LEDS on Audio Power rail

Thoughts and concerns on running a couple LEDS off the +/- 18vdc used for op amps? Is there a clean way to do it or is it to be avoided at all cost?


Edit:
I should add that the leds will be switch indication and not a metering situation.

Also, I realize now that 500 series gear is working this way.

Thanks,

It's good practice to separate non-audio currents.  Run a separate ground return.  Or return to negative voltage and avoid audio ground altogether.

But this is much more important for digital / noisy / switching currents.  An LED indicator is low current and DC so shouldn't be a big problem either way.

If it's not going into audio ground then it should be fine. For example, you could source from + and sink to - (maybe with a small dropper resistor from each rail and a cap to filter / isolate switching transients from the supply rails).

Note that if you string many LEDs in series you can independently short individual LEDs off with a switch (or 2N7000). Because each LED has a voltage drop, you're dropping less voltage across resistors and therefore using significantly less power. Although the voltage drop across the resistors will change depending on how many LEDs are on / off, desired current, etc so the number of LEDs and dropper resistors would have to be carefully selected to prevent noticeable drops in brightness / flaky behavior.

squarewave said:

Note that if you string many LEDs in series you can independently short individual LEDs off with a switch (or 2N7000). Because each LED has a voltage drop, you're dropping less voltage across resistors and therefore using significantly less power. Although the voltage drop across the resistors will change depending on how many LEDs are on / off, desired current, etc so the number of LEDs and dropper resistors would have to be carefully selected to prevent noticeable drops in brightness / flaky behavior.

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This is why MCI do this with a current source for multiple led's in a string. This way the current & brightness is maintained regardless of how many led's are lit.

Rob Flinn said:

This is why MCI do this with a current source for multiple led's in a string. This way the current & brightness is maintained regardless of how many led's are lit.

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yeah - a current source is more efficient solution.
How may switches / leds are you (OP) looking at ?

RSRecords said:

Thoughts and concerns on running a couple LEDS off the +/- 18vdc used for op amps? Is there a clean way to do it or is it to be avoided at all cost?

Click to expand...

This is widely done so does not need to be avoided, as has already been shared you want to be careful about currents corrupting your audio. Modern LEDs are pretty efficient so do not require as much current as back in the old days.

Edit:
I should add that the leds will be switch indication and not a metering situation.

Also, I realize now that 500 series gear is working this way.

Thanks,

Click to expand...

While there are any number of ways to skin this cat, for only a couple LEDs you can pull current from +18V and return it to -18V.

You could even put two or three LEDs in series and turn them on/off with a shunt switch closure shorting out the LEDs one at a time. The change in current between different numbers of LEDs on, should not dramatically alter brightness. This has the additional benefit of not pulling a full current on/off transient from the rails, that could talk into other sensitive circuitry.

This is arguably overthinking a simple non-problem, unless it becomes one....

JR 

If the LED stays on, it is about as offensive as a bleeder resistor.

Current source is not generally "more power efficient". However if you need to turn LEDs on/off it helps to buffer the bumps/thumps.

PRR said:

Current source is not generally "more power efficient".

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I don't understand why a current source would make it more power efficient at all. But using several LEDs in series is more power efficient than giving each it's own current source.

PRR said:

If the LED stays on, it is about as offensive as a bleeder resistor.

Current source is not generally "more power efficient". However if you need to turn LEDs on/off it helps to buffer the bumps/thumps.

Click to expand...

This is why MCI do this with a current source for multiple led's in a string.

Click to expand...

Powering multiple LEDs from a single current source, or resistor is more efficient than duplicating current sources for each, if the multiple LEDs are turned on at the same time. If only illuminated one at a time, there is no current saving, but wiring them in series, and turning on/off by shunting across each individually minimizes the current step amplitude seen by the PS.

