Rectifier behaving unexpetedly

[zitman]

Hi all,

Recently got an oscilloscope and signal generator and was having a play familiarising myself with them both.  Made a bridge rectifier with 4 1N4001 diodes and connected a sinusoidal signal from the sig gen with a 5V p-p and 0v  offset.

Here is the signal from the sig gen:


When I  connect the signal to the rectifier and connect channel 2 of the scope to the output of the bridge rectifier i get this:


2 things I don't understand:

• Why is the input sine wave (yellow) flattened at the bottom
• I expected to see the negative going humps of the input signal inverted on the output and thus not see and flat bits (hope that makes sense).

Any explanations will be greatly appreciated

Ian
Happy new year everyone

[wiss]

Did you short out the generator through one of the diodes?
Remember that all 3 channels (scope and generator) share ground.

[zitman]

Pretty sure I didn't short the sig gen.  Have just remade the circuit from 4 new diodes and also tried a rectifier pack and get the same result.

I don't understand the issue with the earths, can someone explain this please?

Z

[filip_cro]

I don't understand the issue with the earths, can someone explain this please?

Earth (GND) is the same for both gen and oscilloscope.

[JOERGG]

In addition to the above said here is an explanation from Tektronix.

http://www.cbtricks.com/miscellaneous/tech_publications/scope/floating.pdf

On site 5 i found the following and tried it out on your circuit, this works fine for me.

Differential Measurement System
Pseudo-Differential Techniques.The most popular
solution to the need for a
“floating” measurement is the
“A minus B” pseudo-differential technique. Most generalpurpose dual-trace oscilloscopes have an ADD Mode
where the two channels can
be electrically subtracted
(invert CH 2), giving a display
of the difference signal.

To make things clear for you, take a ohmmeter or continuity tester and make a measurement between the BNC ground of the signalgenerator and the BNC ground of both channels of the scope. As mentioned above, they are all at the same potential and a measurement with scopeground on minus side of the rectifier and generatorground on one side on the ac input of the rectifier will definitely shorten one diode.

[zitman]

Right.  I understand now, I did realise both scope channels shared a common ground and once I thought about it it made sense that the sig gen ground would be the same too.  Again with a bit of thought it was obvious that with both scope probes attached it would be shorting one of the diodes out.  I tried the exercise again with a completely separate sinusoidal signal from (a mains transformer) then i started to see what I expected.  A lesson learned for the the future.

It did get me thinking though, could I use an opto-isolator (powered from a battery for instance) to isolate the sig gen output?  The problem as I see it, although I am probably vastly oversimplifying things, is that I would need to use two opto--isolators, one for the positive part of the signal and one for the negative part.  My other thought was to apply a DC bias to the signal, put it though the opto-isolator and then subtract the DC bias (with a DC blocking capacitor) from the output of the opto-isolator.

In theory are either of these possible? And if so how would I actually achieve either of these?  I am at the point where I think I know what I want to achieve but have no idea to do it practically.

Thanks to everyone's for your help to date.

Cheers

Z

[wiss]

You could use two capacitors for isolation, works great to block ground-loop currents from the cable-tv-cable, at low frequency you`d need very large caps thou...