Author Topic: Full bridge rectifier probing question  (Read 975 times)

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Offline Lucky-LukaTopic starter

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Full bridge rectifier probing question
« on: July 28, 2019, 12:13:39 pm »
Hi all
I'm watching an episode from ElectroBOOM channel regarding FBR.
At 2:20 an explosion occour (as always in that channel).

He explained the reson but... I haven't really understood it.
The ground of the probe is shorted to the earth of the power plug (of the scope?)... why?
Cheers
Memento audere semper.
 

Offline StillTrying

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Re: Full bridge rectifier probing question
« Reply #1 on: July 28, 2019, 01:20:21 pm »
He explained the reason but... I haven't really understood it.

A Fool Bridge Rectifier always connects the lowest AC voltage to its -ve output, which is usually used as a circuit's DC GND, For a bare mains connection the lowest AC voltage is -155 or -330v and it's not very a good idea to have your GND at -155V to -330V which is why it went bang when the scopes 0V GND was connected.
Removing the scopes earth connection so you can have the GND at -155 to -330V is a very bad idea. I'm surprised he didn't get electrocuted, but I still live in hope.
« Last Edit: July 28, 2019, 06:51:31 pm by StillTrying »
.  That took much longer than I thought it would.
 
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Offline Per Hansson

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Re: Full bridge rectifier probing question
« Reply #2 on: July 28, 2019, 04:52:44 pm »
EEVblog #279 - How NOT To Blow Up Your Oscilloscope!

 
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Offline Shock

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Re: Full bridge rectifier probing question
« Reply #3 on: July 29, 2019, 01:54:25 am »
I highly recommend you draw the schematic out it really helps. Trace the current path from one half cycle and then the other direction for the other half cycle. Then add where you think the oscilloscope changes the circuit and observe what effect it would have on the current paths.

The main hint I will give you is that mains neutral in many parts of the world is tied (connected) directly to mains ground/earth. Here is a video below that shows this and is helpful to watch, so include that in your schematic path when you connect the oscilloscope.

Some interesting questions to ask yourself is how much current is flowing when you add the oscilloscope and what the diode voltage drops are before and after you add the oscilloscope.

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