It's usually the other way around: exponential at low currents, ohmic at higher currents.
John
- Vote on answer
- posted
12 years ago
LEDs in parallel
Loading thread data ...
- Vote on answer
- posted
12 years ago
Some people still think an exponential has a "knee".
- Vote on answer
- posted
12 years ago
Ha ha, thanks!
-- Bob just used 'canonical' in the canonical way. [Guy Steele]
- Vote on answer
- posted
12 years ago
-- It never really gets ohmic unless you drive the junction hard enough to short it, and once you get past the knee - where a relatively large voltage change results in a small current change - the slope changes so that for a relatively small increase in voltage you get a large increase in current.
- Vote on answer
- posted
12 years ago
Thanks.
-- Bob just used 'canonical' in the canonical way. [Guy Steele]
- Vote on answer
- posted
12 years ago
-- news:86g8i7pot46vjj41o3qlbnl72q65gm157q@4ax.com
- Vote on answer
- posted
12 years ago
Well, lots of people do it and get away with it.
John
--
John Larkin, President Highland Technology Inc
formatting link
jlarkin at highlandtechnology dot com
Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
- Vote on answer
- posted
12 years ago
I executed a red LED to see what it would withstand. The LED just starts to glow at about 1.47 volts and drops 2 volts at 160mA. At 2.7 volts the current is 200mA and brightness is fairly constant over the range of 20Ma to 200mA. At 230mA the voltage rises to almost 5 volts and the brightness falls off. The LED dies a little above that, but it lived a happy life.
-Bill
hnology.com=A0 jlarkin at highlandtechnology dot com
- Vote on answer
- posted
12 years ago
John, I told you there are people who believe there is a "knee" in an exponential function. ;-)
What do you think constant dI/DV means?
- Vote on answer
- posted
12 years ago
Your eye is "fairly constant" over that range. Measure it with a light meter.
- Vote on answer
- posted
12 years ago
And there are people who think that the only kind of resistance is E/I.
Some famous person once said "When all you know is Ohm's Law, everything looks like a resistor."
Well, that was me, actually.
-- John Larkin, President Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
- Vote on answer
- posted
12 years ago
Lots of consumer products do exactly that.
0.7 volts at 1 mA? Where can I get some of those cool silicon LEDs?What color are they?
John
-- John Larkin, President Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
- Vote on answer
- posted
12 years ago
Yeah, efficiency drops at high currents, partially due to heating.
At 230mA the voltage rises to almost 5 volts
Something small like this
formatting link
could be reasonably run from a couple of alkaline cells.
-- John Larkin, President Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
- Vote on answer
- posted
12 years ago
-- Sure, and if they're designed properly, the resistance of the LED and the internal resistance of the battery will limit the current through the LED to safe levels. If not, LED life will be reduced; sometimes drastically.
- Vote on answer
- posted
12 years ago
Something like 350 mA. But there's a lot of variation between different manufacturers' parts.
If it's on a PC board, with a bit of pad+trace to heatsink the leads, it should survive.
At that voltage, it will be well into its ohmic range, namely the current:voltage line will be straight, not exponential. That should be close to the zero TC point too.
Without the series resistance component, pure exponential, increasing the diode voltage from 0.6 to 1.2 would increase the current by a factor of about 10^10.
If you paralleled a bunch of 1N4148s from the same reel, and ran them at, say, 100 mA or so, I'd expect pretty good current sharing.
-- John Larkin, President Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
- Vote on answer
- posted
12 years ago
-- Snipped irrelevant pontificating.
- Vote on answer
- posted
12 years ago
What did you measure?
Electronics is irrelevant to you?
The LT Spice model for the 1N4148 has an Rs value of 0.568 ohms, which looks a tad low to me.
-- John Larkin, President Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
- Vote on answer
- posted
12 years ago
-- 712mA.
- Vote on answer
- posted
12 years ago
That's the static resistance. A more useful value is dV/dI, the slope of the curve in the linear region. That's the Spice 0.568 value. The value from several 1N4148 data sheets is more like 0.7 ohms.
LT Spice shows 0.64 amps at 1.236 volts. Most diode data sheets show about half that current.
-- John Larkin, President Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
- Vote on answer
- posted
12 years ago
-- Since there will always be a disparity between minority and majority carriers in the mix, and charge will be shared between them, there can never be a linear region. Except, perhaps, for the fleeting moment when the TC across the junction equals zero.