LEDs in parallel

Jan 24, 2012 190 Replies
D

Okay, Nym's had his shot.

There are several LED flashlight schemes. The crudest connect 3 series cells to the LEDs, relying on the cells' series resistance and the LEDs resistance. Some add series resistors. If you parallel enough LEDs they can handle 3xAAA in series, but as soon as one LED fails, the rest rapidly die.

Boost converter schemes apply either constant voltage, constant voltage from a current mode boost, or current mode boost, in order of sophistication.

More sophisticated schemes modulate the converter, for multiple power settings.

None use 555s AFAIK.

-- Cheers, James Arthur

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L

Yeah. RCA made some of the earliest mesa parts. Fairchild never made them, but pioneered epitaxy.

RL

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J

oh. good point.

--

John Larkin, President Highland Technology Inc

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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

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K

That's because, as everyone knows, 555s are obsolete. ;-)

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J

Then what stops them?

John Larkin, President Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
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J

Yup. The slope is just about 1 ohm. The dynamic impedance of a PN junction is 1 ohm at 25 mA.

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
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J

Closer to 26mA ;-) ...Jim Thompson

| James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
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G

to

=A0 =A0 ...Jim Thompson

=A0 =A0| =A0 =A0mens =A0 =A0 |

=A0 | =A0 =A0 et =A0 =A0 =A0|

=A0|

=A0 =A0 =A0 |

ide quoted text -

1 ohm is a pretty good number for a small junction. I measured a 2n3904 about the same.

Neat, how the resistance goes to ~1.5 ohms at ~450K.

George H.

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J

[snip]

Precise small-signal slope (assuming no resistive part) is kT/qI, or ~26/I(in mA) at 300°K ...Jim Thompson

| James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
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J

quoted text -

Yup, the ohmic part has a positive TC.

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
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J

quoted text -

Huh? It does, but George was taking about the kT/qI portion...

450/300=1.5 ;-) ...Jim Thompson
| James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
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G

ave to

=A0 =A0 =A0 ...Jim Thompson

=A0 =A0 =A0| =A0 =A0mens =A0 =A0 |

=A0 =A0 | =A0 =A0 et =A0 =A0 =A0|

=A0 =A0|

=A0 =A0 =A0 =A0 |

=A0|

|

.- Hide quoted text -

Opps, I was talking about the resistive part. It should also scale with the absolute temperature, if the resistance is from some hunk of 'metal'.

Figure 3 in the 1n4148 data sheet that someone linked to.

(I'd just been thinking about the Wiedeman-Franz law from Jamie's MAF post.)

George H.

=A0 =A0 ...Jim Thompson

=A0 =A0| =A0 =A0mens =A0 =A0 |

=A0 | =A0 =A0 et =A0 =A0 =A0|

=A0|

=A0 =A0 =A0 |

ide quoted text -

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J

to

quoted text -

Yes, I thought you were. JT's comment didn't make sense.

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
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J

That's British for "whining", isn't it? How is that pronounced?

--

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

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M

Sometimes a troll is just a troll...

You can't have a sense of humor, if you have no sense.
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M

So are Slomans. Museum pieces, in fact.

You can't have a sense of humor, if you have no sense.
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J

to

[snip]

Why? Because you don't understand kT/qI ?:-)

Perfect diode, no resistive term, DYNAMIC impedance scales linearly with temperature (°K). So a perfect diode operating a 26mA at 300°K has a DYNAMIC impedance of 1 Ohm; and 1.5 Ohm DYNAMIC impedance at

450°K.

Perhaps someone would plot 1N4148 forward drop on LOG paper so we can easily see the bulk R breakpoint (and the _dirt_ at low currents, the ISR coefficient in the model). ...Jim Thompson

| James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
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