Paging Bob Cottiers *PIC*

[hongrn]

http://i2.photobucket.com/albums/y48/hongrn/PA080202.jpg


Hi Bob,

I know you'd have the answer to this question. My LM4780 just arrived. As you said in a post earlier, I have to be careful and isolate the chip from the heatsink because of the metal back. The kit includes a mica sheet for this purpose, but shouldn't nylon screws and washers be as well? And will nylon crack over time with the heat? Everyone, if you have done a gainclone, please chime in. Thanks.

Hong.

[chevyman440]

Provided Link: to-220 installation kit from RS (http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F002%5F008%5F000&product%5Fid=276%2D1373)


> Hi Bob,

> I know you'd have the answer to this
> question. My LM4780 just arrived. As you
> said in a post earlier, I have to be careful
> and isolate the chip from the heatsink
> because of the metal back. The kit includes
> a mica sheet for this purpose, but shouldn't
> nylon screws and washers be as well? And
> will nylon crack over time with the heat?
> Everyone, if you have done a gainclone,
> please chime in. Thanks.

> Hong.

Hong... they make a special plastic sleeve that goes through the top tab screw hole that isolates the screw from the transistor case.

Your other option is to isolate the entire heatsink from everything else... it's easier to isolate the chip yes.

See radio shack link to see what I'm talking about! Maybe you can fab something up? Also consider a nylon screw or not actually screwing through the case, but making a clamp to hold it down that doesn't contact the top metal tab.

Hope that helps!
-Matt

[hongrn]

Matt,

Thanks for RS link. I'll check it out.

Hong

[bill]

Hong,

If the screw slots on either side of the chip are plastic, with no metal exposed, then you can safely use metal screws. It looks like the 4780 only has exposed metal on the back mounting surface.

One other caveat: this chip needs a clamping bar to mount it properly. If you just use the screws, as you tighten them the case and the heatsink will distort slightly and the metal slug on the back of the chip package won't make good contact with the heatsink. See the LM4780 datasheet for more info. You can make a clamping bar out of a piece of 1/8" or thicker aluminum; cut it the same width (or slightly larger) than the chip's width, about a half inch high, drill the screw holes, then file away a bit of the area from the outside edges of the bar to past where the screws go through. The filed-away face of the bar goes against the chip; when you tighten the screws the pressure on the chip will then be against the center part of the package, not the ends. I saw something about this on one of the chipamp forums as well.

Also, the mica by itself is not a great conductor of heat. You need a coating of alumina-filled silicone grease on each side of the mica; this fills any voids and helps greatly with heat conduction. If the kit came with the mica insulators, it should also have included some grease. Be sure to use it.

HTH,
Bill

> Hi Bob,

> I know you'd have the answer to this
> question. My LM4780 just arrived. As you
> said in a post earlier, I have to be careful
> and isolate the chip from the heatsink
> because of the metal back. The kit includes
> a mica sheet for this purpose, but shouldn't
> nylon screws and washers be as well? And
> will nylon crack over time with the heat?
> Everyone, if you have done a gainclone,
> please chime in. Thanks.

> Hong.

[hongrn]

Thanks Bill. I'll have silicone grease on both sides of the mica. I also found a site that describes the tensioning bar, as you suggested. Thanks again.

Hong

[jonpike]

Hong..

You are hearing all the right things.

A tip.. you want to pay attention to the area where the part contacts your heat sink. Smooth down any high spots, burrs, large scratches, etc.
If you have a little raised spot, the whole chip package won't be truly flush, and you won't transfer heat as well, even if it is filled with heat sink compound. And, if using an insulating pad, it could poke thru it, connecting the case to the heatsink. After you tighten them down, you want to ohm it out to make sure it's still insulated.

I'd like you to consider another approach, which would be to skip the mica insulator, come up with a insulated way to mount the heatsink, (it will be inside the box, and with a fan?) and just attach the device directly to the heat sink with some thermal compound. This way, you get better heat transfer. Of course, the down side is that you have a "hot" heatsink.

If you are planning on having that heatsink exposed, this would be a safety hazard and you can't do it. But, you might research into thermal pads, Bergquist (sp?) Aavid, and others make insulating pads that have much higher heat transfer than mica does. You could probably get some sample materiel that would do for several amps.

As I recall, the National chip amps are both current limit and thermal protected, they get too hot they will start to clip themselves. Better heatsinking will help you to avoid that.

Let me know if you have any other questions. I think you're in OC like me... I'd be happy to help out, and view the kit and final product!

[hongrn]

Jon,

Really appreciate the advice. I will definitely use a plastic isolation washer in addition to the mica. BTW, where in OC are you? I'm in Placentia, by the Alta Vista golf course. I'll look you up when I'm done with the amp. Thanks.

