Mica versus Silicone pad insulators

That being the case, why is it so easy to burn your tongue on a hot drink or hot food ?

Arfa

Hey Phil,

I have used similar high-performance washers in some of my amplifier designs although I prefer to avoid washers ideally using 'live' heatsinks fwiw.

I can assure you that they absolutely do not have 1.2C/W thermal resistance in practice. It's trivially simple to do the measurements. In one design that comes to mind using the 'rust' coloured washers from Warth, the thermal resistance was measured at ~ 0.3C/W.

Graham

output

critical

control

no

to air

to

quality

vary

so it

several

somewhat

silpad.

lower

in the

I think I will rely on the evidence of my own observations and not performance tables produced by the manufacturer's with an obvious vested interest. I've no reason to believe the original silpads had aged, been affected by WD40 or anything. I will assume they are , all manufacturers, all generically bad until a similar personally conducted experiment, in a real situation, proves to me to be otherwise.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Exactly what is the item of equipment in question?

Ron(UK)

Surely a degree of faith in what manufacturers say, has to be assumed, otherwise we would all go around our whole lives with our heads up our collective arses ! Do you for instance do a complete analysis of every piece of processed food you eat to see if the manufacturer is lying about how much salt it contains or how much calorific value it has ? Do you check your bottles of beer to see if the ABV that the manufacturer quotes is true ? Do you run a complete set of tests on a resistor type before stocking it as a spare to see if what the manufacturer says about its tolerance stability is correct ? No? Neither do I.

I can understand scepticism about 'waved in the air' figures that appear to have little or no foundation in reality, but figures for these washers that quote specifics on the applied pressure to get the results that are published, don't seem to fall into that category to me. I can also understand manufacturers of some products getting into 'mine's bigger than yours' specmanship contests, but silicone heatsink pads ? Do you really believe that these are such a big part of the manufacturer's total output of products, that he needs to engage in the sort of behaviour that involves lying, or at least distorting, the 'true' facts about the product, just to sell a few more than the other guy ?

As it happens, I cannot remember seeing pads distorted in the way that you describe, so maybe they were from a bad batch or something. Maybe, the amp manufacturer torqued them up too much, and the heat did the rest. Maybe the amp was used on midsummer's day in Ibiza at full volume with a blanket over it, and a fault ...

I agree that many amps have output devices that appear to run hotter than we might like, and that the thermal transfer across the insulators seems less than ideal, but if that's the way the manufacturer made it and is happy with, and the item is not on my bench with failed output transistors, then I tend to just note the fact with interest, and work on using the "If it ain't broke, don't fix it" principle.

And to Graham, I HATE amps that have live heatsinks. Just a few days ago, I had a big AV amp on the bench that nearly threw me off my chair when I laid the edge of my hand across the two heatsinks ready to take some measurements with the meter probe in the other hand. This thing had rails of about +/-

Arfa

Considering that you don't know how to measure thermal ressitance I will rely on my own measurements which say that top-quality high performance 'sil-pads' can outperform grease and mica.

Graham

power

have

(heatsink

characters

by

me

piece

much

Do

is

to

that

of

the

over

we

ain't

laid

measurements

Unless big G has a defendable patent on mica then why do I not see like-for-like , in real situations, comparison tables of BigCorp GeeWizz SiilyPad versus mica in the performance figures.

"Live" heatsink amps would be ok if the damn things were labelled with big fluorescent stickes stating such. But of course such amps should only be serviced by their own service agents is what they would say.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

A bit of a zap now and then is sposed to be very good for your heart - if it don't stop it that is ;)

I`m with you on the live heatsink thing, but it does make mounting devices a lot simpler (and cheaper for the manufacturer no doubt).

Has anyone any thoughts on using nylon machine screws to mount devices, instead of the usual tophat spacer and normal nut n bolt?

Ron(UK)

You're not supposed to rest your hand inside live equipment !

I hope you take more care with LOPTs etc.

Graham

You're not a proper TV repair man till you lose all the hairs off the back of your hand!

Ron(UK)

Easy to say, but when you've been doing it for 30 odd years, and are used to heatsinks being grounded, it's hard to change. And as for LOPTxs, I try to avoid even getting the backs off TV sets and monitors these days, unless it's for a mate ...

Arfa

That equivalant in diamond is the best. For best transfer I would get an insulated copper heat sink.

greg

Somebody tried to make a live heatsink on a Luxman amp I am fixing. They left off the insulators.

greg

s/hair/skin/

I had a old Studiomaster mixeramp in for repair a couple of months ago. The owner had removed the side cheeks to rack mount it. In order not to lose the screws, he wound them back into the old holes and one of them touched the heatsink of one of the amplifiers shorting it to earth (that`s 'ground' for the leftpondians).

Bang!

Much replacing of fuses and repairing of fritzzed off pcb tracks later and it`s working again Amazingly no semiconductor devices were harmed in the making of the program!

Ron(UK)

What other part(s) of your body would you be likely to immerse in hot food or drink?

Best regards, Spehro Pefhany

measurements

Here's a live-sink power amp. The transistors are clamp-mounted to nickel-plated copper heat spreaders, which are in turn bolted to the main aluminum heatsink. Output is +-180 volts peak at 100 amps peak.

John

ago, I

laid

measurements

I often arrange spring clips to the body of the trannies especially when using equivalent replacements where the dimensions are different, so mounting hole does not align. Or functional replacement hybrid packs where the mounts don't match. I would never use nylon in these circumstances. I wonder if there is a glass fibre reinforced ceramic screws ?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on