D
Don Klipstein
In <[email protected]>, [email protected]
wrote:
And I follow below with some musings of mine for a homebrew LED table
lamp project using 10-for-$10 eBay 70-80 lumen "1-watt" white LEDs...
If this is going to be a homebrew project, with a $2 used table lamp
and a wallwart, it's starting to sound easy.
I figure, to push 350 mA through ten "1-watt" LEDs @ 1.12-1.19 watts
apiece (3.2-3.4 volts apiece), takes 11.2-11.9 watts, plus a watt or two
for a switching type LED driver - figure 12-14 watts.
The wallwart probably has loss of another 1.5-2 watts if it's a
switchmode type, 2 in the transformer and 1.5 in the rectifier if it's
the older iron core type.
Total power consumption from the wall plug - 13.5 to 17.5 watts to
produce 700-800 lumens of icy cold 6000-6500 K light whose color rendering
indexI guess from experience is probably 70-plus or around 75.
Maybe there is no need for a bulb-shape enclosure for a modification of
an entire table lamp. As for a screw base - burnt out lightbulbs are
free, unblown ones (even of kinds exempted from the upcoming ban) can be
purchased at a dollar store for $1, and I see screw-base fuses at
supermarkets, drugstores, and hardware stores.
=====
As for temperature rise - that is a big problem with LED lighting, but I
see less of a problem in a homebrew table lamp project. Simply get the
heatsink area to 24 square inches to handle 12 watts at the 1/2 watt per
square inch mentioned elsewhere in this thread. (1.12-1.19 watts for each
of 10 LEDs dropping 3.2-3.4 volts at 350 mA)
My experience indicates this will limit temperature rise to 50 C, and
even in a 35 C ambient this means heatsink temperature of 85 C and
junction temperature of "1-watt" LEDs around 95 C - actually quite
tolerable, but expect something like 6% less light than "rated" (which
requires unrealistic junction temperature of 25 C).
I like to have lighting LEDs heatsunk enough to keep their junction
temperatures below 85 C - arbitrary, but that was the max. rated junction
temperature of LEDs back around 1980. At this rate, I like to figure
..4 watt per square inch of heatsink, or 30 square inches of heatsink.
==============
How about a further idea: As cheap as these LEDs are, get 20 instead of
10. Drive them at 175 mA instead of 350 mA. Get 750-870 lumens instead
of 700-800, because usual white LEDs have a nonlinearity where efficiency
increases when they are moderately underpowered. (Efficiency of "1-watt"
and "3-watt" white LEDs with a roughly 1 mm square die peaks at 20-75 mA.)
And, voltage drop at half-current is less than at full current by about
..05 volt - power consumption is decreased slightly. This reduces the
heatsink temperature rise roughly 1.5% - not much, but I like to
consider this along with other minor incremental improvements.
And figure conservatively (after aging, etc) only 30 lumens light gain,
combined with a figure I have of about 300 lumens per watt of "white LED
light" - that's another .1 watt less heat, good for another .8% less
temperature rise of the heatsink. At this rate, heatsink temperature rise
is decreased ~2.2%, maybe close to 1 degree C.
Furthermore, junction temperature is reduced by 3-5 degrees C even at
the same heatsink temperature - 4 to 6 degrees C if considering reduced
heatsink temperature. Shrinking the heatsink to restore the original
junction temperature (if tolerable) is from 30 to 24 square inches, for
85 C junction temperature in a 35 C ambient, the way I see it.
=============
Or, push only 165 mA through each of 20 of these LEDs, for about 6%
less power consumption than at 175 mA, and still get at least as much
light as from 10 of these LEDs at 350 mA each. That lets the above
heatsink be at least 2 degrees C cooler, or at least a square inch
smaller.
====
wrote:
And I follow below with some musings of mine for a homebrew LED table
lamp project using 10-for-$10 eBay 70-80 lumen "1-watt" white LEDs...
