Diecast boxes heat dissipation

"Kevin Barstow"

** What parameter/s are you after ??

And obviously - size matters.

.... Phil

How much power do you need to dissipate, and what is the transistor package? Do you need an insulator?

One nasty thing about diecast boxes is that the sides aren't very flat. That's bad for heat transfer.

John

Keep your thumbs on it and it'll dissipate more heat!

Seriously -- it depends on what environment they're in. The heat has to go somewhere once it warms up the box -- where that somewhere is depends on how much airflow, etc., etc.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

Temporarily fix a contact-cooled resistor (or two) inside the box and run it at various wattages from a bench power supply. To measure the temperature you can use a thermocouple thermometer or (slightly less accurately) a liquid-in-glass thermometer with the bulb wrapped in cooking foil under a small patch of expanded polystyrene.

All your temperatures need to be measured with respect to ambient in still air and you may need to wait for some time for each reading as the temperature stabilises.

Large die cast boxes can exhibit considerable temperature variation between different parts, so it might help to distribute your power-dissipating devices as widely as possible.

As a guide: I have built a stereo 30 + 30 watt amplifier in a die cast box approx.

270 x 170 x 70 mm. It runs from 12v supplies and generates an internal 55v rail to run the TDA7295 output stages. Although it is far from efficient and dumps nearly 50 watts into the box at full power, it only feels warm on the corners after hours of running flat out into a pair of dummy loads. You can see the two output stages well separated in opposite corners:

A mono 60w design for 100v line from 12v supplies, complete with

6-channel microphone mixer is housed in a pair of diecast boxes bolted together.

The main amplifier box is approx. 190 x 120 x 80 mm and the output transistors are well distributed on the shorter sides. The output is transformer push-pull Class-B with two triples in parallel on each side. At prolonged full power into the correct load, the dissipation is around

30 watts and the sides of the box reach about 50 C above ambient.

On one occasion I was faced with an emergency situation which involved running over 100 watts-worth of loading at full power for three hours. On that occasion the sides of the box became too hot to touch, which made operating the mixing controls rather inconvenient. Eventually I enlisted the help of another P.A. engineer and we took it in turns to fan the amplifier vigorously with a large piece of cardboard for the rest of the evening.

--
~ Adrian Tuddenham ~
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www.poppyrecords.co.uk

Yes! And run it for a loooong time, at least an hour per test at the selected wattage. The heat just keeps on building and the temperature climbing until thermal stability where the box dissipates to the surroundings at the same rate as it absorbs from the R.

You should have seen the smoke I got from one such test. At 10 minutes, all was fine in ~ 30 degree F ambient. At 1 hour, not so much. Melting insulation makes a lot of smoke. :-)

Ed

I see you laced some of the cables- I haven't seen this in quite some time.

It's a lost art ...it's also a sure way to discover you need one more cable in the loom -just after you have tied the final knot.

--
~ Adrian Tuddenham ~
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www.poppyrecords.co.uk

Not lost, just misplaced:-

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Some further thoughts:

The dissipation will depend on which way up you use the box and what it is standing on. If it is hemmed-in by other equipment or might be accidentally covered with a blanket, you need to alow for much-reduced dissipation. If there is any sort of risk like that, incorporate a thermal swich.

The colour of the box makes a difference, but not in the way generally supposed. Painting it black will make almost no difference to the heat dissipation because it is not going to be very much hotter than its surroundings, so the net radiant heat loss will be trivial. However, if you are going to use it outdoors in sunshine, painting it black will make it hotter as it will gain heat from the sun (which is at a much higher temperature).

Leaving it with a natural 'silvery' finish will be a slight improvement, but this may not be very reflective to infra-red, so it will still heat up (it gets worse as it becomes old and grey). Some sort of sunshade is the simplest answer.

--
~ Adrian Tuddenham ~
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www.poppyrecords.co.uk

Jeez guys. This is simple. You make an Aluminum block INSIDE the box that is BIGGER than the tab on the transistor (like double or triple wide). You use thermal epoxy to attach that "heat spreader" to the box permanently. You attach the heat source (transistor) to it with ideal means as well. Then, you have to put a heat SINK on the outside of the box to grab the heat that the heat spreader has passed through the box's thin wall now over a greater area due to the heat spreader inside the box. This is the key to success here. The heat spreader. Thin wall boxes suck at delivering heat conduction flows well. If it is custom, you can have the spreader cast in, removing yet one more lossy interface from the system.

The external heat sink can be a finned device meant to remove heat by way of air currents (convection). OR it can be a conduction cooled paradigm, where you only need to insure that a good thermal conductor is against the side of the box that has the heat source. Of course, the whole box will end up with a higher runtime temp, but the key here is conduction of the main heat source to the outside of the box.

