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6400-91 Peltier Cooler Kit
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Installation Instructions for the LI-6400/LI-6400XTPortable
Photosynthesis System
IntroductionThere are two Peltier thermoelectric block coolers
located oneither side of the LI-6400/XT sensor head, beneath the
twofans and heat sinks. The block coolers regulate the tempera-ture
of the air that is circulated through the leaf chamber. Thecoolers
can fail in one of two ways; they can fail completely,whereby there
is no change in the block temperature, or theycan become
inefficient, and fail to modify the block tempera-ture over their
specified range. In the case of total failure, oneor both of the
coolers are physically broken, and exhibit anopen circuit
condition. If the coolers function, but will notchange the
temperature sufficiently, the resistance of thecooler(s) is likely
too high, rendering it inefficient.
This document describes how to test the Peltier coolers
(thisrequires an ohmmeter), and replace, if necessary. The 6400-91
Peltier Cooler Installation Kit includes one Peltier cooler,and
additional parts required for the repair, including:
Qty. Description Part No.1 Peltier Cooler 435-041712 Black
thermal conductor pads 108-033762 Shrink wrap 220-00291
Troubleshooting1. See if the fans spin freelyThe LI-6400XT has
circuitry that senses if the fans for thecoolers are running. If
the fans do not turn, power will not beapplied to the coolers. Blow
on the fan blades and see if theyturn. If they do not spin freely,
they may be jammed; dislodgethe impediment, and the problem may be
solved.
2. Check the fusesThere are two fuses inside the LI-6400XT
console, along theedge of the lowermost circuit board, labeled TEC-
and TEC+(Figure 1). Check these two fuses before proceeding.
Centerthe fuse(s) in the clip holder to make sure it has good
contact;if the sides of the clip holder are flared outward, remove
thefuse and pinch the clip until it makes tight contact with
thefuse.
PPS-274
Figure 1. Check the fuses at TEC- and TEC+, highlighted
above.
3. Test the resistance of the cooler(s)a. Turn the LI-6400XT off
and let it sit, untouched, for at
least one hour so that both sides of the Peltier coolersare at
the same temperature.
b. Remove the three screws that secure the bottom coverof the
sensor head IRGA (Figure 2). You may have topry the back end of the
cover, and lift it enough to clearthe log button and light sensor
connectors on the side.
c. Follow the red and black wires from the coolers, andlocate
the two 4-pin white connectors on the edge of
Note, too, that in addition to these instructions, a
videoshowing how to replace the coolers is available at:
http://www.licor.com/6400cooler-replacement
Tools required:Ohmmeter5/64” hex key (allen wrench)Wire
cutter/stripperSoldering iron and solderPhilips head
screwdriverTweezersHeat gun or butane lighter
435-04171 Peltier Cooler
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the circuit board (Figure 4). The red and blue wires arethe fan
wires, and the other two are the cooler wires.Note that newer
LI-6400XTs have red and black fanwires, so be sure to follow the
correct wires back to theconnector if your fan wires are red and
black.
d. You will need to take a resistance reading for the twocooler
wires at the white connector. Since an ohmme-ter sends a small
current through the device under test,it will cause a good cooler’s
resistance reading to changeas soon as you touch the leads to the
pins on theconnectors. Make sure you have the ohmmeter in aposition
to read the display, touch the leads to thecooler wires (polarity
does not matter), and immedi-ately observe the reading (Figure 4).
A cooler that is‘good’ will read under 5 ohms. If the reading is
between5 and 15 ohms, the cooler will operate, but at a
lowerefficiency. If the reading is over 20 ohms, open (infin-ity),
or in the k ohms range, the cooler has failed andshould be
replaced.
Installing the Replacement Cooler(s)1. Don’t remove the four
screws in the corners of the fan
assembly; it’s much easier to remove the entire fan/heatsink
assembly by inserting the 5/64” hex key downthrough the fan housing
and loosening the two hex headscrews (Figure 5). Try not to let the
screws fall completelyout of the fan assembly; it’ll be easier to
re-install later ifthey’re still inserted into the holes in the fan
assembly.This will remove the fan/heat sink assembly, and exposethe
block cooler.
2. Note that there is a thermal conductor pad on theunderside of
the heat sink (Figure 6). Replacement padscan be found in the kit;
the pads do not need to bereplaced unless they are damaged.
3. Note that the red wire is located on the top of the cooleron
the right side of the sensor head, and on the bottom ofthe cooler
on the left side of the sensor head (Figure 6).This is important,
as the coolers are in series with eachother, and work together; if
one of the coolers in installedbackwards, the coolers will work
against each other.