Do you really get 70 lm/W LEDs for $1 ?
If you intend to build these into the same space as the 60 W bulb, you
are going to have severe heat transfer problems. Quite possibly
needing aluminum based PCBs etc. connected to heat sinks.
Without handling the heat issue properly, you end up with a LED lamp
design that does not even last as long as the ordinary incandescent
bulb.
If this is going to be a homebrew project, with a $2 used table lamp
and a wallwart, it's starting to sound easy.
I figure, to push 350 mA through ten "1-watt" LEDs @ 1.12-1.19 watts
apiece (3.2-3.4 volts apiece), takes 11.2-11.9 watts, plus a watt or two
for a switching type LED driver - figure 12-14 watts.
The wallwart probably has loss of another 1.5-2 watts if it's a
switchmode type, 2 in the transformer and 1.5 in the rectifier if it's
the older iron core type.
Total power consumption from the wall plug - 13.5 to 17.5 watts to
produce 700-800 lumens of icy cold 6000-6500 K light whose color rendering
indexI guess from experience is probably 70-plus or around 75.
Maybe there is no need for a bulb-shape enclosure for a modification of
an entire table lamp. As for a screw base - burnt out lightbulbs are
free, unblown ones (even of kinds exempted from the upcoming ban) can be
purchased at a dollar store for $1, and I see screw-base fuses at
supermarkets, drugstores, and hardware stores.
=====
As for temperature rise - that is a big problem with LED lighting, but I
see less of a problem in a homebrew table lamp project. Simply get the
heatsink area to 24 square inches to handle 12 watts at the 1/2 watt per
square inch mentioned elsewhere in this thread. (1.12-1.19 watts for each
of 10 LEDs dropping 3.2-3.4 volts at 350 mA)
My experience indicates this will limit temperature rise to 50 C, and
even in a 35 C ambient this means heatsink temperature of 85 C and
junction temperature of "1-watt" LEDs around 95 C - actually quite
tolerable, but expect something like 6% less light than "rated" (which
requires unrealistic junction temperature of 25 C).
I like to have lighting LEDs heatsunk enough to keep their junction
temperatures below 85 C - arbitrary, but that was the max. rated junction
temperature of LEDs back around 1980. At this rate, I like to figure
..4 watt per square inch of heatsink, or 30 square inches of heatsink.
==============
How about a further idea: As cheap as these LEDs are, get 20 instead of
10. Drive them at 175 mA instead of 350 mA. Get 750-870 lumens instead
of 700-800, because usual white LEDs have a nonlinearity where efficiency
increases when they are moderately underpowered. (Efficiency of "1-watt"
and "3-watt" white LEDs with a roughly 1 mm square die peaks at 20-75 mA.)
And, voltage drop at half-current is less than at full current by about
..05 volt - power consumption is decreased slightly. This reduces the
heatsink temperature rise roughly 1.5% - not much, but I like to
consider this along with other minor incremental improvements.
And figure conservatively (after aging, etc) only 30 lumens light gain,
combined with a figure I have of about 300 lumens per watt of "white LED
light" - that's another .1 watt less heat, good for another .8% less
temperature rise of the heatsink. At this rate, heatsink temperature rise
is decreased ~2.2%, maybe close to 1 degree C.
Furthermore, junction temperature is reduced by 3-5 degrees C even at
the same heatsink temperature - 4 to 6 degrees C if considering reduced
heatsink temperature. Shrinking the heatsink to restore the original
junction temperature (if tolerable) is from 30 to 24 square inches, for
85 C junction temperature in a 35 C ambient, the way I see it.
=============
Or, push only 165 mA through each of 20 of these LEDs, for about 6%
less power consumption than at 175 mA, and still get at least as much
light as from 10 of these LEDs at 350 mA each. That lets the above
heatsink be at least 2 degrees C cooler, or at least a square inch
smaller.
====