JR

make it more power efficient

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Cooler too.  I went the individual resistor route, 25-ish leds per ch, 24ch's. Seperate supply, trying to keep opa/agnd as clean as possible, but heat and current adds up fast. PRR went so far as to draw me a diagram for CCS. Did I listen? Noooo. Don't be me.  ;D

But for a onezy twozy that are telling you audio's on...shouldn't matter much.

Awesome. Thanks!
Not sure why I was worried. After thinking a bit about it tons of audio uses a single rail. ie every guitar pedal..
  Avoiding the audio ground by using the full 36v (+/-18) makes sense. I'll give it a go.

squarewave said:

I don't understand why a current source would make it more power efficient at all. But using several LEDs in series is more power efficient than giving each it's own current source.

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Yes. Although I hadn't considered using a resistor with leds in series then switching to select leds on.
The designer in me wants to keep the current constant but it may not really be an issue in practice.

Newmarket said:

Yes. Although I hadn't considered using a resistor with leds in series then switching to select leds on.
The designer in me wants to keep the current constant but it may not really be an issue in practice.

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a 2V change wrt 30V or so total is less than 10%... LEDs are not very linear, so less than 1dB current change is not a big deal IMO.

I have used current sources with say 12 LEDs in series for a meter, but 2 or 3 in series for simple on/off indicators don't benefit as much from the extra complexity of a current source.

JR

Undoubtedly there is an IC that has a parallel interface to current sources / sinks that would allow you to turn groups of LEDs on / off together which is nice. And get the obvious benefits of constant current.

Just for the record, personally I wouldn't do any of this. I would use a nice multiplexing, slew-rate limited, current source / sink LED driver IC like MAX7221 which can control up to 64 LEDs separately w/ no resistors (64 resistors would take up a lot of board space) and drive it with a mirco controller. If you use a separate ground for digital and think carefully about layout (return current should follow directly under / over / next to source), it can be perfectly quiet when routed on the same board as audio.

In the MCI consoles they use current sources with strings of up to 16 (I think) led's to do jobs like indicate what buses you have a    channel routed to.  When you unroute the channel from a bus the switch just shorts the led out.  The current source helps to maintain brightness etc when, for instance you may have anything from 1 to 16 led's lit.  They also use them for smaller strings of leds (48v, polarity, eq in etc) in the same way.

JohnRoberts said:

a 2V change wrt 30V or so total is less than 10%... LEDs are not very linear, so less than 1dB current change is not a big deal IMO.

I have used current sources with say 12 LEDs in series for a meter, but 2 or 3 in series for simple on/off indicators don't benefit as much from the extra complexity of a current source.

JR

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Yes – when you run the figures for just a few leds in series the difference is minimal.
I was indeed thinking of a longer string used for metering. Twelve or sixteen leds or so.
Specifically I was thinking of current source driven meter strings on a (never made it to market) digitally controlled analogue desk back in the last century – Novation (not the synthy people) - that I was involved with after it went into administration.
It also occurs to me that you might want a different current through different colour leds (eg green / amber / red ) to equal out the lumens and perceived brightness.

Newmarket said:

Yes – when you run the figures for just a few leds in series the difference is minimal.
I was indeed thinking of a longer string used for metering. Twelve or sixteen leds or so.
Specifically I was thinking of current source driven meter strings on a (never made it to market) digitally controlled analogue desk back in the last century – Novation (not the synthy people) - that I was involved with after it went into administration.
It also occurs to me that you might want a different current through different colour leds (eg green / amber / red ) to equal out the lumens and perceived brightness.

Click to expand...

you could manage that by driving the string with the largest current needed for brightness (green), and use shunt resistors across higher efficiency (red) LEDs, to equalize brightness...  Modern LEDs are higher efficiency than in the bad old days. 

JR

It also occurs to me that you might want a different current through different colour leds (eg green / amber / red ) to equal out the lumens and perceived brightness.

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Gave me an excuse to use a giant, ancient resistor decade my brother loaned me.  ;D

those oldies were nice for using big old wire wound resistors inside.

If you open it up, look for small resistance values that released their smoke from misuse and abuse.

JR