4. Use a pair of tweezers or a small screwdriver to pryaround
the edges of the cooler; it may be stuck to asecond thermal
conductor pad underneath, and may needto be pried off. Loosen the
cooler, but leave it in positionfor now.
5. Use a pair of wire clippers and clip both of the coolerwires
about 1/2” past the black heat shrink, just pastwhere the wires
make a 90 degree bend, as shown inFigure 7.
Figure 2. Remove the three screws that secure the bottom
cover.
Figure 3. Pry the bottom cover off, taking care not to damage
thelog switch and light sensor connectors (highlighted).
Figure 4. Follow the wires from the fan to the connector, and
takea resistance reading at the connector. Polarity does not
matter.
Figure 5. Insert the hex key through the fan housing and
removethe two hex screws that hold the fan/heat sink assembly.
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6. Lay the new replacement cooler over top of the old one,and
bend the wires on the new cooler to match those ofthe old cooler.
Again, make sure the black and red wiresare oriented correctly.
Clip the wires on the new cooler toa length slightly longer than
those that were clipped onthe old cooler. Remove the old cooler and
discard.
7. Strip about 1/16” of the insulation off the two coolerwires.
Unwrap the wires from the fan/heat sink assemblyfor a short
distance, and strip the insulation from them aswell, about 1/16”.
Tin all four exposed wire ends withyour soldering iron.
8. Place the new cooler in position on the sensor head, ontop of
the thermal conductor pad (replace the pad if itwas damaged while
removing the old cooler). Beforesoldering the wires together, it
may be useful to place apiece of masking tape over the cooler to
hold it in placewhile you solder the wires (Figure 8).
9. Place a piece of heat shrink over one of the black and
redwires; they’ll be used later to seal the soldered joints.
10. Use a pair of tweezers to hold one wire steady against
theother as you solder the ends (Figure 9). Place the exposedends
side by side to create a lap joint. Use a freshly tinnedsoldering
iron and solder the ends together. Use thetweezers to slide the
heat shrink over the newly solderedjoint (Figure 10). Repeat for
the other wire joint. Use aheat gun or a butane lighter to seal the
shrink wrap; passthe flame over the heat shrink until it contracts
over thejoint.
11. Note that there are two milled grooves on one side of
theheat sink, as shown in Figure 11. These grooves face theback of
the sensor head, at the 3 o’clock position. Thewires from the
cooler must lay in these grooves wheninstalling the fan/heat sink
assembly, so that the wiresdon’t get pinched.
12. When the cooler wires are routed through the heat
sinkgrooves, put the fan/heat sink assembly into position andstart,
but don’t tighten, the two hex head screws. Alter-nate tightening
between the two screws. Tighten onlyuntil you feel the screws stop;
do not overtighten, as theceramic cooler underneath can be
damaged.
13. Reassemble the lower cover. Make sure the cooler andfan
wires on both sides are not pinched between the coverand the IRGA
block; use the tweezers, if necessary, toposition the wires in the
two grooves on the front edge ofthe lower cover (Figure 12). Make
sure that none of theother wires are pinched, and snap the lower
cover intoplace. You can slide the lower cover back and forth at
theleading edge, nearest the coolers, to see if the wires
arepinched; you should feel metal on metal when the cover
ispositioned properly (Figure 13).
Figure 6. Note the thermal conductor pad on the bottom of
theheat sink. Note, too, that the red wire is on the bottom of
thecooler on the left side of the sensor head.
Figure 7. Clip both wires about 1/2” below the heat shrink.
Figure 8. Place a piece of tape over the cooler to hold it in
placewhile soldering the wires, if necessary.
Figure 9. Use a pair of tweezers to hold the ends together
whileyou solder the exposed wire ends.
Note position of redand black wires
Thermalconductorpad
3
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Inc.
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PPS-274, 7/14
14. Replace the three screws on the lower cover. This
finishesthe cooler installation. Turn the instrument on andchange
the block temperature to verify that the cooler(s)is functioning
properly.
Figure 10. Slide the heat shrink over the soldered joints.
Figure 11. Note the two grooves in one side of the heat
sink;these grooves face toward the back of the sensor head, at the
3o’clock position.
Figure 12. Make sure the wires aren’t pinched underneath theheat
sink before tightening the screws on the fan/heat sinkassembly.
Figure 13. Position the wires in the grooves at the front edge
ofthe bottom cover. Snap the cover into place, rock the cover atthe
leading edge to make sure the wires aren’t pinched, andreattach
with the three screws.
Grooves
Groove
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