dc measuring

[Bakosch]

Which DC can you measure according to picture a, b, c? 20,5V*1,414=28,99V

Is this right? If not need help.

 

[duke37]

To quote the voltage to such precision is absurd since the voltage drop of the diode is unknown and not taken into account.
The two lower diagrams are similar. The only difference is that the diode is placed at a different place in the same wire.

Have you considered polarity?

 

[Harald Kapp]

To quote the voltage to such precision is absurd since the voltage drop of the diode is unknown and not taken into account.

Right, but: One can assume a voltage drop: either 0 V for an ideal diode or e.g. 0.6 V for a typical silicon diode. As long as the assumption is stated in the solution (and lacking more information from the task description) this should be acceptable.

Have you considered polarity?

That is what the difference in the 3 circuits is all about so it needs to be considered.

 

[Audioguru]

On the other website the circuit had some instructions in a foreign language (German?):

 

[hevans1944]

Is this right? If not need help.

It looks right to me, but perhaps you disagree and need help. Do you have a homework question? Some of us here may point you in the right direction, but we will not do your homework for you. Read posts #2 and #3 again.

 

[Harald Kapp]

The original text shown by @Audioguru specifically asks: Which approximate DC voltage can you measure in figures A, B and C
Therefore the voltage drop across the diode seems to be assumed as negligible. The task is to determine the peak voltage across the capacitor (as compared to the rms voltage given as the transformer's output).

Also polarity has to be observed as the image shows Uab = ? which means voltage between points A (positive) and B (negative). We run here a bit into the difference between the German (European?) way of labeling voltages and the American way.
Germans use a defined direction from '+' to '-', regularly shown as arrows on the schematic or indicated as subscript as in this case. We assume a positive voltage between points A (+) and B (-). Then we compute the value and the result may be Uab < 0 V which means that Point A is actually negative with respect to point B.

Is this right? If not need help.

With respect to above statements by others and me, your answer is partially right, but incomplete. You need to state the polarity together with the absolute value of the voltage.