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#31
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green led
Before settling on a specific resistor to get the LED at the brightness
you want, you might want to look at setting up the following circuit. 5V (mobo header) ----.--------------------. | | | | | / 150-ohm | \ resistor | / / | 1500-ohm \ | resistor / / \ 1500-ohm \ / potentiometer /--. | \ | | / | | | | | |------------------------' | | __|__ \ / \ / __V__ | | | GND (mobo header) ---' If you have a patchboard, this is easy to setup for testing. If you want to keep it permanent, solder the 1500-ohm resistor across the outside contacts of the potentiometer and the 150-ohm resistor to the middle contact (that varies with the pot). If you use a small square pot (e.g., http://tinyurl.com/yxgkztas), you could solder on the resistors, bend one up to be on one side of the pot, bend the other to lay against the other side, solder a wire to one of the outside contacts of the pot, solder the other wire to the resistor to the middle contact of the pot, and use heatshrink tubing to encapsulate the package. With the potentiometer (aka rheostat), you can change the effective resistance across the paralleled resistor circuit from 136 to 786 ohms (to change the current from 3.8 to 22 mA). Rheostats go from zero ohms (or some very low value) to their rated resistance hence the 150-ohm resistor that is inline with the rheostat. Of course, you could use a pot that goes from 0 to 750 ohms since the LED is currently working with no resistor (0 ohms), so you could see what you get with a 750-ohm resistance (max for the pot). Then either leave the pot in the circuit or get its resistance and substitute with a resistor with nearly the same resistance. |
#32
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green led
On Fri, 28 Jun 2019 22:06:58 -0500, VanguardLH wrote:
Before settling on a specific resistor to get the LED at the brightness you want, you might want to look at setting up the following circuit. I don't have the parts, and getting them will take more time and money than I want to invest. I like to see the power LED, to know the computer is on, but if I can't fix it with minimal cost, I will live without it. I have soldering iron and heat gun, but I don't keep parts on hand for things like this. I'll spend a couple more bucks trying to lower the LED brightness, but that's as far as my budget goes. |
#33
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green led
On Thu, 27 Jun 2019 15:47:44 -0400, Paul wrote:
Those LEDs will light at 1mA! The voltage will still be at the 2V range. Even if you dropped to 100uA, there will still be around 1.9V or so. But you would need to darken the room, to see a dull green glow. I got a pack of 1k resistors. One didn't help. I kept adding more in series until I was out of alligator clips, at 8k. The LED dimmed some, but it was still too bright. Looks like it needs 20k or more to dim it enough. What I got from this is, one LED is not the same as the next. You need the right one for the job. |
#34
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green led
On Mon, 08 Jul 2019 23:16:52 +0000, T. Ment wrote:
I got a pack of 1k resistors. One didn't help. I kept adding more in series until I was out of alligator clips, at 8k. The LED dimmed some, but it was still too bright. Looks like it needs 20k or more to dim it enough. I bought a resistor assortment pack. It took 68k to dim it enough. This online calculator says: http://www.ohmslawcalculator.com/ohms-law-calculator 5v and 68k ohms gives 0.00037 watts. A 1/4 watt resistor sounds OK to me. Other online calculators recommend a full 1 watt resistor. I wonder why. Those LEDs will light at 1mA! The calculator says current is 0.00007 amp, approx. 1/10 of 1mA. Sound right to you? |
#35
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green led
T. Ment wrote:
On Mon, 08 Jul 2019 23:16:52 +0000, T. Ment wrote: I got a pack of 1k resistors. One didn't help. I kept adding more in series until I was out of alligator clips, at 8k. The LED dimmed some, but it was still too bright. Looks like it needs 20k or more to dim it enough. I bought a resistor assortment pack. It took 68k to dim it enough. This online calculator says: http://www.ohmslawcalculator.com/ohms-law-calculator 5v and 68k ohms gives 0.00037 watts. A 1/4 watt resistor sounds OK to me. Other online calculators recommend a full 1 watt resistor. I wonder why. Those LEDs will light at 1mA! The calculator says current is 0.00007 amp, approx. 1/10 of 1mA. Sound right to you? That's very very good. Perhaps too good by a factor of ten or so. I'd double check the resistor value. A 68K would have an orange third band, while 6.8K would have a red third band. https://www.petervis.com/electronics...alues/68K.html Paul |
