[sdiy] power-question

[harry]

Hi Ulrich: inline

ulrich kehrer wrote:

> hi!
>
> a power supply problem...
>
> i have a 2x18V/2.22A tansformer and after the bridge rectifier i get +/-
> 28V thus my +/-15V regulators get really hot (i know that the input of
> the regulator should only be 3V higher than its output)
> i suppose the 28V are not normal, are they?

Normal would be about 25V for that transformer... I guess it has a light
load,
or the line voltage is high so you get 28V

The regulator MUST have at LEAST three volts to work right. Max is usually
35V so you are safe here.

How much current will you draw from the supply ?  You need to drop the 13
volts
somewhere, but that does not have to all be in the regulator.

My favorite way would be to put a Zener diode in series with the regulator
input...
maybe a 6.8V zener. Then 6.8V is dropped in the zener, leaving 6V for the
regulator... If the input voltage drops, the Zener will go out of avalanche
and the voltage drop will  be mostly in the regulator.

Many diodes in series would give the same effect...

But the Zener could get hot also...

Do you have a good heatsink for the regulators? That is the best way to make
them cool.

H^) harry

>
>
> (the rectifier is a 50V/6A one; i replaced it with 4 1N4005 diodes but
> that didn't change anything)
>
> any help concerning the +/-28V is really appreciated.
>
> regards,
> ulrich