Problem with Glow Plug Current Monitoring using SCXI-1102B

[Y]

Hello,

I have a DAQ system consisting of SCXI -1300 terminal block, SCXI-1102B module, SCXI-1000 Chassis and PCI 6052E. I need to add a feature to it to monitor the current flowing through a glow plug. The + of glow plug is connected to a +24VDC power supply source through a relay, while its - connected to engine body which is GND.  I added a very low resistance shunt between the power supply and the glow plug into the circuit trying to monitor the current through the shunt. The electrical potentials on both ends of the shunt are all close to 24VDC but the voltage drop on the shunt is around 5mV and the current flowing through this shunt is at a few amps. If I add two wires to the both ends of the shunt and send them to my DAQ system, the DAQ system doesn't read the differential voltage (around 5mV) at all -- Looks like each input has exceeded the input range of the DAQ system (+/- 10V?).  Since the glow plug is grounded, there is no way to switch the position of the glow plug and shunt.

Doesn't anyone have any good idea of solving this problem?

Thanks a lot in advance!

[AnalogKid2DigitalMan]

I could not readily locate the manual for the 6052, but can it be set up for differential input operation (not referenced to ground)? I also do not know the max common mode voltage it can take, you may want to look at the specifications in the manual.

5mv is also a fairly small signal to deal with, another option may be to use a galvanic isolation module with some gain (like 5B series from analog devices that NI carries)

[WM_John_Weeks]

I've got a pretty good collection of NI documentation, including the 6052 manual. It says:

Max working voltage (signal + common mode)........................Each input should remain within±11V of ground

So the common-mode voltage in this case is too high. Trying to dredge up information from my past...

how about a Hall-effect current sensor? Here's the first hit from Google:

http://www.allegromicro.com/hall/currentsensor.asp

[Y]

Hi John,

Thanks a lot! I've gotten some Allegro sensors and am going to try it out! I believe it will work!

Appreciatively,

Y

[WM_John_Weeks]

Excellent! My shady past as an experimental scientist pays off.

[AnalogKid2DigitalMan]

Good solution John! I will have to remember that approach.

I came across some app notes today that I'll post for future reference on Linear Technologies current sensing modules, the 6101HV is pretty cool for high side sensing.

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1077,D12479

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1077,D10217

http://www.linear.com/pc/productDetail.do?navId=H0,C1,C1154,C1009,C1077,P12995

[WM_John_Weeks]

That's cool, too. And a nice resource with lots of variations.

The nice thing about Hall effect sensors is that the current is inherently isolated from the output of the sensor, and no shunt resistor is required.