Subject:
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Re: ** voltage dropping and current limitting**
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Sat, 30 Jan 1999 14:27:42 GMT
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Original-From:
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David Kott <dakott@SPAMLESScustomnet.net>
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Viewed:
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991 times
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On Fri, 29 Jan 1999 BMajik5127@aol.com wrote:
>
> I have a transformer, when powered by 110 vac I get 16 vac out.
> I put it through a bridge rectifier and the problem is when I put a cap say
> 20v 1000uf
> across the + - of the rectifier, the voltage jumps to about 20 vdc.
> If I omit the cap and attach a small dc motor I get the same effect. WHY???
> Any info would be much appreciated.
> -Mike
>
>
Well, just off the top of my head, when you say "get 16 vac out" how are
you determining this, a volt meter set for "Volts AC" or some such? Not,
say, an oscilloscope? You will note that almost all meters render "AC
Voltage" as the RMS (Root Mean Square) voltage. That is to say, it is
sqrt(2)/2 the peak to peak voltage. However, when you rectify and low
pass an AC waveform without any much load, you get approximately the peak
to peak voltage of the AC waveform as a DC level, minus twice the forward
voltage drop of the particular diodes you have chosen for your rectifier.
For example, 16 VAC = (16 / (sqrt(2)/2)) = 22.7 VDC. Assuming you are
using a rectifier built with silicon diodes, (which have a .7 volt forward
drop across them) you would measure 22.7 - (2 * .7 ) = 21.3 volts DC.
Now, as to why you are getting lower then that, I would have to suggest
that the cap you are using has a high leakage current, and thus tends to
pull down your output voltage. It might also have been damaged with the
application of more then it's rated voltage.
Is it perhaps one of those light blue bodied Panasonic electrolytics?
They are awful, but cheap.
-d
# rsh -l God universe.all find / -name '*windows*' -exec rm -rf {} \;
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