# Homework Help: Problems with two circuits

1. Sep 9, 2014

### part

Hello!
for the easy circuit 1 i see that the diode does cut out every half of the sinus wave in alternating current.

("Frequenzgeber" means frequency generator)
the device with the peak symbol is an ozilloscop.

i dont understand what happens with the voltage U(t) in circuit 2.
what does the condensator do with U(t) ?

is there any name for it?

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2. Sep 9, 2014

### davenn

hi part

An RC circuit is a filter
now without giving you direct answers do some investigating. Here is one place to start .....

http://en.wikipedia.org/wiki/RC_circuit

have a read and maybe on a couple of other www sites on RC networks and come back and let us know how you got on.
Any mis-understanding? then ask some specific questions pertaining to what you find

Dave

3. Sep 10, 2014

### CWatters

Think of the capacitor/condenser as a charge storage device (bit like a rechargeable battery).

I would also redraw the circuit like this..

http://pics.bbzzdd.com/users/mrdudeman/filter.JPG [Broken]

Last edited by a moderator: May 6, 2017
4. Sep 10, 2014

### part

Yes i know the RC circuit as a filter but isnt it a different setup here? :)
I would say its not functioning as a RC circuit because here R and C are parallel and not in a row.

5. Sep 10, 2014

### davenn

CWatters, you pic doesn't show

part

In my haste and business at work when I first responded, I didn't see the circuit in its other form, which may be what CWatters is referring to

OK your 1st cct is just showing a 1/2 wave rectifier and a resistive load
the 2nd cct, and when viewed more conventionally, as above, the capacitor provides smoothing
or if you like some filtering of the remaining AC signal that is present across the resistor load

have a look at this link and see if it helps ....
http://www.electronics-tutorials.ws/diode/diode_5.html

cheers
Dave

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6. Sep 10, 2014

### CWatters

Not sure why the image didn't show up but yes it was the same as the one Dave posted.

In this case I think it's quite acceptable to refer to the RC as having a filtering function even if the RC is in parallel.

It's always worth looking to see if a circuit can be redrawn to make it look more recognisable. I also prefer to rearrange components so that +ve voltages are towards the top of the page where possible. Examiners on the other hand seem to like drawing circuits to try and fool you into thinking it's more difficult than it is. Some 25 years ago I had a boring summer job working for an examining board checking that examiners had marked every page. It wasn't my job but I couldn't help read some of the physics papers and I spotted that one examiner was marking papers incorrectly. There are several ways to arrange the four diodes of a full bridge rectifier and the examiner was marking some versions as incorrect when in fact they were electrically correct.

This is the diagram I tried to post earlier.

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7. Sep 10, 2014

### davenn

Yes agreed, I even found one place on the all about circuits site that spoke of RC series and parallel ccts and the conversion from one to another when looking at their equivalence

hope that got sorted out so the poor people being examined got their deserved marks :)

cheers
Dave