| Electronics > Projects, Designs, and Technical Stuff |
| Vintage LED's |
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| zingzang:
Thanks for the info guys! I've emailed Fran begging/propositioning her to trade old LED's with me ;D I hope it goes without saying that if anyone comes across any extras they'd be willing to part with, please, speak up or contact me. I'm a reasonable guy. --- Quote from: spec on January 09, 2019, 10:04:49 pm ---Hi zingzang, I can remember when LEDs first became available at a reasonable price. Not sure if you already know this, but the best and most reliable way to drive a LED is with a constant current, rather than a constant voltage. --- End quote --- I'm not sure if I know that - you mean DC verse AC but I was under the impression that DC current is constant and that the only issue in my situation would be not to feed too much voltage into them. I'm a novice with electronics so feel free to teach me what I'm missing! --- Quote from: jeroen79 on January 09, 2019, 02:03:05 am ---Resistors to limit the current would be most practical. Adjusting the input voltage to be in exactly the right spot on on the diode's I/V curve would be much more complicated. A 3V coin cell has an internal resistance high enough to limit the current. --- End quote --- That's why I love em! I've tested many older LED's with a 3v without many negative consequences. One exception is a TIL 209 LED made by Texas Instruments whose epoxy cracked after a second of contact at 3v. Needless to say I'll be more careful with the remain 209's. Talk about brittle epoxy! --- Quote from: mvs on January 09, 2019, 02:12:12 am --- --- Quote from: zingzang on January 09, 2019, 12:26:58 am ---Anyone find any really old specimens? --- End quote --- I have some where soviet red and green 0.1-0.2mcd LEDs in metal can (AL102 series). They were produced in '80s, but design is from the mid '70s. --- End quote --- I come across these on ebay quite often. It looks like the soviets had a "if it ain't broke don't fix it" mentality with LED's as there seems to be 100x as many older style soviet LED's than there are US manufactured ones. |
| james_s:
--- Quote from: zingzang on January 10, 2019, 01:46:27 am ---I'm not sure if I know that - you mean DC verse AC but I was under the impression that DC current is constant and that the only issue in my situation would be not to feed too much voltage into them. I'm a novice with electronics so feel free to teach me what I'm missing! --- End quote --- No, AC vs DC has nothing to do with it, LEDs are DC, ignoring special purpose types. Perhaps a better term to use here would be "limited" current, you don't feed voltage into an LED, the voltage pushes current through the LED. You have to have a resistor to limit the current or the LED will try to draw as much as the source can provide. To calculate the resistor value you need to know the forward drop of the LED in question, then you subtract that from the supply voltage and plug the results into Ohms law. If you don't know the forward drop, you can measure it by using a resistor that will limit the current to something you can be reasonably certain is safe, like under 1mA then just measure the voltage across the LED. |
| zingzang:
--- Quote from: james_s on January 10, 2019, 01:57:44 am --- --- Quote from: zingzang on January 10, 2019, 01:46:27 am ---I'm not sure if I know that - you mean DC verse AC but I was under the impression that DC current is constant and that the only issue in my situation would be not to feed too much voltage into them. I'm a novice with electronics so feel free to teach me what I'm missing! --- End quote --- No, AC vs DC has nothing to do with it, LEDs are DC, ignoring special purpose types. Perhaps a better term to use here would be "limited" current, you don't feed voltage into an LED, the voltage pushes current through the LED. You have to have a resistor to limit the current or the LED will try to draw as much as the source can provide. To calculate the resistor value you need to know the forward drop of the LED in question, then you subtract that from the supply voltage and plug the results into Ohms law. If you don't know the forward drop, you can measure it by using a resistor that will limit the current to something you can be reasonably certain is safe, like under 1mA then just measure the voltage across the LED. --- End quote --- Ah gotcha! Thank you for the info. I knew I'd need resistors but I see what your saying now. I'll try the test you suggested on the mystery LED's (or the ones I can't find data sheets for.) Incidentally, I only learned about forward voltage a few months ago while hunting. I came across a strange LED (still in packaging) that was backwards. It was indicated on the packaging so it wasn't a manufacturing error. Turns out it's likely a TI led that is yellowish-green at higher volts but turns orange-red as voltage decreases. The guy who told me about the TIL209 mentioned he'd heard of such an odd LED when he was starting out there back in the day. I'm guessing it was intended to be a cheap/simple battery meter? No expensive logic to deal with and only one LED needed... could see it being used in a calculator or radio to let the user know the batteries were running low. Maybe? |
| spec:
--- Quote from: zingzang on January 10, 2019, 01:46:27 am --- --- Quote from: spec on January 09, 2019, 10:04:49 pm ---Not sure if you already know this, but the best and most reliable way to drive a LED is with a constant current, rather than a constant voltage. --- End quote --- I'm not sure if I know that - you mean DC verse AC but I was under the impression that DC current is constant and that the only issue in my situation would be not to feed too much voltage into them. I'm a novice with electronics so feel free to teach me what I'm missing! --- End quote --- OK No problem. No not DC versus AC- just DC. The forward voltage of a LED for a given current is indeterminate and varies from sample to sample even of the same LED type and from the same batch. The forward voltage is also dependent on color, age, and temperature, especially with the earlier LEDs. The main objective with a LED is to have it look as you like, in terms of brightness etc. But once again, there are more variables in this because the efficiency of LEDs also varies quite a bit. With early LEDs the variation in brightness is quite large as is the beam angle. So the best way to illuminate a LED is to feed a defined DC current through it (in electronics this is called a constant current). The LED will then be able to have whatever voltage across its pins that suits it at a particular brightness and temperature. You can generate a semi constant current by putting a resistor in series with the LED and connecting the circuit to a DC voltage, but this is not very accurate unless the voltage is relatively high. Or you can use a simple circuit to generate an accurate constant current. Note that although the current is constant, it can be set to any value that you like, within reason that is. But once set the current stays constant irrespective of the LED forward voltage and any changes in the LED forward voltage with temperature etc. --- Quote from: zingzang on January 10, 2019, 01:46:27 am ---Adjusting the input voltage to be in exactly the right spot on on the diode's I/V curve would be much more complicated. --- End quote --- It would be harder than that. :) --- Quote from: zingzang on January 10, 2019, 01:46:27 am ---A 3V coin cell has an internal resistance high enough to limit the current. --- End quote --- Afraid this is not true. :) |
| spec:
+ zingzang Just to get the scope of this project: * Can you say what the maximum number of LEDs on display will be? * Will you want the LEDs to be illuminated one at a time or simply all on, or perhaps any number of LEDs on and any number off in any combination? * Will you want to vary the brightness of the LEDs. If so, all together, or individually in any combination? * Will you be displaying any modern high-power LEDs (say 1W) as a contrast with the older LEDs? * What will be the power source: battery or mains? |
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