Cutting off a piece of an LM3886 (yes, with a saw) - diyAudio
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Old 4th January 2008, 12:13 PM   #1
indoubt is offline indoubt  Netherlands
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Default Cutting off a piece of an LM3886 (yes, with a saw)

I've checked the datasheet but cannot find anything with respect to this matter.

Here is the question:
Is the thinner part of the LM3886TF chip, the part where the screw hole is located, empty and only used for mounting or are there any electronics in this part?

The reason being that I would like to cut a part of for mounting purposes. The chips will be mounted with a heat spreader bar and I will not use the screw hole. In stead I have just not enough space for the height of the chip and if I can saw the upper part, say upto the mid of the screw hole, off than I can fit the amps in the desired case.

Maybe a strange question though I cannot see a reason not to try
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Old 4th January 2008, 12:22 PM   #2
Nordic is offline Nordic  South Africa
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Well for sure it is electricaly connected to the chip... guess you can cut some off, but I would derate the unit a little then... and aim to keep power a little bit further away from maximum as you now have a smaller heat transfer area from the case to the heatsink...
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Old 4th January 2008, 12:36 PM   #3
indoubt is offline indoubt  Netherlands
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I will use the chips with 18V tranny (25V DC) so they are not up to the max power potential. There will be an aluminium heat spreader bar or cooling element on the front of the chips to keep them as cool as possible under the circumstances. My guess is that most of the heat is generated in the area where the electronics are
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Old 4th January 2008, 12:48 PM   #4
Nordic is offline Nordic  South Africa
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Yep... but its not realy designed to go through the plastic would be my guess... most heat will move through the metal parts... to front part is more like a dome over the chip...

Oh well even if it burns up... you can probably still afford a few meals for the month...
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Old 4th January 2008, 12:51 PM   #5
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There are no electronic components in the thin area. Given your proposed solution for mounting, I would say you have a high probability of a successful installation.

Two things to consider after you make the cut.

1. You will be exposing a metal tab underneath the plastic which is electrically connected to the chip. Make sure the exposed metal has enough clearance from your heatsink / enclosure to avoid a short.

2. After you cut, bevel the cut edge with some sandpaper or a file so that burrs from the cut plastic and metal do not prevent the part from sitting flat on the heatsink.

Good luck!

David
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Old 4th January 2008, 01:27 PM   #6
indoubt is offline indoubt  Netherlands
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Thanks Guys,

It is a TF chip, so no metal parts currently exposed. I will isolate the metal parts if they become exposed. If it is only the metal part I'm not worried, I was worried about electronics that might be exposed and/or damaged.

Calculated maximum power dissipation will be at 28 watt maximum with this relatively low voltage and the predicted loudspeaker impedance(4 ohm) and a maximum power rating of 50W output per chip.

I would not try this ussually but I want to pack the amplifier with PSU and remote controls for source selection and volume control in a really small case
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Old 4th January 2008, 01:28 PM   #7
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I can't see any problem in cutting off part of the exposed metal. What you do want to be careful of is warping the part, as that will crack the die mounting. Look at the torque specs for mounting flat pack devices- they're quite low for a reason. I'd not try this with a hack saw, but use a jewelers saw with a medium blade. Far less stress on the part.
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Old 4th January 2008, 04:10 PM   #8
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fwiw. I attempted to cut the tab off of a voltage regulator a while back to make it fit a case. Unfortunately it did not work after the event.
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Old 4th January 2008, 05:01 PM   #9
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Default Less sinking?

Couldn't you do this...In addition to sinking on the back half as usual , make up for the loss by running a sink on the front half?
__________________________________Rick............ ...
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Old 6th January 2008, 06:39 PM   #10
indoubt is offline indoubt  Netherlands
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There is no way back now

It looks like the operation was successful. It will be a while before I can test them though but I will let you know when and if it works

The chips will be in between aluminium, there will be a small heatsink on the front, a part of an 486 processor cooler.

Pics to follow
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