Electronics > Beginners

Class A Amplifier Heat Sink Requirements!?

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Robbie010:
First off, apologies for the long winded post but I’m really struggling here....

I’m about to build a 40W RMS Class A power amplifier split in to two small(ish) enclosures as monoblocks, but I am struggling to work out what heat sink setup I can get away with. I am trying to balance the design and look of the amplifiers with the requirements of the circuit design which means I don’t want to just bang in any old massive heat sink which may be overkill.

The amplifier design calls for 4 TO3 power output transistor per channel in a complimentary darlington set-up and as such will run fairly hot. My plan was to mount each transistor on its own 10cm x 10cm multi-finned heat sink to the outside of the enclosure, two on the left and two on the right and then cap all 4 transistors with a mini heat sink to maximize dissipation.

See pictures for the type of heat sink & cap I was planning to use.

Now, I am completely bemused by all the calculations / equations in trying to work out whether my planned heat sink setup will be up to scratch. One thing that I am unsure of is whether to account for the side walls on the enclosure as well as the main heat sink and the cap / mini heat sink in any calculations???

The side walls of the enclosures will have the heat sinks mounted directly to them and are 4mm thick aluminium, measuring roughly 30cm x 12cm, amounting to approximately 720 Sq cm (360 Sq cm per face) of surface area, which could potentially have a major effect on the dissipation of heat and subsequently the calculations / equations.

Can anyone comment on whether I should account for the side walls of the enclosure in any calculations??

Also, is there any easy way to work out whether my planned heat sink set-up will be sufficient?

Thanks in advance

Noob.

Jwillis:
I wouldn't rely on the enclosure for added heat sinking. Just calculate for the heat sink only. You can get pre drilled finned TO-3 heatsinks for 4 transistors and they work very well at peak power of a 2N3055.You can never have to much cooling.

GigaJoe:
Long cables to power output transistors are kinda bad idea. you may get oscillation. I would think about active heatsink like computer CPU, large fan usually noiseless, but an active airflow significantly decrease heat-sink size.

TERRA Operative:
Here's a good resource on heatinks, written in an easy to understand way:

http://sound.whsites.net/heatsinks.htm


Also, as GigaJoe says, fans are good if you can use them, even a minimal amount of airflow can have a big impact on heat transfer.

Calvin:
Hi,

Nelson Pass is rumoured to have said: " Just make ´em big and heavy".  :-DD

Anyway, some heatsink manufacturers like Fischer Elektronik offer guides on their websites for heatsink dimensioning.

regards
Calvin

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