[sdiy] dissipator for power supply

[Linium]

On lundi 2 Août 2004 22:04, you wrote:
> 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


Thx for the time you put on the answer.

Really i should read the specs of the 7815 (the one i have are rated 1.5A max) 
and try to understand the requirement.

(All the deductions you made in your post are quite right. I didn't gave a lot 
of details, i intend to build a classic bipolar 15v supply for a modular. It 
makes 1A max per rail)

BTW, just one question you said :

> 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.

Why do you say that the rectified will be 1.4142 time 18V or 25V ?
I think that the 18v are just rectified to 15v, and only 3 volt to dissipate ?
This could be my ignorance about transformers, may be the 18v given is an 
average value, and the peaks are more important ?

Anyway thank again for your answer (and first post, you are welcolme ;-) 
 

Linium