[sdiy] dissipator for power supply

[Niels A. Moseley]

At 21:00 2-8-2004, you wrote:
>Hello,
>
>I am hesitating between 2 kind of thermal dissipator for a new power supply
>(powered by a 36VA 2*18V transformer block). It will be regulated at +/-15 V
>and the dissipator are for the regulator 7815/7915.
>
>i have choice between 3.9°C/W wich i know is quite good (and big and $$) and
>another only 7.1°CW.
>
>Would this second one be enough ? i have no clue. It is intented to power a
>good bunch of modules...

Hi,

This is my first post to this list.. so here goes.. 8)
First, I cannot answer you question because I don't know the thermal 
resistance of the 7815/7915 you are going to use. The amount of heat 
dissipated by the voltage stabilizers depends on the current you want out 
of them and the voltage-drop across them.

This brings me to your transformer... Assuming that your 2*18V is the 
_unrectified_ voltage, the rectified output voltage will be 1.4142 times 
that, or 25V. This means that the stabilizers will have to drop about 10 
volts for 15V output.

As your transformer is rated 36VA, the max. output current is about 2 amps. 
Let's say you're going to run 1.5 A and keep a safe margin. (You will need 
(high-current) 78S15/79S15 for this).

As power is voltage times current, each stabilizer will have to dissipate 
15 (W). Which is quite a lot.

According to the datasheet: http://www.hep.upenn.edu/SNO/daq/parts/lm7815.pdf
for a 'normal' 7815, the max. dissipation at 25 C with a 10 CW sink is 
around 7 W.
This figure gets _worse_ as the ambient temperature rises. So you would 
need a considerable better heatsink than 10 CW..  As the max. dissipation 
with an infinite heatsink is 20 W, in my opinion, you might be better off 
with a transformer that matches the required (rectified) voltages more closely.

Hope this, sorta, answers your question!

Kind regards,
Niels Moseley