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EspoTek / Labrador

EspoTek Labrador is a USB device that transforms your PC or smartphone into a fully-featured electronics lab. This repo holds all of the source code!
http://espotek.com
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Scope reads 4.5Vdc but multimeter reads 3Vdc #148

Closed macktion closed 3 years ago

macktion commented 3 years ago

Hi. I'm monitoring a small 4.5Vdc 1A PSU used on some tiny Christmas lights. The Labrador zoomed-in shows quite smooth 4.5 (0v offset)but the multimeter shows only approx 3.15V max/min/RMS. shouldn't the scope and meter values be close?

I also haven't had luck measuring current with the Lab. trying to verify/improve the accuracy of the 200Ma I measured with my discrete meter that created a significant/unwanted series load that dims the lights. I could not even get the Lab to allow the string of lights to turn on while indicating current as high as 1.5A at 0ohms series.

EspoTek commented 3 years ago

The multimeter measures the difference between CH1 and CH2. If nothing is attached to CH2, it'll float towards VCC/2 (around 1.65V). Please see documentation here: https://github.com/EspoTek/Labrador/wiki

Current measurements with a low burden voltage is not a standard feature of most multimeters, including Flukes. It's possible to use an external amplifier circuit to lower burden voltage to near-zero, though. Dave Jones did a video on it a while back: https://www.youtube.com/watch?v=jx0dryLmUEQ

macktion commented 3 years ago

Thank you. Very helpful.

lkoepsel commented 3 years ago

This issue is happening for me. If one follows the following process, it is easy to replicate.

  1. Open Labrador application, press M to bring up Range Dialog and change Vmax to 5V.
  2. Connect Oscilloscope CH1 to Power Supply (Positive) and Oscilloscope CH2 to Power Supply (GND) Note the CH1 line is about 4.8V and is solid, no noise.

Screen Shot 2021-03-27 at 7 38 46 AM

  1. Uncheck Oscilloscope CH1 and check Multimeter, all of the values (Max, Min, Mean and RMS) will be identical and incorrect. In this case, 2.96V. Note CH1 line has now moved to 2.96V, continues to be solid, no noise

Screen Shot 2021-03-27 at 7 34 55 AM

  1. Uncheck Multimeter and check Oscilloscope then check Paused and uncheck Paused. Note the CH1 line moves up to 4.9V and once Paused is unchecked, the line appears with a slight amount of noise
  2. Uncheck Oscilloscope and check Multimeter and the values:
    • Are correct when compared with a DMM
    • Are not all identical, they vary with a little bit of noise.

Screen Shot 2021-03-27 at 7 35 25 AM