This blog posting concludes the evaluation of Enercell’s AC/DC 300 mW power supply, with 7 selectable output voltage between 12 V and 1.5 V. Part 1 of the series looks at the power supply’s important features and discusses their benefits to the electronics hobbyist. Part 1 is also an introduction to the test setups and measurements frequently used in evaluating power supplies. In Part 2 the output voltage of each selectable output voltage is measured and characterized, the open circuit voltage accuracy and the peak-to-peak voltage ripple. Lastly, in this third and final post of the series, the manufacturer’s claim that the power supply will produce 300 mW off each of the output voltages is tested. This performance parameter is tested by varying the load presented to the power supply and monitoring the output voltage and calculating the current through the variable resistor at points before and after the voltage compresses.
Characterizing The Maximum Current Supply
To evaluate the maximum current capability of the different supply rails the resistance of the load is gradually reduced until the power supply goes into current limit. In each iteration of this experiment the peak-to-peak ripple of the output voltage is measured with Oscium’s iMSO-104 oscilloscope and calculated from the power dissipated in the resistive load.
The Measurement Setup
In the setup sketched below, the AC input block and DC output block represent the 300 mW Enercell power supply, and the load resistor, R1, represents a decade resistor box. The output current is calculated using Ohm’s Law, V = I*R, and the power dissipated in the load is P = I^2*R.
Resistor Decade Box
The figure below shows a typical resistor decade box. It works by selecting the desired resistance in each of the places of the dial. This is a link to the manufacturer’s datasheet for the decade box used in this test. http://www.ietlabs.com/pdf/Datasheets/RS-CS-LS.pdf
DMM VS Oscilloscope: The observations discussed here would be very different if a DMM were used in these measurements and not Oscium’s handheld oscilloscope. For example, voltage ripple seen with an oscilloscope would look like the DMM fluctuating between measured voltage values and not quite settling on an exact value.
Output Voltage VS Load: The output voltage waveform passed through four phases as the load increased from minimum to maximum. The figure below pictorially describes these phases and includes a short summary.
- Event #1: The well-regulated output was characterized by a fairly flat output at the nominal voltage level, within some amount of peak-to-peak voltage tolerance.
- Events #2 and #3: The unregulated output has increased voltage ripple at the output. The peak-to-peak ripple increases as the load on the power supply increases. The ripple is caused by the power supply struggling to source enough current to the load and failing to regulate the voltage within the same peak-to-peak voltage range as the “well regulated” case described above.
- Event #4: Over current protection mode is a low duty cycle, pulsed output. The power supply enters this mode of operation to protect itself from a short circuit at the load. As the voltage starts coming up the regulator senses the over current, or short circuit, and shuts itself down for a predetermined amount of time before repeating the cycle.
If you’ll remember back to the first part of this posting, the purpose for evaluating the maximum current capability of this supply was to verify the manufacturer’s 300 mW power rating for each of the selectable output voltages. The measured results for this experiment are recorded in the table below, divided the table into two parts. The first part, recorded under the “Ripple” heading records the load resistance at the time the power supply output moved from “well regulated” into the “unregulated” modes described above. The second data group, recorded under the “Over Current” heading, is the projected current and power dissipation in the load that would occur IF the power supply was still regulating. However, the second data group is simply speculation because at the point the power supply enters Over Current mode, the voltage to the load drops to less than 0.2 Volts.