#36
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green led
On Sat, 20 Jul 2019 22:15:27 -0400, Paul wrote:
5v and 68k ohms gives 0.00037 watts. A 1/4 watt resistor sounds OK to me. Other online calculators recommend a full 1 watt resistor. I wonder why. Those LEDs will light at 1mA! The calculator says current is 0.00007 amp, approx. 1/10 of 1mA. Sound right to you? That's very very good. Perhaps too good by a factor of ten or so. I'd double check the resistor value. A 68K would have an orange third band, while 6.8K would have a red third band. https://www.petervis.com/electronics...alues/68K.html Paul It has 5 bands: blue, gray, black, red, violet. Here's what I used: https://www.digikey.com/en/resources...or-code-5-band I checked it with my multimeter too. It reads 68.0 on the 200k setting. Two 20k in series made the LED brighter. That proves the single resistor is more than 40k. Hard to believe? |
#37
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green led
T. Ment wrote:
On Sat, 20 Jul 2019 22:15:27 -0400, Paul wrote: 5v and 68k ohms gives 0.00037 watts. A 1/4 watt resistor sounds OK to me. Other online calculators recommend a full 1 watt resistor. I wonder why. Those LEDs will light at 1mA! The calculator says current is 0.00007 amp, approx. 1/10 of 1mA. Sound right to you? That's very very good. Perhaps too good by a factor of ten or so. I'd double check the resistor value. A 68K would have an orange third band, while 6.8K would have a red third band. https://www.petervis.com/electronics...alues/68K.html Paul It has 5 bands: blue, gray, black, red, violet. Here's what I used: https://www.digikey.com/en/resources...or-code-5-band I checked it with my multimeter too. It reads 68.0 on the 200k setting. Two 20k in series made the LED brighter. That proves the single resistor is more than 40k. Hard to believe? That LED must have a hell of a lens on it. You can get them with a lens, like 15 degrees to half power. Which is a 30 degree cone of light, and then there isn't much light off axis. That's one way to juice up the result, on-axis. The best I can find at my local electronics store, is 10,000 MCD at 20mA in green, with a 15 degree lens on the end. At 1mA that would be half a candle. Which would be pretty bright. Only problem with my electronics store, is they want $3.00 for what should cost at most $0.20. I got one of their special LEDs once before, a 5mm LED that could take 100mA, and it didn't really like running at 100mA and it cost $3.00 as well. I checked Newark, and the brightest one I can get there, at 20mA, is 18,500 to 45,000 MCD at 20mA, with a forward voltage of 3.2V to 4.0V. Which is way too high and almost suggests two LEDs in series. That's a higher forward voltage than the white LEDs I've got, which are blue inside and run at 2.5V. 3.2V would be enough to do ultraviolet. (The forward voltage is proportional to color, and the slope is Plancks constant.) Such a high forward voltage would also affect your math a bit. If you're not careful, 5V may not be enough to run one properly. (For a Power LED, when the computer is in sleep mode, a transistor switches the LED on and off, and Vce could be 0.4V or so. Now you've got 5V - 0.4V - 4.0V on the LED, or only 0.6V across the 68K resistor (which is getting close to zero current). That's if your LED happened to have a Vf that high. Paul |
#38
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green led
On Sun, 21 Jul 2019 00:01:16 -0400, Paul wrote:
That LED must have a hell of a lens on it. I finally got the resistor wired into the case. The LED color is not a perfect match for my original, but it's not bad. The 68k resistor leaves the LED a little brighter than my original, but at least now it doesn't drown out the hard drive yellow LED below it. $5 of parts took $1,000 of time, but who's counting. It's the challenge to see if I can get it done. My next big project is to learn wilderness survival skills. Computers will be useless when the next Carrington event kills the power grid. No power, no computers. It took 100 years to build the power infrastructure. If enough main transformers get knocked out, I doubt there's enough inventory to fix things in a hurry. And without power nationwide, how do you build new ones? All manufacturing depends on electricity now. Chicken and egg problem. For God it's not a problem, He created chickens first, then they laid eggs. But we can't even make a tree. |
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