LEDs in parallel

It's usually the other way around: exponential at low currents, ohmic at higher currents.

John

Reply to
John Larkin
Loading thread data ...

Some people still think an exponential has a "knee".

Reply to
krw

Ha ha, thanks!

--
Bob just used 'canonical' in the canonical way.  [Guy Steele]
Reply to
Adam Funk

--
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
 Click to see the full signature
Reply to
John Fields

Thanks.

--
Bob just used 'canonical' in the canonical way.  [Guy Steele]
Reply to
Adam Funk

--
news:86g8i7pot46vjj41o3qlbnl72q65gm157q@4ax.com
Reply to
John Fields

Well, lots of people do it and get away with it.

John

--
John Larkin, President       Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   
 Click to see the full signature
Reply to
John Larkin

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

Reply to
Bill Bowden

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?

Reply to
krw

Your eye is "fairly constant" over that range. Measure it with a light meter.

Reply to
krw

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   
 Click to see the full signature
Reply to
John Larkin

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   
 Click to see the full signature
Reply to
John Larkin

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   
 Click to see the full signature
Reply to
John Larkin

--
Sure, and if they're designed properly, the resistance of the LED and
the internal resistance of the battery will limit the current through
 Click to see the full signature
Reply to
John Fields

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   
 Click to see the full signature
Reply to
John Larkin

--
Snipped irrelevant pontificating.
Reply to
John Fields

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   
 Click to see the full signature
Reply to
John Larkin

--
712mA.
Reply to
John Fields

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   
 Click to see the full signature
Reply to
John Larkin

--
Since there will always be a disparity between minority and majority
carriers in the mix, and charge will be shared between them, there can
 Click to see the full signature
Reply to
John Fields

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.