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Old 8th September 2009, 01:06 PM   #2461
Salas is offline Salas  Greece
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How many mA is your consumption? I know better than battery, liittle shunt with 3 components.

Quote:
Originally Posted by AndrewT View Post
let's assume that we have V+ = 5V.
Vf = 2V
R from 5V = 100r
R to each LED = 100r.

What is the maximum current that can flow to any LED in worst case conditions?

Imax = [ V+ - Vf ] / [ 100r + 100r ] = 15mA

One cannot overload any of the LEDs using these values for the resistors. The pots can be any value from 0r0 to 10M and the LEDs will work.

Fine tuning of these added resistors may well achieve a nice attenuation rate in the pot rotation. Keep trying and report back.
Andrew, is that 30mA max for two channels?
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Old 8th September 2009, 01:43 PM   #2462
AndrewT is offline AndrewT  Scotland
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the 15mA current limit due to 3V drive and 100r + 100r in series is the highest current to any single LED.

If two LEDs are in parallel and each has a 100r in series then the worst case current becomes 3000mV / (100+50) =20mA shared between the two parallel LEDs, i.e. worst case of a two channel set up with a common 5V + 100R supply is 10mA per LED.

If one wants a slightly wider range of attenuation one may want to look at the possibility of reducing the series resistors to ~82r or maybe as low as 68r.

3000 / (68 + 34) = ~30mA total or 15mA per LED

If one leaves the lightspeed on 27,7 and leaves the volume setting at max or min then two LEDs will be passing 15mA each for 6000 to 8000hrs per annum. That could be a good reason for not using 56r series resistors.
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Last edited by AndrewT; 8th September 2009 at 01:47 PM.
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Old 8th September 2009, 01:52 PM   #2463
Salas is offline Salas  Greece
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Its a CCSed mini shunt idea. OK, it would be 3 components if it was under 10mA so a NJFET could be the CCS and one string could take the current, still simple though for 46mA max. If you don't have 2.5Vf LEDs in hand, it can be lifted by using a diode between the LEDs and earth. Also a 470uF capacitor from +5V output to ground can be added if proves subjectively beneficial. It just needs 8-10V rectified and capacitor filtered DC for input.
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Old 8th September 2009, 10:42 PM   #2464
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Will try that on my next build. Neat job Salas


fran
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Old 8th September 2009, 11:01 PM   #2465
rocosr is offline rocosr  United States
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Default Aging Issues with LEDs and LDRs

Is anyone worried about aging effects which afflicts devices of this type. All LEDs show a decrease in light output over time. I don't know what the LDRs do over time, but here is what PerkinElmer has to say about these devices:

"Life expectancy of an AOI is influenced both by the input and output
devices. Isolators which use an LED have long life since LED lifetimes
are long: 10,000 to 200,000 hours, depending on the application. LEDs
normally show a decrease in light output for a specified bias current as
they age.

The photocell output elements in AOIs show an increase in output
resistance over time as they age. With a continuous input drive current
and with voltage bias applied to the output, the output resistance will
generally increase at a rate of 10 percent per year. The aging rate is
lower with intermittent operation. Figure 5 shows the trend line for
output resistance under typical operating conditions. Other AOIs using
different photoconductive materials show similar trends..."

http://optoelectronics.perkinelmer.c...oisolatorB.pdf
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Old 9th September 2009, 12:21 AM   #2466
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I know about the Perkin Elmer ones, I found the Silonex to be a better one (NSL32SR2S) for this application, they told me they have 200,000hrs to 300,000hrs use, if on for 24/7 that equates at the minimum 200,000 to 22years!!!, and if run at half power (lets then give them 300,000hrs) as I state in the instruction that come with the production Lightspeed Attenuator, that equates to 34 years!!!!!!! life expectancy.
The hundreds that I have built to date, the oldest are over 5 years old are still going as strong as the day they were built.

Cheers George
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Old 9th September 2009, 01:17 AM   #2467
rocosr is offline rocosr  United States
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George, I am referring to the aging rate, not the lifetime. What does Solinex specify for aging? I guess I'll have to give them a call to find out.

Have you measured the resistance transfer function of an LDR over extended periods of time?

I'd like to build this, but I think that long term, this design needs to be tweaked periodically, or there needs to be some form of automatic adjustment mechanism.

roco
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Old 9th September 2009, 04:00 AM   #2468
tinitus is offline tinitus  Europe
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Who wants to listen to the same gear forever
I sure dont hope devellopment stops here

Anyway, I have my reg supply now
Just need to order trafo
Build a box and assemble
Im getting pretty tired of my worn out remote


Last edited by tinitus; 9th September 2009 at 04:04 AM.
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Old 9th September 2009, 04:37 AM   #2469
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As I said in the post above I have hundreds out there over 5 years old, and I tell all my custormers to leave them on 24/7, and not one has wandered from it's original spec from when it was calibrated new on my test bench. What more can I say.

Cheers George
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Old 9th September 2009, 05:17 AM   #2470
rocosr is offline rocosr  United States
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Sounds like I will have to buy a few of these and measure them over a 1-2month period. This is pretty easy to do.

In the meantime, I'll see what the manufacturer has to say about the stability of LDRs.

Oh, btw, it's not that I intend to use this forever. It's just that I'm trying to decide between this and a passive preamp with resistor arrays and relays...I think this is the best solution.

George, I am somewhat surprised that you have no hard data to share on aging after working on this for so long.
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