Re: What is this cirquit used for?

jasen wrote:

On 2006-12-28, Markus Zingg <m.zingg@xxxxxx> wrote:

Thanks for the help so far. I made a little test and turned my lab
power supply to 5.5V and went up to 9V like the 6V rated spare wall
wart I have outputs without load.

Behind the formentioned regulator cirquit the voltage drop get's a bit
smaller (starting at ~5.2V) the higher the input voltage, but it still
goes up to 6V (at 9V input). So it apears to me that this simple
voltage regulation cirquit is not doing so well. Lacking a 110V AC
power source I can't know what the original wall wart really would
output, but asuming it would be lower than my 6V supply, I can savely
asume that the output voltage would also be lower.

I therefore ask if it's not better to completely replace the cirquit
with a low drop fixed voltage regulator (LM2940CT in it's standard
configuration acording to the data***) and feed it from the 6V wall
wart I have? I have the regulator and the two capacitors needed here
so that would be a quick solultion. Is this a good or bad idea?

get a 6V regulated wall-wart off the shelf an stick a 1n4001 in series
with the output (get approx 5.4V)

or a 7805-based 5V one rated for a little more current than you need
and bootstrap it by putting a 1n914 in series with the gournd leg of
the 7805. (result about 5.6V)

either way should beat the specifications of your original...

Bye.
Jasen


The 7805 needs TWO volts more at the input than the output voltage. A
small unregulated supply at 9 volts no load will sag below the required
voltage, and the output voltage will drop below what you need.


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