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Author Topic: lm3886 heatsinks/thermal grease etc.  (Read 8569 times)

troublerat

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lm3886 heatsinks/thermal grease etc.
« on: June 15, 2009, 11:00:37 PM »
I have read the posts on RDV lm3886 amp. I want to see if I"m understanding this properly as I am about to build one myself. The chip needs thermal grease to be properly coupled to the heatsink. The heatsink needs to be properly isolated from any other metal parts [case]. The heatsink needs to be properly sized for the chip. Now what I am not clear about is the mica and other substances mentioned. Are they to be used with the thermal grease or an alternative . The heatsink should not come in contact with other pieces on the circuit board. Any help would be appreciated.

J M Fahey

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Re: lm3886 heatsinks/thermal grease etc.
« Reply #1 on: June 16, 2009, 09:34:22 AM »
Hi TroubleRat.
To begin with, you must *always* use heat transfer grease, on all surfaces between the chip and the heatsink.
The "Tab" (metallic back of the chip case) is connected to the V- (negative voltage) pin, so there are 3 possibilities:
1) Single supply (Only V+ and ground, output capacitor between output and speaker)
In that case V- and ground are the same, use only grease, don´t need mica. *possible*
2)Split supply: V+ , V- ,  and ground; output directly connected to speaker. You must use grease *and* a suitable mica, to electrically insulate them, and be free to mount the heatsink anyway you wish. You may even use the chassis (if made of aluminum) as a "free" heat sink. *recommended*
3)Split supply, you want to save around 1 cent by not using a mica. If you wish .... but then insulating the full heatsink from the chassis becomes a pain in the .... well, you know where. *not recommended*
There are available some rectangular pieces of grey birthday party balloon rubber (well, I don´t find them very different to that), called "silpads", which claim to replace both mica and grease. Many use them, to save some cents in assembly, or, for the better reason (this one almost convinces me) of avoiding the *messy* operation of greasing, but I trust *much* more traditional grease/mica, in insulation, heat transfer, and puncture-resistance.
There is an LM3886 *insulated* version, but the dissipation penalty is heavy and I just don´t find it worth paying.

armstrom

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Re: lm3886 heatsinks/thermal grease etc.
« Reply #2 on: June 16, 2009, 02:26:46 PM »
One additional option is to buy the version of the LM3886 that has electrical insulation on the "tab". That will eliminate the need to electrically insulate the connection between the chip and the heat sink. If the tab on your chip is black you have the insulated version and can get away with simply using good heat sink compound for thermal transfer. If your tab is silver in color then you have the non-insulated version and will need to take one of the precautions outlined by the previous post.
-Matt

troublerat

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Re: lm3886 heatsinks/thermal grease etc.
« Reply #3 on: June 17, 2009, 12:33:55 AM »
I have been reading up on heat sinks at Elliot sound products and have a little better understanding. The chip needs thermal grease/ the smoother the finish on the flat side of the alum. the better mating of the two. I also have been looking back at some post at diy stompboxes. R G writes that an  lm3886 will work on an unregulated power supply. So I have an trans. with 24 -0 - 24 secondaries and I"m going to use an 25a bridge rect. with some largish caps [between2000 and 4000 uf] with those snubberizer caps. I"m not too interested in getting max vol. or even gigging with it. Forgive me for being a little thick on this thermal grease /heat sink thing. What we want is to pass the heat from chip to the heat sink but not the electrical characteristics of the chip itself. With an + - power supply. The pos volt connected to the pos. pin the neg. volt connected to the neg pin. The surface of the chip and heat sink are the grnd ?

phatt

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Re: lm3886 heatsinks/thermal grease etc.
« Reply #4 on: June 17, 2009, 05:31:01 AM »
Forgive me for being a little thick on this thermal grease /heat sink thing. What we want is to pass the heat from chip to the heat sink but not the electrical characteristics of the chip itself. With an + - power supply. The pos volt connected to the pos. pin the neg. volt connected to the neg pin. The surface of the chip and heat sink are the grnd ?
Hi troublerat,
                  NO That would be a very bad assumption!
You do get chips that are "Case Ground" but most are NOT.

Heat sinking is not hard to get your head around just think BIG and FAT,
You don't even need the fins in some cases,, The Thickness of *Where* the chip mounts is the important thing, not the fancy fins.
That Chip/Power Transistor whatever has to get rid of excess heat instantly and the only way to do that is to have Alloy with some girth/Mass (thickness) right there next to it.  Fancy Finns help but more often than not they just look pretty.

To get some idea of what goes on look at the heatsinks on computer chips,,, some actually come with a *Copper* centre!! Yes the Copper pulls the heat out faster than alloy.  Then the alloy,,,, Then the fancy finns, then the fancy Fans.

I use  a 6mm thick Alloy Angle plate 100 x 100mm by 200mm long (with no fins) 
Dissapating a 50 watt amp and it is only ever warm.

A rule of thumb I use is if the alloy is *to hot to touch for more than 1 sec* then your in trouble.  though some pro gear runs this hot it's far from ideal.
Phil.