The footprint of the transistor tab alone is not enough because there are multiple interfaces to lose conduction through, and the tab just isn't enough. The heat spreader makes GOOD conductive (thermal)coupling with the transistor tab, and SPREADS that heat out, so that the attachment to the can is greater, and therefore the efficiency of any external heat sink is greater.

No math needed here. It is simple mechanics. Big solder tips heat faster than small solder tips. It is about thermal mass, and how you present that to where you want the heat to go.

If you want to remove heat from a source inside the thin walled can, you need to spread that heat out INSIDE the can so that you can more effectively couple it to the outside of the can, so that you can then carry the heat away by convective sink attachment or conductive cooling sink attachment mass which you mount it against..

I had assumed the OP was interested in getting rid of the heat once it was in the box walls. With portable equipment which has to be chucked in with other kit, finned heat sinks are a bit of a liability, so they are best avoided if the heat problem is not too great. They can be protected by a perforated cover, but now the equipment is no longer a simple box and may be inconveniently unwieldy.

You will notice that my 60W amplifier had multiple output transistors. That was not just to cope with the high currents, which could have been handled by fewer, bigger devices; it was to spread out the heat generation and to have more parallel paths in contact with the box walls. (It also made the output transformer design easier, for reasons which are irrelevant to the present thread.)

I did mention that the devices need to be spread about, so as to transfer the heat to as large an area of the box as possible without their hot areas overlapping. That is another way of heat spreading without having to make a custom-built spreader or a special box.

I did use your heat-spreader idea on an earlier design of 30W amplifier with just a pair of big output devices; in fact I put heavy aluminium bridge pieces across the corners so as to transfer the heat to two walls instead of one. It worked quite well, but the 60W design without a spreader was perfectly adequate and I doubt if a spreader would have helped it to get rid of enough extra heat to allow me to uprate it significantly.

--
~ Adrian Tuddenham ~
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www.poppyrecords.co.uk

In , Adrian Tuddenham wrote (edited for space):

The colour of the box makes a difference, but not in the way generally

One factor for heatsinks, especially without good convecting fins or forced air cooling, is how well they radiate thermal infrared.

How good a thermal radiator of a particular surface is, of a particular wavelength, is same as how well that surface absorbs the same wavelength. Bare metal tends to reflect thermal IR well, so bare metal is a poor emitter of thermal IR. This is why aluminum heatsinks are often anodized, especially smaller ones that have low anticipation of good air flow.

Oxidized metal tends to radiate thermal IR better than bare metal does. For that matter, heatsinks without fins or forced air cooling tend to get rid of heat better if they are painted or covered by a layer of masking tape - despite the thermal resistance of such a coating.

One more thing - non contact thermometers don't read bare metal surfaces well. Adding a masking tape covering tor a layer of spray paint to a bare metal heatsink tends to result in it reading hotter than before, while it is actually cooler than before. You can try treating a small area of a heatsink to get a good reading. After that, try treating the whole heatsink to see the effect on the accurately-reading area.

--
 - Don Klipstein (don@misty.com)

That doesn't matter either. The thickness (thinness) of those boxes means that they are not going to pass the heat correctly along the area needed to sink the heat in a timely manner.

Even if one gets the whole box temp down to say 60C, the source transistor inside will probably be frying due to the poor interface to the actual heat sink mass.

Since the box wall is then, the heat needs to be spread INTERNALLY first, then the thermal current flow through the box wall will be fast enough to cool the device well enough to keep it from running away.

Trying to spread the heat of a 1cm square source OUTSIDE the box will not work. Doing so inside the box allow the sinking done outside the box to have a high thermal conduction efficacy.

He IS trying to get rid of it there. But getting the heat there (outside) efficiently is also important.

That is the critical factor. If the'radiator' is not particularly hot or some of the surrounding surfaces are even hotter, the net result could easily be a heat gain rather than a loss. Black heatsinks may be all right in some circumstances, but outdoors on a sunny day (which is where you might be using your waterproof die-cast box) they could be a real liability.

--
~ Adrian Tuddenham ~
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www.poppyrecords.co.uk

In that case, use white spray paint. That absorbs little of the solar radiation that makes it through Earth's atmosphere, but it is a good thermal IR emitter.

--
 - Don Klipstein (don@misty.com)

Kapton tape is almost black in the thermal IR. So is black whiteboard marker.

John

...but for outdoor use, you don't want it IR black, you want it IR reflective, otherwise it will heat up.

--
~ Adrian Tuddenham ~
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www.poppyrecords.co.uk

Actually, one would want this to be reflective of solar radiation (nearly all getting through the atmosphere is .3 to 3 micrometers).

And preferably "black" in "low temperature thermal IR" - mostly wavelengths 4 to 40 micrometers.

White spray paint, anyone?

--
 - Don Klipstein (don@misty.com)

It will also then REFLECT the heat from within BACK in.