This is Part 2 in the blog series. People sometimes wonder which instrument is better to use for measuring a voltage, an oscilloscope or a hand held digital multi-meter (DMM). One answer to that rhetorical question is “It depends on what you want to know.” In the image below, a DMM and an oscilloscope are measuring the voltage output of the power supply from Part 1 of this series.
Compare DC Voltage with O’scope and DMM
In the foreground of the image above you can see the oscilloscope probe and ground clip connected in parallel across the leads of the supply (not pictured) with the red and black clip probes of the DMM. An iPad 2 paired with Oscium’s iMSO-104 functions as the oscilloscope for the measurement.
Basic Setup for the Measurement
The figure below is a depiction of the equivalent circuit for this measurement. The oscilloscope and DMM (connected in parallel) are measuring the output voltage of the power supply. During the experiment, the power supply is set to each of the seven output voltages and the resulting output is measured with the iMSO-104 handheld oscilloscope and the DMM.
Dependent Variable: The Load Resistance
Two trials are performed in this experiment. In the first trial the voltage characteristics for each of the seven outputs is measured with an open circuit (infinite resistance) load. The second trial used a 1 kOhm resistor as the load (pictured below). Voltage regulator manufactures publish datasheets containing information about loading and its effects on performance. The open circuit trial presents the lightest load possible to the power supply. The 1 kOhm resistor, on the other hand, presents a moderate load to the power supply that is typical of general application circuits.
Measured Data, Unedited
The measured data for the experiment is recorded in the first pair of tables, immediately below, for the open circuit and 1 kOhm loads. For the oscilloscope the mean value for each of the seven voltages and the corresponding peak-to-peak voltage are recorded in the tables.
Analysis of Average Value Measured Data
The first part of the analysis looked at the accuracy of the power supply’s regulated output voltages. The raw data is recorded in the tables above. The accuracy of each output voltage is represented in the tables below as an error percentage. For example, look at the data contained in the first row of the tables below: the DMM measured the 12 V supply within 0.2% of the nominal 12 volts and the oscilloscope was about 0.7%. The calculated data is different for the two instruments because they use different topologies for making the measurement.
Analysis of Peak-to-Peak Value Measured Data
The second part of the analysis is performed only on the oscilloscope data. The peak-to-peak variation for each voltage recorded in the previous tables is brought forward for this analysis, they are compared to the average value for each voltage output, and then used to calculate the equivalent resulting minimum and maximum voltages. Lastly, each of the nominal output voltages are compared to the calculated minimum and maximum values to represent the accuracy as another error percentage.
- Load resistance reduces peak-to-peak voltage ripple. As expected, when using a 1 kOhm resistor for the load improved the power supplies voltage ripple performance. For all seven voltages, the peak-to-peak ripple dropped approximately from 200 mV to 80 mV. The smaller voltage ripple reduces the minimum and maximum excursions from the mean voltage measured with the oscilloscope.
- DMM readings were largely unaffected by the increased load on the power supply.
- The calculated maximum output voltage with the 1 kOhm load measured by the oscilloscope more closely resembles the voltage measured by the DMM. This observation supports the claim that the power supply needs some minimum load for it to function properly. Most regulators operate inefficiently, oscillate, or fail to meet the specified performance when the load it too little.
Next Time: Maximum Current Characterization
In the next blog of the series, Part 3, a decade box is used to vary the output load on the supply and monitor the output voltage. The power supply manufacturer, Enercell, advertises a 300 mA current rating for all seven voltages and this experiment will show just how much current it really can source.