Getting heat out of a box

Sounds like the only viable approach, short of oil-filling the whole thing. Try to do your best to reduce dissipation in the electronics. I suggest to avoid electrolytic capacitors or similar components where the MTBF significantly drops with rising temperature.

The AC-DC power supply presents a challenge here. Not because you can't get enough efficiency but because it requires at least one electrolytic capacitor. Any chances to at least send in DC, at whatever voltage?

I know a company (client of mine) that can probably do the heat transfer simulations but it's not going to be cheap. Would be strictly as an engineering service, they don't make boxes. I don't think a box makers would do that or even have the SW tools and engineering know-how to do that.

If interested send me an email. Clicking on the reply-to address works.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

Thanks Joerg. I'm going to do a bit of hunting first, if I can get into the right ballpark with an aluminium extrusion manufacturer and get some sample boxes I may even protoype something using dummy resistors and do some temperature profiling myself. This might not catch hotspots in the real thing of course, but it'll give me a good idea of how far off a solution it might be overall. Thanks for the offer, I may take you up on it later :)

Mark.

I use the Dale RH-50 series for that a lot. You can even get resistors in TO-220, should nicely mimic power transistors and large Schottkys:

Another method that easily lets you "dial in" the dissipation without needing a large variety of power resistors is to take a bunch of LM317 and then use them as constant current sources into ground. It takes only one (smaller) resistor to set the desired current and thus dissipation. The LM317 comes in TO-220 and all sorts of other packages, including some that can sink heat into a chunk of copperclad so you can mimic a circuit board that heats up.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

ses

If the box has >30 square inches of external free-air surface, a circulating fan inside the box would do it. Or, does it have to be small?

Do you mean and area of the sidewalls of the box externally > 30 square inches (that's equavalent to 5.48" x 5.48" for ease of visualisation), with an internal fan, will get rid of 30W? What's the max temp rise above ambient internally do you think, and would I need to put heatsink fins inside the box as well as outside?

The box is going to need to be big for the components, probably 12" x 4" x

3" external dimensions or so. That's a good 120 square inches available if you ignore the base and the two end plates. Those other free-air surfaces can also have quite large surface area heatsinks on them too.

Having said that, I would really like it to be rugged and not have any moving parts in it at all. What do you think the chances I could keep all internal temps below, say, 20 degrees above ambient if I were to bolt the high powered stuff directly to the box walls inside? The 30W is spread out a bit component wise, but 20W of that comes from the PSU.

Mark.

So if we understand you, you have a 10 watt load and a power supply that is loosing 20 watts in the conversion to DC?

That doesn't sound too "green" to me. :)

tm

--- news://freenews.netfront.net/ - complaints: snipped-for-privacy@netfront.net ---

ses

e

ch

r

I would go with conductive cooling. That means not only attaching a source device to the wall of the case, but you need an internal integrated block of Al that connects a sizable mass and conduction path between all the heat sources, and then couple that heat block to the case too. That way, all temps are held homogenously together you can use conductive pads to clamp it all up together. I would also conformally coat everything and make sure that thermal mating faces are bare afterward. This is what aircraft avionics do because the air at 70.000ft is so thin that it does not perform much cooling work. At least not efficiently so. I had to retrofit an external HD enclosure with a system recently. It was about 3/4" bigger than a laptop drive on all sides. Hardware encrypted, so it was actually a computer in the box. Probably the most advanced small form factor computer around currently. Absolutely secure, so even a lucky water fountain that ends up with one tossed behind it after being missing for several days would not even cause so much as a flinch.

.

ve

" x

ble

ll

he

out

is

Better to supply it via dongle and keep the device all device.

No, this design is actually a custom power supply. The total heat dissipation inside my box is 30W, there's an AC-DC converter in there which is dissipating 20W of this, and is powering another circuit internally which is dissipating the remaining 10W. This additional circuit provides an output from the box and delivers power to another box. The overall efficiency of my box is about 70% (series combination multiple of the efficiencies of the AC-DC supply and my additional circuit), so basically it is consuming 100W and delivering 70W to another box.

