AVR-Chat

On Saturday 25 September 2004 01:32, Dave VanHorn wrote:
> At 12:23 PM 9/24/2004, jay marante wrote:

> >i don't know the rating of my heatsink. anyway, my final board is
> >already done. i was just curious why such problem and maybe i can
> >work on it without changing my board. maybe the only way to solve
> >it is to find a 5V relay or lower down the input voltage.

> The regulator heat issue is simply the result of using a linear
> regulator in a high current application, with high input-output
> differential. Heat = (Vin-Vout)*Current

Indeed. I was building a benchtop power supply from a kit and noted
that the LM317 (TO3 package) used to provide the variable output
voltage was tapped off the transformer secondary at 30 V. This was
OK for low current work (less than an amp) at low voltage (<6V) and
at medium current above 18V. The regulator would get very hot and
simply shut down.

I read the data sheet on the regulator and figured out the solution.

I added in a switch to the transformer tap to use a 15V tap from the
same transformer.  I was then able to draw full current (always over
2A) over the entire voltage range; by flicking the "current boost"
switch. The transformer is rated at 2A... so heavily regulating at
greater current.

A centre-tapped input to the rectifier bridge would have been better
requiring larger diodes, but somewhat too complicated to implement
at the time; more than 20 years ago when I started tinkering, due to
graduate unemployment. 

As a solution to the issue of having alread made PCB's and then
finding that a linear regulator generates too much heat; one could
replace the linear component with a simple switching regulator on a
daughter-board that connects to 3 pads on the PCB designated for the
linear reg.. The input cap and heat sink can probably be down-sized
to make room for the daughter-board if space is tight.

-- 
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