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Old 16th March 2007, 09:04 PM   #1
Minion is offline Minion  Canada
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Default LM3886 Isolation Problem......

Hi, I am building a Bridged/Paralell Chip amp useing 2 LM3886 Chips but I am haveing a Major problem Isolateing the Chips from the Heatsink......

I have the 2 Chips mounted to the Same aluminum Heatsink and I am useing Silicon Thermal Pads and a Bolt to Fasten the Heatsink to the Chips going through the Hole provided on the Chip ,and I have a Rubber washer between the head of the Bolt and the Chip......

The Problem is that when I use my DMM I can see that there is still a electrical connection between the -v Rail and the Heatsink It"s self......I do not see how it can be unless the Bolts threads are touching the Inside of the mounting Hole in the IC??? But how do you stop this??

I have even Tried Painting the the inside of the Mounting hole but there still seems to be an electrical connection from -V to the Heatsink......

I usually use the Isolated versions but this time I couldn"t get them so I have the un-isolated version......

How do I stop this from happening??

Would it burn out the Chips if they were not Isolated from each other?? I don"t think it should because they are both recieveing the Same -V, It"s just if the heatsink comes in contact with 0v or the +v rails that the short curcuit would orrur...Right??

I was wondering if I can"t fix this problem would it work as long as I made sure that the Heatsink didn"t touch Ground or +v???


Thanx a Lot.....

Cheers
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Old 16th March 2007, 11:14 PM   #2
ttan98 is offline ttan98  Australia
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I use to experince the same problem as you do when I was building an amp using transistor based on TO3 package.

You can do the following:

1. repalce the rubber isolato with an elongated plastic isolator between the bolt and chipamp. I think you can still find them. You may still need to adjust the chipamp so that the is no electrical contact.

or

2. use a metal bracket that can cramp to the body chipamp with 2 screws at opposite end, refer to this site:http://www.shine7.com/audio/bpa300.htm
as an example.

hope this helps..
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Old 17th March 2007, 12:52 AM   #3
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Hi,
use the insulating bushes, they use in transistor mounting kits.
http://www.maplin.co.uk/module.aspx?...30386&doy=17m3
and make shure that you didnīt use a conductive cooling pad
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Old 17th March 2007, 10:10 AM   #4
raindog is offline raindog  Romania
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As long as your heath sink is isolated from ground ( and chassis ) and V+ you don't need to use anything between the chip and the heath sink ( except some thermal conductive paste ) and you get better heath transfer . This is what i did with my amp and I've had no problems... Or you could use the TF version of the chip.
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Old 18th March 2007, 10:27 PM   #5
Minion is offline Minion  Canada
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Thanx for the Tips.....I am going to try a Few things like Painting the Bolts and see if that helps or matbe finding a easy to build Bushing for the inside of the hole for the Chip ......


Thanx Guys
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Old 18th March 2007, 10:44 PM   #6
Nordic is offline Nordic  South Africa
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If you have an old dead PC power supply somewhere you can rip it open, it has some TO220 diodes stuck to heatsinks useing the broper isolating plastic bushes... you can buy kits with the pads and bushes online from places like RS components...
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Old 19th March 2007, 12:27 AM   #7
BWRX is offline BWRX  United States
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In the past I've used some heat shrink tubing around exposed parts of screws to insulate them from a metal chassis. Electrical tape works too but isn't as secure as heat shrink tubing.
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Old 28th March 2007, 01:23 AM   #8
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I would recommend against merely painting the bolts. This may not be a long term solution, you should either isolate the heatsink from the chassis (if metal chassis) since 2 chips using same supply negative rail will not be a problem in electrical contact with same heatsink, or use the proper isolation bushing as already mentioned.
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Old 28th March 2007, 02:34 AM   #9
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I like to take an old PC heatsink, cut it down, and drill two holes in it. Use it to strap down the chip. It's safe and actually helps cooling a little.
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Old 28th March 2007, 03:51 AM   #10
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go to the hardware store and buy an aluminum plate..use the plate to clamp the IC to the heatsink...
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