PROBLEM WITH RECTIFIER DISPLAY IN LABVIEW

[petersrule2003]

hi guys , i have a rectifier circuit built and i am sending the rectified output through an ADC to the PC serial port to be displayed on labview but i keep getting the waveform attached below. I want to know if it is because i need to do some calibration , or why is it not displaying a normal fullwave graph on the waveform graph VI.

Thanks for ur prompt response.

cheers

[ShaunR]

You sure its not a 1/2 wave rectifier?

[ccie]

Hi shawn , im sure it is a full wave circuit , i am just wondering why it dosent display curves as a normal oscilloscope , what im trying to say is how do i interprete this graph to my supervisor at my final year project defense

[ShaunR]

QUOTE (ccie @ May 3 2009, 11:45 PM)

Hi shawn , im sure it is a full wave circuit , i am just wondering why it dosent display curves as a normal oscilloscope , what im trying to say is how do i interprete this graph to my supervisor at my final year project defense

Interpret it as "it doesn't work"...lol.

Can't make out much from your graph. The time is in arbitrary units (ms, seconds, light-years?) and you haven't told me what what you are expecting (pk-pk voltage 10-100V? 20ms period?).

You say the A2D comes through the serial port. How? Does it have a serial interface and your reading AScii characters? Or are you using the serial port as a digital IO and expecting a bit pattern representing the voltage. The source would also tell us what you are trying to do and expecting.

Assuming it's not a software issue. I would check the polarity of the diodes. Getting one round the wrong way is easy to do and easy to check. Then I would probably stick a sig-gen up the A2D and see what was displayed. If you then get what your expecting, its your rectifier. If not its either the A2D or the PC (software or hardware).

With all these things its a process of elimination. Segment the system into testable parts and iteratively narrow it down 'till you find the culprit.

[petersrule2003]

Hi shawn , im sure it is a full wave circuit , i am just wondering why it dosent display curves as a normal oscilloscope , what im trying to say is how do i interprete this graph to my supervisor at my final year project defense

/* i have two accounts ccie and petersrule2003 */

[BobHamburger]

Shaun asked you some reasonable questions, the answers to which might help us all diagnose the difficulty you're facing. To his questions I would add: what is the sampling rate, range, and resolution of your ADC? And how are you inputting the results of the A/D conversion, through the serial port, into LabVIEW? What is the serial representation of the numeric results of the ADC? Can you show us your measurement setup and your code?

Unless you're doing a whole bunch of things correctly, there won't be any meaningful way that you can "...interpret this graph to my supervisor..." with any confidence. What your graph may be showing you is that your circuit is miswired, or your program isn't working the way you expect it to. We'd really like to assist you, but simply repeating your call for assistance over and over again, without providing any meaningful additional information, doesn't enable us to provide the help you're requesting.

[mross]

QUOTE (petersrule2003 @ May 3 2009, 01:57 PM)

hi guys , i have a rectifier circuit built and i am sending the rectified output through an ADC to the PC serial port to be displayed on labview but i keep getting the waveform attached below. I want to know if it is because i need to do some calibration , or why is it not displaying a normal fullwave graph on the waveform graph VI.

Thanks for ur prompt response.

cheers

Looks like a much too low sampling rate (your rate should be 10 times faster than the frquency of the signal of interest). And a full wave would not go to zero, so there is something wrong with the way you are wiring it up.

You can count on this: the graph is showing you the voltages you have applied to the terminals of the DAQ card. I bet it you put a real scope on the same points, using the same scale and sampling rate you will get the same sort of readings.

Also, if you are putting -120 volts directly on the DAQ board (you did not mention which of the many possible DAQ boeards you are using) then you have destroyed it by now. If you have some signal conditioning in line with the signal you should share details about it.

There is a paper on field wiring you should read:

http://zone.ni.com/devzone/cda/tut/p/id/3344

[petersrule2003]

sampling rate = 62.5khz

8 bit ADC

interface is via serial port

my labview program is attached , pls help me edit it if dere is any error in my conversion.

i am expecting about 9V peak-to-peak

[mross]

QUOTE (petersrule2003 @ May 4 2009, 08:52 AM)

sampling rate = 62.5khz

8 bit ADC

interface is via serial port

my labview program is attached , pls help me edit it if dere is any error in my conversion.

i am expecting about 9V peak-to-peak

I am sorry, I overlooked the information about this being a serial comm setup.

You should verify that the serial read operation is providing useful data by looking at that data directly. Wire an indicator or attach a probe to the output of the VISA Read. I can't see any data to review the conversion because I cannot run your application. The data is lost when you save the VI without making the data in the the various indicators default so it persists when you send the VI to us.

All serial data is encoded in some manner and you have to decode it properly. If the serial read data is not correct, then you need to find out why, and I cannot do much to help unless you provide the data.

You have a note that talks of a boolean array, but there is no boolean data. The data from the Read is string data. You are converting that in to a 32 bit integer.

My best general advice to you is to remember that the graph is showing you exactly the information it was given, and showing it in the way you have told it to show. If the data is right and you set up the graph correctly (bring up the graph properties to change things) you will see the desired output.

You can create a numeric indicator on any wire, do this on the wire that feeds the graph and look at the numbers directly to see if they make sense. You need to work your way back to where the data becomes good data and find out why it is not correct beyond that point. You can do this with probes and indicators in LabVIEW. This is normal debugging procedure.

On the data source end of the exercise you should always have an oscilloscope to make sure you have a good understanding of the signal that is wired to the DAQ hardware. You can waste a huge amount of time thinking there is a programming issue when it is really hardware, wiring, and signal related.

If the signal applied to the DAQ hardware is verified to be useful, and the serial output is not correct then you know the problem is in the DAQ card somewhere, or in the wiring (see the link to Field Wiring I sent earlier)