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Rigol DG4102 issue????

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Grosseinator:
To start I am a second year electronics student with some new toys and I am still learning.  Hopefully my problem I am having is related to my inexperience or something stupid and not faulty equipment.  So...

 I am using the Rigol DG4102 func gen as well as the Rigol DS2102 scope (the new toys).  I built a simple (standard) full wave bridge rectifier with four diodes(1N4004, tested as all working) and just a 1.5k 1/4W resistor across the output for a load.   I used the func gen to inject a sine wave of 60Hz and 12Vpp into the bridge. I first placed the scope probe across the output of the func gen (while connected to the circuit) to confirm the signal and I get the correct full sine wave displayed on the scope. I next placed the ch 1 (I only use the one channel probe) probe across the resistor, but instead of a fully rectified sine wave I get the equivalent of a half wave rectified sine wave.  I used a fluke 87 V and confirm the average V and presence the right amount of DC Voltage for a half wave rectified sine wave.

 I took the circuit to school and used their equipment and confirm the circuit is working perfectly.  So I took the ac supply (it's just a step down transformer with a fuse and a switch plugged into the wall outlet) home and used that to input a sine wave into the circuit (to test my scope is not the problem) and I do in fact get the fully rectified sine wave one would expect.  So (sorry for the length of explanation) it seems the fault lies in the func gen or my ability to use it.

Does anyone have any ideas?

Also a week ago I was trying to pass a square wave through an AND gate (yes it works) using the func gen and had equally frustrating experience.  My guess is that the func gen is not working, but don't want to jump to that conclusion just yet.

PA0PBZ:

--- Quote from: Grosseinator on August 31, 2013, 07:57:00 pm --- I first placed the scope probe across the output of the func gen (while connected to the circuit) to confirm the signal and I get the correct full sine wave displayed on the scope.

--- End quote ---

So the FG seems to work fine.


--- Quote --- I next placed the ch 1 (I only use the one channel probe) probe across the resistor, but instead of a fully rectified sine wave I get the equivalent of a half wave rectified sine wave.  I used a fluke 87 V and confirm the average V and presence the right amount of DC Voltage for a half wave rectified sine wave.

--- End quote ---

Did you use the 87 with the scope still attached? I think what is happening is that the FG and the scope use a common earth, so you are shorting out a part of your bridge.


KedasProbe:
Yes, I'm also pretty sure that that is the problem.
Using your transformer disconnected you from earth hence you had only the scope earth.

Use both your probes to make a differential measurement, scope earth not connected to your circuit.
I think Dave also has that somewhere in a video.

edit: question 23
http://www.allaboutcircuits.com/worksheets/scope1.html


edit2:

Grosseinator:
Thanks for the help.  The solution pictured below (assuming the picture displays) doesn't seem great.  The rectified sine wave seems ugly.  That is assuming I am doing it right.  I used A-B since A-(-B) = A+B.  I'm guessing isolating the Func Gen from earth ground is not a good solution to the problem.  Is that correct?  Does this common ground cause further problems when probing a circuit with a wave generated by the FG?  That would seem to negate the usefulness of using both together.  Maybe I need to study more the use of the scope and FG in actual use and not just the theory.

Wim13:
If you make drawings of your setup. mostly you can see /  tell what is wrong,

See picture 1, with the earth connections of the DSO and the SG, you short D5

A kind of solution is to put in a little audio transformer between SG and the circuit,
see picture 2.

Edit, convert to JPG..

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