Hong

[bill]

Use one insulator or the other, not both. Thermal resistance adds like electrical resistance; if you use two insulators stacked together, you add their thermal resistance. The chip willget way too warm and it'll go into thermal protection at a much lower output power.

Best regards,
Bill

> Jon,

> Really appreciate the advice. I will
> definitely use a plastic isolation washer in
> addition to the mica. BTW, where in OC are
> you? I'm in Placentia, by the Alta Vista
> golf course. I'll look you up when I'm done
> with the amp. Thanks.

> Hong

[hongrn]

http://i2.photobucket.com/albums/y48/hongrn/IsolationWashercopy.jpg


Bill,

I found these instructions for a discrete amp on the net. Note that the transistor (TIP147) has a metal back like the LM4780. The use of mica and silicone grease is similar to what all of you recommended above. Please look at the arrow pointed at the plastic isolation washer. Now can I use the same type of washer on the LM4780? It looks like the washer only insulates the screw from the chip. Thanks.

Hong

[bill]

> Bill,

> I found these instructions for a discrete
> amp on the net. Note that the transistor
> (TIP147) has a metal back like the LM4780.
> The use of mica and silicone grease is
> similar to what all of you recommended
> above. Please look at the arrow pointed at
> the plastic isolation washer. Now can I use
> the same type of washer on the LM4780? It
> looks like the washer only insulates the
> screw from the chip. Thanks.

> Hong

Hong,

I wouldn't use the isolation washer for two reasons. First, based on the datasheet info, the screw slots are completely insulated; the metal slug doesn't touch the slots. Since you have the chips in front of you, you can confirm this. The plastic is non-conductive; only the metal slug on the back of the package is of concern.

Second, you need to use a tension bar, as we discussed earlier. The screw heads need to hold this on. Plastic washers can cold flow; the areas under pressure thin out, causing the screws to loosen. That's why the mounting diagram shows the isolation washers used in conjunction with a spring washer. Your tension bar will serve this function to some extent, although you do have to be careful not to overtighten the screws.

Third, since the package isn't designed for isolation washers, the holes (slots) are sized for the correct size screw. If you use isolation washers, you'll have to go down a screw size.

So, I don't think it's a good idea to use the isolation washers in this case.

HTH,
Bill

[jonpike]

> Jon,

> Really appreciate the advice. I will
> definitely use a plastic isolation washer in
> addition to the mica. BTW, where in OC are
> you? I'm in Placentia, by the Alta Vista
> golf course. I'll look you up when I'm done
> with the amp. Thanks.

> Hong

Aha... I'm in Irvine, not too far. I'm starting to think seriously about building a chipamp as well. Even thinking about maybe breaking down and getting SoundEasy, and trying the filter simulation. I'll need a few channels of amps for that!

I agree with Bill that you shouldn't need the isolating washer, and it might create some mechanical problems. I also think the force spreading bar is a good idea as well. Kind of like using a slightly bowed piece of hardwood to clamp something much wider than the depth of your bar clamp's reach... to apply the force out in the middle rather than just at the edges. That part is kind of wide, and you're not pressing directly on the center.

[robertcottiers]

> Hi Bob,

> I know you'd have the answer to this
> question. My LM4780 just arrived. As you
> said in a post earlier, I have to be careful
> and isolate the chip from the heatsink
> because of the metal back. The kit includes
> a mica sheet for this purpose, but shouldn't
> nylon screws and washers be as well? And
> will nylon crack over time with the heat?
> Everyone, if you have done a gainclone,
> please chime in. Thanks.

> Hong.

Hong:I just tuned in..Looks like you have plenty of good suggestions to go with.

Enjoy Bob C.

[envisionelec]

> Hong,

> I wouldn't use the isolation washer for two
> reasons. First, based on the datasheet info,
> the screw slots are completely insulated;
> the metal slug doesn't touch the slots.
> Since you have the chips in front of you,
> you can confirm this. The plastic is
> non-conductive; only the metal slug on the
> back of the package is of concern.

> Second, you need to use a tension bar, as we
> discussed earlier. The screw heads need to
> hold this on. Plastic washers can cold flow;
> the areas under pressure thin out, causing
> the screws to loosen. That's why the
> mounting diagram shows the isolation washers
> used in conjunction with a spring washer.
> Your tension bar will serve this function to
> some extent, although you do have to be
> careful not to overtighten the screws.

> Third, since the package isn't designed for
> isolation washers, the holes (slots) are
> sized for the correct size screw. If you use
> isolation washers, you'll have to go down a
> screw size.

> So, I don't think it's a good idea to use
> the isolation washers in this case.

> HTH,
> Bill

Wow, the flood of incorrect information astounds me.

Use two screws with spring washers and a mica insulator with thermal compound.

THAT'S IT.

You can use a tension bar if you want, but it's not 100% necessary.