Various current transducer output voltages

[Sojo]

This will be my first experience on using daq cards and transducers so pardon me if I ask dumb questions and any essential info is appreciated related to topic. I will use NI USB 6128 daq card. It's Analog Input range is  ±10 V, ±5 V, ±1 V, ±0.2 V. There are various current transducer output voltages like; 1.65V ± 0.833V , 2.5V±0.625V , ±10V , 4Vrms,  2.5V±0.5V (these are LEM's values) ...  ±10V seems the most suitable but they tend to be expensive. What if I use a transducer which has 4Vrms output voltage? Will I lose precision, if yes how much?  

Secondly, I have solar panels and I need to measure surface temperature. Which sensor type will be most suitable and how can I read temp data in labview. As I know USB 6128 can't see thermocouple or rtd and their modules are expensive is there any other way to measure temperature and read the values in labview.

Thank you

[Christian Mergl]

Hey,

It's always best to use the voltage range which fits closest to the measurement source voltage range.

I think you mixed up some numbers, because there is no 6128 card, so I assume you have a 6218.

In that case you can calculate the resolution via the voltage range and the 16bit of the ADC. E.g. for the +/-10V range you can say 20V divided by 2^16 is your smallest voltage-step.

For temperature measurements I would suggest to simpley use a Pt-100.

Hope this helps,

Christian

[Sojo]

Thanks  Christian

You are right. It is USB 6218.

So according to your calculation if I use 4Vrms which is in  +/-4V range, my smallest voltage step will be 8/2^16 and it will be 2.5 times smaller than +/-10V range. And what I understand from your post is that step count is independent from source voltage range(2^16), only voltage value between two steps differs, so in theory +/-4V and +/-10V ranges are the same according to accuracy but in real world maybe smaller voltage value between two steps can be a problem. Am I right?

Is there any other way than purchasing this module "http://sine.ni.com/nips/cds/view/p/lang/en/nid/205189" to measure PT100? It is clear that USB 6218 Daq card can't read RTD because it doen not have a current source. What about thermocouple? Its module is much more cheaper "http://sine.ni.com/nips/cds/view/p/lang/en/nid/201881".

[AnalogKid2DigitalMan]

Sojo:

4VRMS = +/-5.656V peak (scale factor is 1.414) if it is a sinusoidal mwaveform..

-AK2DM

[Sojo]

AnalogKid2DigitalMan you are right rms is an AC term but most of the current transducers can measure both DC and AC so for DC we can say that +/-4V

http://www.lem.com/docs/products/has50_600p_e.pdf

For the electronic measurement of currents: DC, AC, pulsed, mixed, with a galvanic isolation between the primary circuit (high power) and the secondary circuit (electronic circuit).

VOUT Output voltage (Analog)@ ± IPN, RL = 10 kW, TA = 25°C ± 4 V ± 40 mV

[AnalogKid2DigitalMan]

Sojo:

Very good, and thanks for the link! I have added LEM to my bookmarks for future needs.

-AK2DM

[Christian Mergl · Accepted solution]

Hey Sojo,

Yes, you have always the same voltage level count because the ADCs resolution is the same. 16bits means 65536.

These 65536 steps will be spread over your measurement range via the internal amplifiers gain. So if you set a smaller measurement range you get a higer resolution. E.g. is the smallest step 3.05mV with +/-10V range and 1.22mV using +/-4V range.

And you can measure RTD if you use an external Excitation Voltage, and you can also measure Pt-100 and Thermocouples without additional hardware. However, using the appropriate Signal Conditioning hardware is always better in terms of accuracy. 

I would suggest you to contact your local NI office and take to an internal sales representiv who can give you the best hardware suggestion for your application.

Christian