create LED warmup time with Capacitor
Jun 11, 2009 at 12:54 AM Thread Starter Post #1 of 26

nullstring

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So, I am gonna build a starving student millet hybrid, and.. I would like the LED's to warm up as the tubes do.

I read somewhere that you can use a Capacitor to create this...

How long do tubes take to warm up...?

And how do I figure out the capacitor size for my LED to take a certain time to warm up?

This is my LED:
EL-264-7USOC/S530-A4

I don't know what the forward voltage and current are >_>
maybe you can help with that too.

I might just have to buy a few capacitors to play around with.
Because they have nothing to do with audio.. I'm sure i can buy cheap ones

EDIT: forward voltage is 2V @ 20ma
 
Jun 11, 2009 at 12:56 AM Post #2 of 26

logwed

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This is a good idea, I'm very interested in what others have to say! I thought tubes generally had pretty long warm-up times, over 10 minutes, and up to 30 for some tubes. Seems to me that you'd need a pretty large capacitor.
 
Jun 11, 2009 at 1:00 AM Post #3 of 26

Tridacnid

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Well, for most small signal tubes, the operational point is about 10 seconds. I actually inadvertently did this when I made a small CCS to drive the LED in my amp (see my sig) for the fun of it. I had a spare CCS chip, a capacitor, and a couple resistors, so I threw it together. A side effect that it pretty cool is the warm up time. It takes about 5 seconds for my LED to get to full brightness.
 
Jun 11, 2009 at 1:07 AM Post #5 of 26

nullstring

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Tridacnid: whats a CCS chip?

Punnisher: I want a gradual warm up.. not one that is instant after a few seconds.
 
Jun 11, 2009 at 1:09 AM Post #6 of 26

logwed

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Quote:

Originally Posted by nullstring /img/forum/go_quote.gif
Tridacnid: whats a CCS chip?

Punnisher: I want a gradual warm up.. not one that is instant after a few seconds.



Probably some kind of common circuit, but my google searches aren't turning much up.
EDIT: ne'ermind.
 
Jun 11, 2009 at 1:39 AM Post #7 of 26

tangent

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CCS = Constant Current Source

Gives a linear capacitor charge curve instead of exponential (?) as you get when you charge a cap through a resistor.
 
Jun 11, 2009 at 1:39 AM Post #8 of 26

nux

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Quote:

Originally Posted by nullstring /img/forum/go_quote.gif
Tridacnid: whats a CCS chip?

Punnisher: I want a gradual warm up.. not one that is instant after a few seconds.



CCS is a constant current source. LED brightness depends on the current, not voltage. So you cannot simply use a capacitor to have the brightness ramp up over time. You would need to have an adjustable current source that goes from 0-20mA in a logarithmic fashion, since the brightness/current relationship is not linear.

Another option would be a PWM circuit.
 
Jun 11, 2009 at 1:49 AM Post #9 of 26

nullstring

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I would rather have the perceived-brightness/time relationship be logarithmic..

What is the perceived-brightness/current relationship? exponential?
 
Jun 11, 2009 at 2:36 AM Post #11 of 26

nullstring

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The only reason I thought it would be exponential is because you said the current source should be logarithmically dependent.. in order to get a linear brightness/current relationship.. hmm

I'll look into PWM circuits and dependent current source circuits..

Sorry for needed to be spoon fed a bit there.
 
Jun 11, 2009 at 2:55 AM Post #12 of 26

Emooze

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Quote:

Originally Posted by nullstring /img/forum/go_quote.gif
Sorry for needed to be spoon fed a bit there.


It's the best way to learn sometimes. I don't get this kind of exposure in class unfortunately...
 
Jun 11, 2009 at 3:32 AM Post #13 of 26

DKJones96

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If you use a CCS for your tube heater you can tap into that for your gradual LED.

The only drawback to CCS for tube heaters is they take longer to warm up. I've played with amps using the 6922 with rectified constant transformer voltage at 6.3 that are good to go in 8-10 seconds. Mine using CCS takes at least twice that long since the regulators start at 3.x volts and gradually increase as the resistance through the heater increases.

When I wanted a slower led light up I just used a bigger cap.
 
Jun 11, 2009 at 6:32 AM Post #14 of 26

cerbie

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Quote:

Originally Posted by nullstring /img/forum/go_quote.gif
The only reason I thought it would be exponential is because you said the current source should be logarithmically dependent.. in order to get a linear brightness/current relationship.. hmm

I'll look into PWM circuits and dependent current source circuits..

Sorry for needed to be spoon fed a bit there.



I'm not sure how to match it with tubes, here, but figure this, for a nice 5mm LED:
.1ma: you can see it in a dark room
1ma: nice indicator
10ma: bright indicator, but is it really 10 times as bright?
20ma: that's really double?
50ma: OK, finally it's really bright! (do not run your average indicator LEDs at this current--it can cause gradual dimming)

The thing is, 1ma could be 2V, 10ma could be 2.5V, and 50ma could be 3.5V--not a nice neat voltage scale (it's logarithmic, as is your perception of light). So, with a plain RC circuit, you would be ramping voltage, the LED would appear to turn on late but quickly.

If you ramp the voltage logarithmically, it will appear as if it is lighting up close to linearly. If you ramp the current linearly, it will appear to light up slightly logarithmically (however, it will probably be subtle enough that it will "look" exponential or linear).

Actually doing any of that, short of a 555 or using a uC...I'll leave that to the experts. I mean, a PICAXE can do it easily enough, but it's kind of overkill, and hardly in the spirit of tubes, much less an amp designed for cheapness and simplicity
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