PWM output measurement help

Discussion in 'General Electronics Chat' started by Scott Manning, Mar 15, 2015.

  1. Scott Manning

    Thread Starter New Member

    Feb 12, 2015
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    ello Everyone

    I was in the need of a variable power supply but did not want to spend heaps of money. I had an old lap top power supply with output specifications 20VDC, 3.5A which is heaps for my needs.

    The next thing was to make it variable. I found a variable motor speed controller on ebay for a little over 5 bucks which I could not go past. The specifications of the speed controller are below (I don't understand most of it).

    Working Voltage: DC 12V - DC 40V.
    Control Power:0.01 - 400W.
    Static Current:0.02 A ( Standby ).
    PWM Duty Cycle:10% -100%.
    PWM Frequency:13 KHz.

    Link to speed controller on ebay.
    http://www.ebay.com.au/itm/161450586494?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT

    I connected the laptop power supply to the motor speed controller and everything works great.

    Now I want to be able to display the output voltage of the speed controller. As this is a PWM output my understanding is that I am looking for the mean measurement of the on / off switching (if that makes sense).
    I purchased a basic digital voltage display from ebay so I am ready to go except I don't really know what to do next. Specifications of voltage display below.

    Measuring range: DC 0V--30V.
    Input range: DC 5V-30V(Max Input: DC 30V. The device can be damaged if input is over 30V).
    Working Current:< 13mA.
    Display Color: Blue.
    Display Mode: Three 0.36 "digital tube.
    Measurement accuracy: ±1%.
    Refresh rate: about 300mS / times.
    Input impedance: > 100K.
    Operation Temp: -10℃-+65℃.
    Size:2.8cm x 4.7cm x 2.1cm.
    Wiring:
    • Red: Power supply +.
    • Black: Power supply -, measure -.
    • White: measure +.
    Link to voltage display on ebay.
    http://www.ebay.com.au/itm/201085258336?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT

    So I need some advice on how I can use the display I have to read the PWM output as a voltage. I understand I may need to build a filter perhaps? Worth noting is that I did try measuring voltages on the speed controller board at different points and I found several that would range from 0VDC to about 11VDC depending on the position of the potentiometer. Am I incorrect in thinking I should be looking for a reading range of 0VDC to 20VDC?

    Kind Regards from Australia .... Scott
     
  2. DickCappels

    Moderator

    Aug 21, 2008
    2,653
    632
    You will probably get something close to 0 to 20 volts, depending upon the the design of the controller and the load.

    With a 13 kHz pwm frequency you should get good reading if you filter the input to the voltmeter by passing the voltage through a 10 k resistor followed by a 20 uf capacitor to ground for a 100 ms time constant. If the meter's response feels too sluggish try 2.2 uf.

    Another thing to consider is that if your controller can go below the meter's required power supply voltage of if the meter needs a fixed power supply, you will probably have to supply a separate power supply for the meter.
     
  3. Scott Manning

    Thread Starter New Member

    Feb 12, 2015
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    Thankyou DickCappels. Could I trouble you to show me a diagram of what your suggesting? Sorry to be painful.
     
  4. Scott Manning

    Thread Starter New Member

    Feb 12, 2015
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    Forgot to add that I intended to power the meter at the same point the speed controller receives its 20VDC from the laptop charger and measure the output with the remaining white wire. Is there anything obviously incorrect there?
     
  5. DickCappels

    Moderator

    Aug 21, 2008
    2,653
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    In the diagram below "input" comes from the controller output and "output" goes to the input of the meter.

    [​IMG]
    "Input range: DC 5V-30V(Max Input: DC 30V. The device can be damaged if input is over 30V)"

    Noting obviously wrong with your intended setup.
     
  6. Scott Manning

    Thread Starter New Member

    Feb 12, 2015
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    Thankyou very much for your help. I will build the circuit and report back.

    Thanks again.
     
  7. Scott Manning

    Thread Starter New Member

    Feb 12, 2015
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    Ok, so I purchased a 1w 10k resistor and a 22uF, 63v capacitor and built the circuit as shown but I am getting a constant voltage reading of 18 volts. Not sure what I have done wrong.
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    What load did you have on the supply when you got the 18V reading?
     
  9. Scott Manning

    Thread Starter New Member

    Feb 12, 2015
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    It's supplying a dc motor just free spinning without and mechanical load.
     
  10. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    1,251
    Several people on ebay sell power supply modules that are adjustable DC/DC switching regulators with a built-in output voltage display.

    ak
     
  11. Scott Manning

    Thread Starter New Member

    Feb 12, 2015
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    Thanks AnalogKid

    At least I have options if I can't get this working. Considering I have what I have I would like to persist with trying to get it working for a bit longer. Not sure if it's relevant but the output varies when tested with my multimeter while the Ebay voltage display remains steady at around 18vdc
     
  12. Scott Manning

    Thread Starter New Member

    Feb 12, 2015
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    Any ideas anyone?
     
  13. DickCappels

    Moderator

    Aug 21, 2008
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    That makes the central question one of why your panel meter acts differently from your DVM.

    If you connect your DVM directly across the input to the panel meter, do the readings on the two meters differ?
     
  14. crutschow

    Expert

    Mar 14, 2008
    13,009
    3,233
    Are you sure the meter power pin and the measurement pin are going to the proper spots.
     
  15. Scott Manning

    Thread Starter New Member

    Feb 12, 2015
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    If I measure the output at the same point with both the multimeter and the panel meter the results are different. The volt meter displays a voltage representative of the position of the potentiometer (0 to 20vdc) the panel meter shows 18vdv with the additional circuit installed or just 20vdc without. This is static regardless of the potentiometer position.

    I have connected the supply and measure of the panel meter to the output with the same result.
     
  16. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    That suggests the panel meter
    1) is incorrectly wired,
    2) has some internal storage/latch to retain the measurement until reset, or
    3) is faulty.
     
  17. Scott Manning

    Thread Starter New Member

    Feb 12, 2015
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    I will confirm again the operation of the panel meter tonight but I did use it on another source where it worked perfectly.
    I did consider the possibility that the + and measurement wires were wired incorrectly so I reversed them. No luck. Baffling.
     
  18. Scott Manning

    Thread Starter New Member

    Feb 12, 2015
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    0
    I have no idea what is happening but have decided to stop wasting time and I purchased a DC voltage regulator from Ebay with a voltage display for 10 bucks.

    Thanks everyone for your help.
     
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