicon junctions are eternal, but I guess they must have a life expectancy. The amp is currently open on the bench and I note that the LEDs are quite dim - much dimmer than a new red LED passing 3 mA; some have almost an unoticeable glow. Furthermore they are not all equally lit between halves of the bipolar supply on each current source section. Note that the transistors including the power finals and all other diodes don't seem to suffer aging...
Information on these old amps is scarce but once I stumbled upon an article on upgrading a member of the same series and vintage of amps, which has basically the same circuit but with scaled-down components and power (I have an exemplar of this model as well). The author, which apparently had a good deal of experience in restorations and upgrades, wrote that he systematically replaced current source LED references.
So I guess I should too, or was he over-zealous?
How about measuring the bias current? You can do that by measuring across the resistor connected to the collector of the CCS transistor. If the CCS provides the specified current I wouldn't worry about replacing the LEDs.
Older LEDs weren't very efficient and if they're run current starved I would imagine that their brightness could vary quite a bit. But you don't care about the brightness. You care about the bandgap voltage and resulting CCS output current. That's determined by semiconductor physics.
If it ain't broken, don't fix it.
Tom
Older LEDs weren't very efficient and if they're run current starved I would imagine that their brightness could vary quite a bit. But you don't care about the brightness. You care about the bandgap voltage and resulting CCS output current. That's determined by semiconductor physics.
If it ain't broken, don't fix it.
Tom
Avago reliability data now 200,000 hours https://docs.broadcom.com/wcs-publi...eliability-data-sheet/259/141/av02-0385en.pdf
Heat is the LED's enemy.
Heat is the LED's enemy.
Thanks for the data. So a red LED can be expected to last at least 224 000 hours, about 25 years and a half. This amp is 30+ years old, I've bought it 20 years ago and of these I have run it on permanently for like 20 years. I do not know how long the previous owner ran it, but at any rate that's close to the figure.
A peculiarity of the amplifier is that each 1 % change of the current from the current source changes the collector current of the input transistors by about 8.7 %, see the thread https://www.diyaudio.com/community/...m-mc-discrete-phono-stage.405380/post-7504533
You could try asking @Mark Tillotson or @Elvee They seem to know a lot about device physics.
If you only want to know whether your amplifier input stage is still biased correctly after all these years, measure the voltage drop across their collector resistors (R223 and R218), making sure you don't cause oscillations. One way to reduce the chance of oscillations is to connect a 1 kohm or so resistor to the test point and to connect the long lead to the meter to the other side of the resistor.
You could try asking @Mark Tillotson or @Elvee They seem to know a lot about device physics.
If you only want to know whether your amplifier input stage is still biased correctly after all these years, measure the voltage drop across their collector resistors (R223 and R218), making sure you don't cause oscillations. One way to reduce the chance of oscillations is to connect a 1 kohm or so resistor to the test point and to connect the long lead to the meter to the other side of the resistor.
It IS broken now...
However that's not the LEDs but the electrolytics that are starting to fail - have to do a comprehensive overhaul of the thing.The question of LEDs biasing is relevant mostly to biasing in the phono stage discussed in another of my threads quoted by Marcel above, although the power section has them too.
Ah. Well in that case...
If you want to replace the LEDs, measure the forward drop of one that is working. Check by measuring the CCS output current. Then find a replacement LED that has the same forward drop at that current. Ideally, you'd also want an LED with a low dynamic impedance, i.e., one that shows little change in the forward drop vs forward current.
Given the age of the amp, the LEDs are likely to be plain vanilla red ones. Not low-current, high-brightness, or anything like that. That should allow you to narrow down the search criteria enough that you can find a replacement.
I still lean in the direction of not broken, don't fix. The CCSes aren't broken, right?
Tom
If you want to replace the LEDs, measure the forward drop of one that is working. Check by measuring the CCS output current. Then find a replacement LED that has the same forward drop at that current. Ideally, you'd also want an LED with a low dynamic impedance, i.e., one that shows little change in the forward drop vs forward current.
Given the age of the amp, the LEDs are likely to be plain vanilla red ones. Not low-current, high-brightness, or anything like that. That should allow you to narrow down the search criteria enough that you can find a replacement.
I still lean in the direction of not broken, don't fix. The CCSes aren't broken, right?
Tom
I'm with you, but measurements wouldn't hurt perhaps on the more critical phono stage. Rest assured I'm not the type of guy to replace parts for fun.
They don't wear out, but some grow whiskers!
I have a keen interest in LED's as we use them in one lab for curing polymers that have an incorporated a photo-initiator, i.e. causing them to crosslink when exposed to a certain wavelength.
LEDs do wear out, there is a phenomenon of so called dark junctions that don't emit light.
LEDs also have a problem due to the semiconductor material, which unlike silicon, does not have a stable oxide protecting the surface, so the chip is exposed to the environment in its non hermetic plastic package.
LEDs also have a problem due to the semiconductor material, which unlike silicon, does not have a stable oxide protecting the surface, so the chip is exposed to the environment in its non hermetic plastic package.
Ñot so sure about that Nigel. I remember to have seen a datasheet in the seventhies of a TO3 cased 2N3055. It included data for life expectation in relation to junction temperature. The max juntion temperature was given in relation to 5000 hours of life expectation. I remember that clearly because at the time i was very suprized that it was only half of the life expectance of a SQ tube. I would thinck that in order to be able to specify max ratings it has to be in relation to life expectance, do you thinck otherwise?
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When you take a deep submicron MOS transistor, put its nominal supply voltage between drain and source and put its gate halfway, hot electron degradation will shift its threshold voltage so quickly it will be unusable in a couple of months - and you don't need overvoltage or excessive temperatures for that, although those often can speed up the process.
"A failure is any LED that is open, shorted, or failed to emit light."
This isn't helpful in this circuit, where the LED's voltage drop is used in the circuit, not whether it emits light. I'm supposing the voltage drop at a fixed current (approximated by the resistor feeding it) remains unchanged, especially at a low current of 3.6mA and being near room temperature, but that reliability data doesn't tell us.
Yes, heat is the enemy of many or most electronic parts, but that doesn't seem to be a problem here.
Measure the voltage across these old LEDs and compare with a new (same color) LED running the same current. If they're significantly different then I'd be tempted to replace with something other than LEDs, maybe a couple of 1N4148 in series, maybe add a bat54 to get to the right voltage, then never have to worry about a LED going out. But at this current and temperature I think a LED is as reliable as any other semiconductor.