Mark.

ses

e

ch

r

Sounds like a job for heat-pipe. The heat generated in the box boils water (at sub-atmospheric pressure) which condenses on the inside of the heat-sink extrusion that forms the outside of the box, and wicks back to the heat source where it boils again.

The heat transfer rates can be remarkably high, as long as water vapour is the only gas in the heat-pipe.

We got ours from Isoterix of Woolmer, Northumberland, back in England in 1992. There are other people who make and sell heat-pipe assemblies.

-- Bill Sloman, Nijmegen

The

What's your airflow outside the box? There's not much use getting the heat to the skin of the box if it doesn't have a place to go after that.

--

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

Scary. A square inch of surface in still air can have theta over 100 K/W.

John

Fins inside would certainly help. I'm rusty at my thermo, but the way you'd calculate this would be to figure out the thermal resistance for each metal to air interface (component to air, air to box inside, box outside to air) and add them up -- you'll find that the thermal resistance of the box's aluminum itself is insignificant, unless you're really aggressive about fins.

There are nice tables of area vs. airflow vs. thermal resistance which are helpful, but figuring out the airflow is impractically difficult.

I used to work in a group that had a very similar problem to yours, compounded by the fact that most of the parts inside the "box" were on a moving platform and thus _had_ to dissipate heat to the internal air. The mechanical group always did lots of fancy calculations _second_, but _first_ they populated the space with resistors, light bulbs, fans and thermocouples to get a baseline for their numbers.

20 degrees above ambient sounds pretty ambitious in itself. It depends significantly on airflow on the outside of the box -- convection makes a huge difference to cooling, you can't do this calculation without taking the outside of the box into account.

Using convection inside the box is certainly convenient, but it multiplies the number of metal-to-air junctions by three -- instead of having the box outside to air junction, you have the component to air, the air to box inside, then the box outside to air. How much this will increase your overall thermal resistance depends heavily on how well stirred the air is inside the box and the airflow outside the box.

--

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

[snip]

Configure a "endless" tunnel (with internal fins), fan driven. Fins on the outside. And pray a lot ;-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

                   Spice is like a sports car... 
     Performance only as good as the person behind the wheel.

Well everyone, I've found this site which if I can learn to drive it might be really useful. It's got some seriously interesting stuff in there, including online calculators:

Mark.

.

re

with

bient

he

Total bullshit. He said no moving air inside.

All radiation must be external.

Remember the SR-71B? It dropped 50=B0F when they went from bare skin to matte black IR radiative paint... on the OUTSIDE of the airframe. Even though the skin was the source of the heat.

Conduction cooling relies on operating AT elevated temps, and designing to reliably do so within a window of EXTERNALLY radiated energy acceptability. So, it needs to be mounted onto a conduction plate externally, or operate at a temp where it can be free standing and still run.

The hardest part is usually trying to comply with the upper end of any claimed thermal operating envelope.

are

with

mbient

the

=A0 =A0 ...Jim Thompson

=A0 =A0| =A0 =A0mens =A0 =A0 |

=A0 | =A0 =A0 et =A0 =A0 =A0|

=A0|

=A0 =A0 =A0 |

Sounds like a speaker enclosure I once saw an amazing example of. Klipsch. What an amazing piece of art and engineering.

Something like this?

ftp://jjlarkin.lmi.net/L650_HS_proto.JPG

ftp://jjlarkin.lmi.net/L650_HS_pretty.JPG

John

Please quote a post. The one I can find said that he'd 'really prefer' no moving _parts_. Which is understandable -- fans wear out. It'd be interesting to do an MTTF calculation for the assembly with electrolytics (and elevated temperatures) but no fans, and with electrolytics and fans (and hence lower temperatures).

Yes, that's basic thermo.

Passive heat flow itself requires a temperature differential. So _any_ cooling "relies" on operating at elevated temperatures.

Again, you assume radiative or conductive cooling externally, when (unless I missed a post) the OP has not said anything at all about the environment he's putting his electronics into. Should he take a Colman cooler, aluminize the inside, put his electronics into that, and shut the lid, then he won't get good cooling no matter what he does with the _inside_ of the box.

True. Unless you don't pay any attention to the lower end of the envelope, in which case you'll be getting calls from your first Alaskan customer come winter.

--

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