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Really Old LEDs - 3 volt Forward Voltage?
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ghporter
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Join Date: Apr 2001
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Jan 15, 2017, 09:43 PM
 
I have found a stash of 5mm red LEDs that I got a LONG time ago (about 35 years ago!) after they were pulled from OEM keyboards that went into larger assemblies. I want to use some of these for a couple of projects, but since I don't have any data on them, I'm trying to test them to figure out what I have so I can use them instead of just frying them.

Testing these guys, I found that they light up with 3 volts applied from a coin cell - and haven't seemed to be harmed by it. With a beefy coin cell (CR 2450) they light up OK, and with a CR 2025 they light up much dimmer... I haven't had a chance to try these with an adjustable power supply yet, but data I can find for modern LEDs seems to indicate that 3 volts should destroy these, yet they have not blown up.

If these had integral resistors, would I be able to see them? Were that kind of devices available in 1982?

Were LEDs in 1982 that much sturdier than today's? Or have I just not yet run too much voltage through them yet to blow them up?

Thanks for the insight.

Glenn -----OTR/L, MOT, Tx
     
reader50
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Jan 15, 2017, 11:07 PM
 
I tossed my old Radio Shack catalogs from that time. If memory serves, some LEDs were sold with resistors pre-soldered, but I don't recall true integrated resistors.

They were rated by max current, not by voltage. Your button battery may be run down, with enough internal resistance to limit current.

If you have an adjustable current supply, I suggest sacrificing several. Gradually run up the mA until she blows. Note down each value, then rate them at 50-75% of the lowest blow value. Whatever round figure fits best within that range. If one blows far earlier than the others, discard that datum as a bad one.
     
ghporter  (op)
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Jan 16, 2017, 10:16 PM
 
Excellent points. My button cells are certainly low current, and that's a factor I didn't consider.

I have a variable voltage supply handy (a buck-down unit that I can adjust to whatever I like below the applied voltage), and I think I'll take a whack at seeing how these gadgets do with 1.5v or so applied, then play around with how much current they pull (assuming my ancient Fluke 77 can actually still measure low currents).

Glenn -----OTR/L, MOT, Tx
     
   
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