# differential probe with oscilloscope

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#### vishjay

##### Member level 2
Hi,

I want to measure the pole voltages for three phases of inverter b6 bridge.
Can anyone tell me shortly how to measure the pole voltage of R phase using differential probe connection to oscilloscope to see square waveform (switchings) of R phase?
in this case do i need two differential probes? since we have two gates i.e., one pole has two mosfet's.

Thanks

#### KlausST

##### Super Moderator
Staff member
Hi,

Honestly, I don't understand the question.

Differential voltage...measures the voltage between two points. Let's name them point A and point B.
A differential voltage probe has two connections for measurement. Connect them to A and B respectively.

Klaus

#### FvM

##### Super Moderator
Staff member
In practice you'll measure pole voltage either referred to DC bus minus or plus.

##### Super Moderator
Staff member
three phases of inverter b6 bridge.
...
differential probe connection to oscilloscope to see square waveform (switchings)
...
two mosfet's.
I presume this resembles your setup. My simulation is 3-phase delta. Each phase created from 2 transistors. The gating signals are square waves. Shoot-through does not occur. Waveform to loads is called modified sine waves, although they are square waves with brief dead time.

How many channels on your oscilloscope? If 2 channels then you can attach the ground clip to any of the load nodes, and read two load waveforms directly. To figure the third load, however, you need to do math in your head. (This method creates uncertainty about displaying current directions in proper relation.)

If you wish to examine current directions, then you need 3 differential scope probes. A simulator makes it easy but real hardware needs real oscilloscopes.

So as a handy trick, make 2 anti-parallel led's as each load. Reduce frequency to 1 or 2 Hz. This makes it easier to watch the led's indicate current direction and timing.

#### vishjay

##### Member level 2
I want to measure switching waveform at one phase(i.e., for two mosfet’s). In this case do i need two differential probes for connecting at two gates and two sources of two mosfets?

#### FvM

##### Super Moderator
Staff member
You are asking about "pole" voltage. Why do you want to measure the gate voltages? They can be of course interesting, but in this case I count three probes: low- and highside Vgs + lowside Vds, if Vdc can be considered constant, thus highside Vds has same waveform as lowside.

#### vishjay

##### Member level 2
You are asking about "pole" voltage. Why do you want to measure the gate voltages? They can be of course interesting, but in this case I count three probes: low- and highside Vgs + lowside Vds, if Vdc can be considered constant, thus highside Vds has same waveform as lowside.
As you said high side vds has also same waveform as low side vds. But in the negative half cycle right?

#### FvM

##### Super Moderator
Staff member
Vds,hs = Vdc - Vds,ls (if measured drain to source in both cases). Thus it's inverted and DC shifted, but basically the same AC waveform, as long Vdc is constant. In a real bridge inverter, Vdc may carry some ripple, making Vds,hs looking different from Vds,ls. But that's beyond the original question of "measure the pole voltages".

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