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Old 26th January 2013, 02:53 PM   #1
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Default Adequate cooling

Hello,

I am planning on building a power supply (posted this in another thread), consisting of 8 stages. I will not go into further details, but provide the following information: the maximum continuous DC total wattage that these 8 LM317/LM350 are going to dissipate, is around 36 watts.

In short, I am going to ensure a 6 volt difference between input and output, and the two LM350's will provide 1.5A current max each, while the 6 LM317's will provide 0.5A max each. So, total wattage on the regulators = 2*6*1.5+6*6*0.5 = 36 watts.

Now, I was thinking of mounting them directly on the aluminium chassis for cooling. The chassis dimensions could be approximated by 50cm x 15cm x 10cm, and it will be 3-5mm thick.

Do you think it would be adequate as a heatsink? The transformer powering the whole system will be a toroidal mounted inside the same chassis.
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Old 26th January 2013, 09:51 PM   #2
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barely ...
a square horizontal 25cm x 25cm x 3mm bare aluminum plate has Rth approx. 2.5 K/W for a single heat source in it's center
the body temperature of a single 36W device in the center would therefore be at 90C above room temp
... don't know whether multiple devices distributed is better or worse but it seems that you need something like 1 K/W heat spreader with fins on the outside or backside of your box
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Old 27th January 2013, 11:16 AM   #3
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Then, how about using 5mm aluminium along with mounting a 0.5 K/W heatsink on the back side of the chassis? It is going to be on the opposite side of the regulators, but will it help anyway?
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Old 27th January 2013, 04:30 PM   #4
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sounds reasonable
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Old 17th September 2013, 07:47 PM   #5
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What is better in terms of cooling as far as positioning the components is concerned?

I mean, is vertical mounting worse than typical horizontal? I have a 5W wirewound resistor that I want to place on my pcb, and positioning it vertically will save me much space. Will it also aid cooling of the resistor or the opposite?
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Old 17th September 2013, 08:53 PM   #6
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Quote:
Originally Posted by audiostrat View Post
What is better in terms of cooling as far as positioning the components is concerned?

I mean, is vertical mounting worse than typical horizontal? I have a 5W wirewound resistor that I want to place on my pcb, and positioning it vertically will save me much space. Will it also aid cooling of the resistor or the opposite?
Quote:
Vertical power resistors: The use of these in power amplifiers appears at first attractive, due to the small amount of PCB area they take up. However, the vertical construction means that any impact on the component, such as might be received in normal handling, puts a very great strain on the PCB pads, which are likely to be forced off the board. This may result in it being scrapped. Single-sided boards are particularly vulnerable, having much lower pad adhesion due to the absence of bias.
"Audio Power Amplifier Design Handbook" by Douglas Self, Chapter 18.
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Old 17th September 2013, 09:01 PM   #7
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Thanks for the answer! I have read that part too, I was primarily aiming at whether it would benefit cooling or not.
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Old 18th September 2013, 09:31 AM   #8
AndrewT is offline AndrewT  Scotland
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The air inside of the chassis will probably get as hot as the base plate. Can all your other components survive high ambient temperature?
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Old 18th September 2013, 04:29 PM   #9
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If you refer to the consumption posted above, I have changed my plans, so that about 15W at most will be dissipated across the chassis. I will also use an aluminum corner that will be the basic sink.

Now I was thinking about mounting the 5W resistors vertically to cut the box size, but since there seems to be no other benefit I will not, after all it is not that critical.

I will use 105C capacitors. And biggest DC dissipation across each 5W is designed not to exceed 0.67W. I can touch them at such a dissipation - hot, but not untouchable for seconds (minutes) on end. So I assume that the inside temperature shall not exceed 20C above ambient.
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Old 24th November 2013, 12:32 PM   #10
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As far as heat exchange is concerned, is it better to have the inside of the chassis painted black or not?

Painted only the outside matt black will be good, or must the inside also be black?

I don't know what applies to contacting metals - an aluminum corner mating with the chassis should be both left unpainted, or black color would enhance heat transfer?
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