Re: What is this cirquit used for?

Michael A. Terrell wrote:
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.


I once mightily p!ssed off the Principal of a well known Electronics College by proving that his first term class project *could* *NOT* meet the specification of 5V @ 1A using a 'vanilla' 7805 and the supplied 6.3V transformer. He tried to get out of the hole by throwing more reservoir capacitance at it, but I was able to show by a graphical construction that the reservoir capacitance required was ridiculously large.

By this time, we had attracted an interested audience of the college lab technician and the instructors for the digital and analog modules (and a decidedly bored audience of the other students in the class). An attempt to demonstrate that the supply did in fact work as designed failed miserably when the current reached about 200 mA with a slight voltage droop which when I insisted a scope was used, proved to be the regulator dropping out and following the bottom of the ripple.

I pointed out that if he really wanted to meet the spec, He'd need 78S05 regulators, a mimimum of 8 V rms transformers and heatsinks. Since he'd bulk ordered the parts this did *not* go down well. It turned out that previous classes had 'cooked' their lab reports. As a face-saving compromise, and knowing that we were supposed to be using these PSUs for our digital labwork, I suggested derating the supplies to 500 mA and fitting a much larger reservoir capacitor.

Next week, the lab notes had the title "A 5 V, 500 mA Regulated Power Supply" and the larger caps had been purchaced :-) The Lab tech and myself got the assignment of upgrading the class set of these PSUs that got used by the 'foundation' course. We were both amused that a professional engineer could make such a goof on regulator headroom and basic PSU design and I remained close friends with the tech till his death a few years ago.

The moral of the story? Even with a humble 78xx series regulator *LOOK* at the data *** at least once in your career and preferably again before designing it in to anything that isn't a one off project. And *DONT* embarrass the head honcho in public unless you *enjoy* doing scutwork . . .

--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
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