Measure ripple of negative voltage using oscilloscope properly

Thread Starter

Thao Vu

Joined Apr 27, 2017
13
Hi all,
I just want to have a question regarding to the measured technique.
I observed an engineer, he measure ripple on the negative source.
The way he probes is ground of the probe connects to the negative side of the capacitor and probe tip to the positive side of the capacitor, and nothing happens. I understand we can't swap on the positive output because it will create short circuit when GND touch positive output. But here, on the negative output it doesn't.
May some one explain, why it is not creating short circuit on the negative output?

Thank you!!!
1634105419127.png
 

MrChips

Joined Oct 2, 2009
26,790
If you connect one probe only, there is no short.
If you connect both probes at the same time then yes, you create a short across C2.

Note that since the power supply circuit is floating you need to connect the ground clip and this can be placed any where in the circuit. Both ground clips must be at the same place in the circuit.
 

Ian0

Joined Aug 7, 2020
5,839
You circuit can only have one ground! Connect the clip to ground. If the voltage is negative of ground, move 0V to the top of the scope screen using the position control
 

ronsimpson

Joined Oct 7, 2019
2,250
Both GNDs connected to the center point. Probe Tip to "-" and "+". Most scopes the a "invert" function on at least one channel. I use "AC coupled" so the DC disappears and only the ripple is seen.
1634129053357.png
Even if you don't the a invert function this will work.
 

BobTPH

Joined Jun 5, 2013
5,467
Note that, if the power supply is grounded to your electric supply ground, you are correct about the short. In that case, the oscilloscope ground can only be connected to the power supply ground.

Bob
 

tautech

Joined Oct 8, 2019
278
Both GNDs connected to the center point. Probe Tip to "-" and "+". Most scopes the a "invert" function on at least one channel. I use "AC coupled" so the DC disappears and only the ripple is seen.
View attachment 250137
Even if you don't the a invert function this will work.
But you won't need Invert with AC input coupling for ripple measurement only if you are needing to examine the ripple waveform in detail for clues as to where any ripple/noise is originating from.

Using both probe reference clips (GNDS) is unnecessary in this case as a linear PSU is not likely to kick out lots of noise where both might be needed however many scope users often do use both as they are unaware the reference leads can be easily unclipped from the probe so not to dangle into live circuits and create shorts.
I remember a repair tech we sold a scope to some years back wouldn't use probe reference leads for precisely this reason and instead had a mains earth bench lead coming from his scope that he could connect to his DUT and not need probe reference leads. This methodology does introduce noise into measurements however experienced techs can mainly disregard it except for low level measurements.
 

ronsimpson

Joined Oct 7, 2019
2,250
To measure noise you need to have the ground clips connected. If there is a switching power supply or some RF then a ground clip is not enough.
1634169468709.png
 

Thread Starter

Thao Vu

Joined Apr 27, 2017
13
If you connect one probe only, there is no short.
If you connect both probes at the same time then yes, you create a short across C2.

Note that since the power supply circuit is floating you need to connect the ground clip and this can be placed any where in the circuit. Both ground clips must be at the same place in the circuit.
Yes. i used only one probe at a time. The attachments is confusing because i drew 2 probes at same time.
 

Thread Starter

Thao Vu

Joined Apr 27, 2017
13
Both GNDs connected to the center point. Probe Tip to "-" and "+". Most scopes the a "invert" function on at least one channel. I use "AC coupled" so the DC disappears and only the ripple is seen.
View attachment 250137
Even if you don't the a invert function this will work.
I used only one probe to measure ripple. Sorry for the photo makes confusion. i should draw only on negative output.
Thank you for correct me.
 
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