Is is a proper way to solve the problem of high common-mode voltage?

[xcuresme]

Hi, guys~

I am now using PCI-6251 to sense the voltage Vsense in figure 1, whose amplitude ranges from -5mV to 75mV.

figure 1

As we can see when T2 and T3 are closed, the common –mode voltage of Vsense is as high as 160V!!! But the working voltage of PCI-6251 is only 11V.

So I plan to change the Vsense from a grounded signal to a floating signal. That means the DC voltage source of 160V is provided by an isolated AC/DC power supply as shown in figure 2. In this way the “ground” of the Vsense is different from the ground of DAQ.

Figure 2

Could anyone tell me whether this solution can work safety?

Thanks in advance for any assistance!

[crossrulz]

You might be better off trying to find a high side current shunt amplifier that can handle the high common mode voltage.

[johnsold]

I agree.

Consider the Hall effect type of current sensor.  Some of those have isolation ratings ~1 kV.  The bandwidth is about 100 kHz on some that I have used.  If your switching rate is faster than that, you may have a variety of problems getting good measurements and good isolation.

Lynn

[xcuresme]

Hi, crossrulz~

What I'm doing is to develop a current sensor based on shunt and amplifier. So using the PCI-6251 is to examing the performance of the amplifier, and the high common-mode voltage is one of the tough problems.

 I'm not very sure about the method I proposed and afraid of the DAQ to be damaged.

Do you have such experience or any advice?

Thanks a lot!

[xcuresme]

yeah johnsold

Just because the Hall effect type of current sensor have such problems you have mentioned, I'm now developing a current sensor based on shunt and amplifier. So using the PCI-6251 is to examing the performance of the amplifier, and the high common-mode voltage is one of the tough problems.

I'm not very sure about the method I proposed and afraid of the DAQ to be damaged.

Do you have such experience or any advice?

Thanks a lot!

xcuresme

[johnsold · Accepted solution]

Essentially, the advice both crossrulz and I are giving is that a shunt resistor in a configuration which has a wide and rapid variation in common mode voltage is a poor choice for use with most DAQ devices. Both because of possible damage to the device and because of the difficulty of making good measurements.

If you can float you entire test circuit and ground one end of the shunt resistor AND DO IT SAFELY, then you could use something like your Figure 2 from yesterday. Remove the ground at the capacitor. Ground at AI-. Use a single ended configuration fro the DAQ device - differential is no longer needed. Put the isolated DC power supply, your switching circuit, the load, and the shunt in a protective enclosure with suitable interlocks for safety.

Do you get large spikes when switching the inductive load? What kind of transistors/thyristors are your using for the switches? What is the switching rate? How are you controlling the switches?

Lynn

[crossrulz]

A thought I just had.  Another option for you would to use a digitizing DMM.  Double check the spec of the DMM to make sure it can handle that much common mode voltage.  I want to say the PXI-4070 can handle 300V of common mode.  Again, check the specs to make sure.

The DMM will not be able to constantly capture data like the DAQ, so you are going to need some type of trigger system if you decide to go this route.

[xcuresme]

Hi crossrulz~

Thank you for your advice! I have think of using DMM and oscilloscope. But the oscilloscope isn't precise enough and the sample rate of DMM is too low. The rising time of the signal is only about  400us and the duration of the pulse is below 100ms. So the DAQ may be the only device suitable for this case I think. And due to the budget, PCI-6251 is the only one DAQ I can use. So I'm very careful about it.

xcuresme 

[Henrik_Volkers]

Do you have to put the shunt rigth at the load?

Sensing GRND-current after the H-bridge would be to easy?

 

[xcuresme]

Hi, Henrik.

There is something wrong about the Figure 2. The ground connected with the capacitor should be removed.

The position of the shunt is ok. Though low-side current sensing can save much time, it can't sense the current sometime in my circuit. So I have to connect the shunt in series with the load.

I have posted a new thread about this topic here,(http://forums.ni.com/t5/Multifunction-DAQ/Can-floating-a-signal-be-a-proper-way-to-solve-the-problem... which is more detailed. Waiting for your further suggestion!

Thank you!