rectifier question

Thread Starter

automagp68

Joined Nov 13, 2011
81
Hello

Quick question on a test i have going

I am testing 3 different bridge rectifier all solid state full wave

One radio shack unit and another i built out of 4 diodes

Signal source is a function generator simulating 60 HZ AC Signal

It works but it look like half wave as there is no Frequency doubling. Just a flat spot in the wave where negative should of been folded up.

Any idea what my issue is?

It does this on all 3 rectifiers i have, both the store bought and the home made diode one.
Thanks
 

jimkeith

Joined Oct 26, 2011
540
My guess is that you have a ground fault--signal generator common connected to scope common--isolate ground pin on one unit and see what happens--or connect scope as a differential input, if it has that feature
 

Thread Starter

automagp68

Joined Nov 13, 2011
81
Ah yes i know what u mean now i read it wrong

No i am not sharing the ground

Rectifier has 4 pins

The Signal generator is on two of them + and -
The scope prob on the other + and -

No idea what differential input is?

Its a Tektronix 2205
 

Adjuster

Joined Dec 26, 2010
2,148
The Tektronix 2205, like most oscilloscopes has coaxial inputs. The outer connection of the inputs will be linked to mains ground for safety.

It is possible that your function generator has a similarly grounded output, which will create a short-circuit in your experiment.
 

Thread Starter

automagp68

Joined Nov 13, 2011
81
so how does one get around this?

If this is the case then how does one use the scope for testing, if i cant see the doubling?

And even if so i don't understand why this matters,
I should only be looking at postive side of the sine wave any way, I understand the scope is grounded.

Basically the scope says zero in the middle of the wave where it should have a peak. Like a half wave

It is not possible to test rectifiers on a scope?

Thanks
 

Adjuster

Joined Dec 26, 2010
2,148
You could do it using a source which is isolated, perhaps by passing the signal through an isolating transformer.

Alternatively, you might do a differential measurement with two inputs subtracted, if the Tek 2205 scope supports that function. Edit: it seems likely that it does: there is an "ADD" vertical display option and Y channel 2 can be inverted.

NB for correct differential mode operation, the Y channel sensitivity settings must be equal. Before engaging "ADD", but with Y2 already inverted, ensure that both channel displays are entirely on the screen.
 
Last edited:

w2aew

Joined Jan 3, 2012
219
The problem is that the output of your signal generator is grounded too. So it's connecting one node of your bridge to ground, and the scope is connecting a different node to ground.

In most applications of full wave bridge rectifier circuits (power supplies), the AC drive to the bridge is coming from a transformer - neither of the two connections are grounded. If you transformer couple your signal generator to the bridge, it will work fine.

The other thing you can do is make the scope measurement differentially. Use two inputs to the scope. Use channel one to measure between ground and the + output of the bridge, and CH2 to the - output of the bridge. Then, set the scope to invert CH2, and set the vertical mode to ADD. This will create a pseudo differential measurement across the + and - outputs of the bridge.
 

w2aew

Joined Jan 3, 2012
219
You could do it using a source which is isolated, perhaps by passing the signal through an isolating transformer.

Alternatively, you might do a differential measurement with two inputs subtracted, if the Tek 2205 scope supports that function.
Hah - you were faster on the keyboard than me!
 

Thread Starter

automagp68

Joined Nov 13, 2011
81
The problem is that the output of your signal generator is grounded too. So it's connecting one node of your bridge to ground, and the scope is connecting a different node to ground.

In most applications of full wave bridge rectifier circuits (power supplies), the AC drive to the bridge is coming from a transformer - neither of the two connections are grounded. If you transformer couple your signal generator to the bridge, it will work fine.

The other thing you can do is make the scope measurement differentially. Use two inputs to the scope. Use channel one to measure between ground and the + output of the bridge, and CH2 to the - output of the bridge. Then, set the scope to invert CH2, and set the vertical mode to ADD. This will create a pseudo differential measurement across the + and - outputs of the bridge.

Hi thanks for the idea

So instead of using two channels i just made a common ground instead , because u said it was connect to a different ground.

That did not work of course or im sure u would have suggested it.
Why is that

Also so i set up the scope like u suggested but can u explain what goes on channel 2

Probe to the - output of the bridge and where does the ground go?

I cant seem to get that to work
 
Last edited:

Adjuster

Joined Dec 26, 2010
2,148
So instead of using two channels i just made a common ground instead , because u said it was connect to a different ground.

That did not work of course or im sure u would have suggested it.
Why is that
I don't really know what you mean by that, but whatever ground arrangements you try to make, the mains ground is still there. (And it has to stay there - don't even think of trying to disconnect it as this is extremely dangerous. )

Try drawing the circuit on paper, including all the ground connections. The cause of the short-circuits will probably become clear to you.

I have to go now.
 

jimkeith

Joined Oct 26, 2011
540
One probe goes to the anode of the bridge (+), while the other probe goes to the cathode (-).
Both channels must be set at the same sensitivity and channel 2 must be inverted.
The scope probe ground leads are not connected in this case.
 

w2aew

Joined Jan 3, 2012
219
Can any one explain where channel 2 goes for this type of measurement
Your function generator most likely has a coaxial output that you are connecting to the AC ports on the bridge. The shield connection of the coaxial output of the generator is GROUND. Thus, you are connecting GROUND to one side of the AC inputs of the bridge.

The scope inputs are coaxial also. The shield connections of the CH1 and CH2 are also GROUND. Connect these GROUNDS to the same exact place as that you connected GROUND of the function generator. The center connections of CH1 and CH2 then connect to the + and - ports of the bridge. Then, invert CH2 and select the ADD vertical mode (both are settings on the scope)

I don't know how to make it clearer than that.

Of course, this will only work IF the function generator swings above and below GROUND.
 
Hi
please put the scope input on your signal generators output - no other things- be sure that the output of signal generator is symmetrical around zero volt, and the output is ok as you expect
 

spinnaker

Joined Oct 29, 2009
7,837
The other thing you can do is make the scope measurement differentially. Use two inputs to the scope. Use channel one to measure between ground and the + output of the bridge, and CH2 to the - output of the bridge. Then, set the scope to invert CH2, and set the vertical mode to ADD. This will create a pseudo differential measurement across the + and - outputs of the bridge.
Wow now I have a use for that Add button on my scope :)
 
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