Why do I blow a fuse when I scope a bridge rectifier?

Thread Starter

Involute

Joined Mar 23, 2008
106
I want to evaluate this full-wave bridge rectifier for a project I'm working on. Because of the high-voltage involved (120 VAC), and because the part is SMD and I'll be swapping out various filter capacitors, I built a test setup that's a little more substantial than a breadboard (see top and bottom photos below). Rectifier test - top.jpg Rectifier test - bottom.jpg The part accepts AC fine (via the ~ input terminals), and outputs 106 VDC (RMS) via the output terminals (+ / -), as read by a DMM. It even shows a rough approximation of the expected waveform (well, half-wave rectified) when I attach an ungrounded scope probe to the + (red) output terminal (below).1.png
As soon as I touch the probe's ground lead to the - (black) output terminal, however, I blow a mains fuse and the rectifier dies. What am I doing wrong?
 

AlbertHall

Joined Jun 4, 2014
12,345
Because you are connecting a diode across the mains supply.

In the diagram below the thin red and blue lines represent the mains input to the bridge rectifier. The thicker red line represents the current flow when you connect the 'scope ground (which is connected to mains earth) to the positive output of the rectifier and the thicker blue represents the current flow when it is the negative rectifier output that the 'scope ground is connected to.

When working with mains power without the isolation provided by a transformer you must take great care and you cannot connect the ground of a 'scope or any other test equipment to the circuit.
Clipboard01.jpg
 

ericgibbs

Joined Jan 29, 2010
18,766
Hi Albert,
I know that.!
But the TS should be made aware of the point.

EDIT:

Added a circuit showing the result if the mains Neutral is NOT close to Earth/Gnd
Hope this makes it clearer.

AA1 25-Oct-18 19.41.gif
 
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ericgibbs

Joined Jan 29, 2010
18,766
hi @AlbertHall
With due respect your drawing in post #4 is misleading, it shows no Earth/Gnd connection on the mains incoming supply.
If that was the case the Scope Gnd could be connected without a problem.

It is the presence of a Earth or 'close' to Earth of the Neutral that provides a current path that causes the fuse to blow via the Scope Earth/Gnd
Also it makes the project potentially lethal for the 'tester'

An isolation transformer breaks that direct connection to Earth of the Neutral supply line.

Eric
 

BobaMosfet

Joined Jul 1, 2009
2,110
I want to evaluate this full-wave bridge rectifier for a project I'm working on. Because of the high-voltage involved (120 VAC), and because the part is SMD and I'll be swapping out various filter capacitors, I built a test setup that's a little more substantial than a breadboard (see top and bottom photos below). View attachment 162361 View attachment 162362 The part accepts AC fine (via the ~ input terminals), and outputs 106 VDC (RMS) via the output terminals (+ / -), as read by a DMM. It even shows a rough approximation of the expected waveform (well, half-wave rectified) when I attach an ungrounded scope probe to the + (red) output terminal (below).View attachment 162363
As soon as I touch the probe's ground lead to the - (black) output terminal, however, I blow a mains fuse and the rectifier dies. What am I doing wrong?
Because you are shorting through the ground on your scope. STOP!. If you do not carefully evaluate and understand exactly how your ground is connected through your scope, and how A/C travels through your probes, scope, and this ground-- you can damage your scope, your probes, etc.
 

ericgibbs

Joined Jan 29, 2010
18,766
It is a drawing of the setup that TS had.
OK, but you must agree that the your drawing of his circuit, with the incoming mains, that the Neutral line should have included an Earth path.
else the fuse would not blow.
 

AlbertHall

Joined Jun 4, 2014
12,345
OK, but you must agree that the your drawing of his circuit, with the incoming mains, that the Neutral line should have included an Earth path.
else the fuse would not blow.
That all depends on the electricity distribution system which is different here to over there. I didn't think including the substation would really help.
 

Danko

Joined Nov 22, 2017
1,829
Use “Floating” the Oscilloscope or “A minus B” Measurement Techniques
“A minus B” Measurements
(Also known as the Pseudo-Differential Measurements)
The “A minus B” measurement technique allows the use
of a conventional oscilloscope and its passive voltage
probes to indirectly make floating measurements. One
channel measures the “positive” test point and another
channel measures the “negative” test point. Subtracting
the second from first removes the voltage common to
both test points in order to view the floating voltage that
could not be measured directly. Oscilloscope channels
must be set to the same volts/division; the probes should
be matched to maximize common mode rejection ratio...
...........................
 

Attachments

DNA Robotics

Joined Jun 13, 2014
647
Thanks guys
Oscilloscope channels must be set to the same volts/division
That includes the variable volts/division should be off or the top & bottom of a square wave look like a tilde. ~~~~~~
the probes should be matched to maximize common mode rejection ratio.
Different probes didn't matter on mine but I was testing a 60 Hz square wave.
 
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Danko

Joined Nov 22, 2017
1,829
Real oscillograms of diode bridge voltage, using A minus B technique.
Channel A - Red, Channel B - Yellow, A minus B - Green.
Probes: X10.

1. Oscilloscope is grounded:
A-B_Ground.PNG

2. Oscilloscope is not grounded:
A-B.PNG
 
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Phil-S

Joined Dec 4, 2015
238
Bear in mind that many new supply company connections to domestic installations are TNCS or PME (Protective Multiple Earthing) where no earth is provided in the conventional sense e.g. lead-sheathed cable, but rather the neutral conductor is earthed at intervals by the company. In my property, only two insulated cables come in (live and neutral) and the house earthing conductors are split off the neutral at the supply fuse/meter installation. Neutral is essentially ground (TNCS = Terre, Neutral, Combined, System)
 
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