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Original question:

TMCS1107EVM: EValuation module

TMCS1107EVM: Evaluation Module Questions

Angelo Kako
Prodigy 20 points
Part Number: TMCS1107EVM
Other Parts Discussed in Thread: TMCS1107

I am using the A3B (200mV/A) EVM and have connected an AC lamp as my load (See attached diagram for connections). 

I am powering the EVM with a +5VDC power supply at VS but I am not seeing any change on the output side when measuring with a multimeter and I am getting what is to me odd results with a scope( See attached scope shots). 

  • With all components OFF (NO POWER): 
    • I read 25.1mV with multimeter, 
    • and with the scope, I read a pktopk voltage of 880mV

  • With ONLY the DC Power Supply ON(+5VDC) and load OFF:
    • I read 2.474V with a multimeter,
    • and with the scope, I read a pktopk voltage of 2.13V

  • With PS ON(+5VDC) and the load ON: 
    • I read 2.474V with a multimeter,
    • and with the scope, I read a pktopk voltage of 4.38V

My Questions/Confusion: 

  1. I thought the output should be a DC value, why do I see a sin wave with the scope -- this would imply to me that I'm seeing an AC signal?...
  2. Why are my multimeter readings the same with the load in ON state and OFF state? The lamp  is rated for 500W @120V, that's ~4Amps of current -- Using the A3B EVM, I should see at least an 800mV change with that much current, right?..

Am I doing something wrong or not understanding something completely? 

Thanks in advance for any help!

  • 0
    TI__Intellectual 1811 points

    Angelo,

     

       Thanks for considering TI product's in your application. To answer your question, consider that the bandwidth of the TMCS1107 is 80Khz. The 60Hz AC signal you are running through the input pins is easily tracked by the TMCS1107. It appears as an AC signal on the output. I assume your meter is set to read DCV. What's present on the output pin is actually an AC signal. When your meter is set to read DCV you are basically reading 0V or in this case mid supply.

       Going a bit deeper the first two scope shots are unexpected. The presence of any signal at all in the first two scope shots is a bit puzzling.  It seems that the device is going below ground and that’s not possible.  I suspect a scope coupling issue.  I’d set the scope coupling to DC.  This could be ground issues or residual voltage or current from the AC outlet.  I’d try the same setup but unplug the lamp from the AC outlet.

       You might find the link below helpful.

    https://masteringelectronicsdesign.com/how-to-derive-the-rms-value-of-a-sine-wave-with-a-dc-offset/

     

      Please let me know if there is anything further I can assist you with.

     

    Regards,

    Tommy Santoyo

  • 0 in reply to Tommy Santoyo
    Prodigy 20 points

    Hi Tommy, 

    First, Thank you for your response. 

    It turned out it was an issue with my DC power supply causing that noise (I believe it was a grounding issue with the supply). I reran the test and these were my results: 

    • With all components OFF (NO POWER):

    • With ONLY the DC Power Supply ON(+5VDC) and load OFF: 

    • With PS ON(+5VDC) and the load ON:

    Using the site that you provided, I used the formula below to calculate Vout,RMS(OFF) and Vout,RMS(ON).

    a0=RMS output offset value

    a1=peak voltage output @ offset

    I was able to calculate VoutRMS(OFF) as 2.541V by: VoutRMS(OFF)=sqrt(2.541^2 + 0^2/2)

    and I calculated VoutRMS(ON) as 2.67614V by : VoutRMS(ON)=sqrt(2.541^2 + 1.18750^2/2)

    I believe this is what I should expect from the EVM, is that correct? 

    If it is: 

    I am planning on reading the Vout value through a development boards A/D. I would prefer to convert the AC signal into a DC signal that the A/D can read. Do you have a suggestion for a simple IC that I can use to convert this signal? 

  • 0 in reply to Angelo Kako
    TI__Intellectual 1811 points

    Angelo,

        The ADCs I am familiar with do not calculate the RMS of a signal but digitize the signal and normally process the data to get the value you need. You can find a variety of ADC here (https://www.ti.com/data-converters/adc-circuit/overview.html) there are too many options and you would need to see what meets your requirements. Depending on ADC you may need something else to drive the ADC from the TMCS1107. At lower frequencies such as 60Hz you may not to worry about it. https://www.ti.com/lit/an/sboa443/sboa443.pdf

    Regards,

    Tommy Santoyo

  • 0 in reply to Tommy Santoyo
    Prodigy 20 points

    Tommy, 

    Can I put a bridge rectifier on the Vout of the EVM to be able to read a DC value with the ADC? 

  • +1 in reply to Angelo Kako
    TI__Intellectual 1811 points

    Angelo,

    Since the output will only be between 0 and 5V, I don't think that will work, but can you show me a schematic of how you would connect it?  Also can you share more info on what your ultimate goal is? 

    Tommy

  • 0 in reply to Tommy Santoyo
    Prodigy 20 points

    Hi Tommy, 

    I was really just trying to avoid having to sample an AC signal a bunch of times but I figured out how to do that so no need for the rectifier anymore. 

    Thank you for your help, this issue can be resolved.