J M Fahey

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Re: lm3886 heatsinks/thermal grease etc.
« Reply #5 on: June 17, 2009, 09:15:59 AM »
Hi TroubleRat. Please re-read carefully the answers, it´s all there.
Quote
>"The "Tab" (metallic back of the chip case) is connected to the V- (negative voltage) pin,"<
In a split supply, "V-" is *not* the ground.
Quote
Split supply: V+ , V- ,  and ground; output directly connected to speaker. You *must* use grease *and* a suitable mica, to electrically insulate them,
Quote
I have an trans. with 24 -0 - 24 secondaries and I"m going to use an 25a bridge rect. with some largish caps [between2000 and 4000 uf]
Good !!  :tu: , you'll have a powerful +/- 35Vd.c. supply, your amp will be LOUD and you´ll definitely be able to gig with it.
Your transformer must be able to supply at least 4A, preferably 5 or 6, just to play it safe.
Buy those 4700uF rated at 50V (at least 40V, no less)
The heatsink suggested by Phil is *excellent*, and will probably be easier to find locally and cheaper than a big finned one. Anyway, you might get a good traditional one in EBay, just look around. Fenders usually use only the aluminum chassis as heatsinks, but mount their transistors on a thick aluminum bar (1 1/4" square or thereabouts) to provide that "up close girth" Phil mentions.
I myself use a 2mm aluminum chassis (i *hate* steel), with a supplementary extra plate, 4" by 10" , greased and riveted to the main chassis, plus 1/8" aluminum angle to which the actual transistors are bolted. So the thickness tapers: 7.2mm>4mm>2mm as you go away from the "hot spot".
The "finger-test" is good: we can´t support 70ºC (don´t know how much that is in ºF) for more than a few seconds, 60ºC is *very* uncomfortable for the (more sensitive) palm of our hands.
If your "wet" finger sizzles .... get ready to buy new chips/transistors.
Good luck with your project.
I think your neighbours will hate you. :trouble

Enzo

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Re: lm3886 heatsinks/thermal grease etc.
« Reply #6 on: June 17, 2009, 07:48:48 PM »
One cheap source of heatsinks is dead home stereo amplifiers.  FInd an amplifier or receiver someone is discarding and strip it for the parts.

Thermal grease is NOT an insulator.

If i recall without looking it up, the LM3886 with the insulated plastic tab is the LM3886TF.  Correct me if I am wrong please.

troublerat

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Re: lm3886 heatsinks/thermal grease etc.
« Reply #7 on: June 17, 2009, 08:09:18 PM »
   Hey I get it, I have a history of making fairly simple or unkown things very difficult to understand.  I went around the mountain several times  when I went on the " lets understand these transformer kick. And then the light turns on and all of a sudden " ok that was fairly simple after all. This is a part time hobby for me after all, it all started when I realized that very soon that I would not be able to buy any more gear and if I wanted anything then I better learn how to make some of this stuff. Heres a sad story, at one time I had no less than three amps, four guitars, pedals out the ying yang, effects processors [ three of them]  hell man I couldnt even make up my mind which one to turn on. This is my story I was an gearoholic. Long after the fall I find myself plugging in to an 1.75 realistic 12 watt power amp and one of the few things I kept,an 4/10 univox cab. One day I was looking at the tiny hacked up power amp and telling myself this cant be that hard to make. But its all good now. By the way everything that was said made good sence and thanks for the help..........stripped stereos   :)Hey thats where I have been acquing parts and storing them until I found out what to do with them. LM 3886 tf I think thats the one I ordered.

joecool85

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Re: lm3886 heatsinks/thermal grease etc.
« Reply #8 on: June 18, 2009, 05:32:31 AM »
I used a insulated version of the lm3886 with no issues myself.  I drove an 8 ohm load at 50% volume for hours on end and never over heated the chip.  That said...I did have a bit large of a heatsink.  But, if it was the chips thermal dissipation you were worried about, it wouldn't matter how large of a sink I had.
Life is what you make it.
Still rockin' the Dean Markley K-20X
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troublerat

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Re: lm3886 heatsinks/thermal grease etc.
« Reply #9 on: June 18, 2009, 08:47:17 PM »
http://www.epemag3.com/index.php?option=com_docman&task=cat_view&gid=56&Itemid=38  I understand now what RDV was speaking about in some post I"ve read on shorting out the chip. If you go to this site and read the pdf file on the practise amp utilizing the tda2030. It describes using the metalic back of the chip as an direct connection to -ve and should be isolated from everything else. Also it describes that thermal grease may not be needed. This sounds to me that its an recipe for disaster. Especially someone like myself  being [just a little ignorant of the facts] maybe. Iwant to do this right the first time. I am going to assume that this a top order build [all though its not] but pretending that it is hopefully that being a deterent to future mistakes...... Now on the note of connecting -ve via the back of the tda 2030 what theyre doing is using that back instead of the -ve pin on the chip which would be the safer route. Am I close to being correct? I mean if you use the pin for -ve and  utilize thermal grease and mica on thr back of the chip then your fairly well protected.