This blog series uses Oscium’s iMSO-104 handheld oscilloscope and a digital multi-meter to evaluate Enercell’s selectable output voltage power supply. This inexpensive, commercially available “wall wart” power supply is a terrific addition to any hobbyist’s arsenal of electronic gadgets. Follow this blog series to learn how this power supply tested and see if it is something you’d like to incorporate in one of your projects.
Product Features and Specifications
The device under investigation in this blog series is Enercell Universal 300mA AC Adapter, model number 273-315. Enercell must be a store brand exclusive to Radio Shack; the technical information about Enercell products is only available at Radio Shack websites. Follow this link to the product page for the Enercell 273-315. Although the 273-315 is an AC power adapter, in this series of blog posts it is referred to as a selectable voltage output power supply because it's being evaluated for use in hobby applications as cost effective alternative for traditional DC power supplies.
Begin by summarizing the product’s specifications and features that might be relevant in a prototyping or hobbyist setting.
- Adaptaplugs E, F, H, J, L and M are included (others sold separately)
- Powers products requiring 1.5, 3, 4.5, 6, 7.5, 9 or 12VDC requiring up to 300mA
- Regulated, filtered output reduces hum and noise
Adaptaplugs: The various sized adapters that come with the 273-315 make it easy to use as a replacement for other power converters. There is also a plug-in accessory that terminates in solder tinned leads. This accessory makes the 273-315 nice to use with custom applications and screw-down terminals that at are found on prototyping breadboards.
Multiple Supply Voltages: The range of output voltages allows for powering all kinds of electronics, from amplifiers (high power amps need the +12V) to digital logic circuits (+1.5V), and 300 mA of current is decent.
Conditioning: Another strong feature of this supply is the output conditioning. Only the cheapest and dirtiest commercial power supplies would come without some kind of filtering and voltage regulation. Or for you Do-It-Yourself’ers, try rectifying 110V AC and building your own DC supply; you’ll quickly appreciate how much work goes in to eliminating the 60 Hz “hum” and regulating the output voltage over a dynamic current load.
There were no formal published specifications or datasheet like you’d expect to see with a semi-custom or industrial grade piece of equipment. The Radio Shack website and the “user guide” PDF on the site simply restate everything you already know about this power supply: it has several output voltages you can choose from, and its maximum current rating is 300 mA. This series of blogs will look at the tolerance around the nominal supply voltages, discover just how much each voltage rail can source before it crunches, and find out what happens when the load demands more current than the supply can source (you’re in for a surprise).
The Fun Stuff – Seeing How It Works
Characterizing Open Circuit Output Voltage
For average voltage measurements use a DMM that looks at the voltage over a period of time and sends the running average to the display. Oscilloscopes, on the other hand, are necessary for looking at time varying voltages. For the measurements in this blog, Oscium's iMSO-104 handheld oscilloscope is used to measure the mean voltage and the peak-to-peak variation around the mean.
In the figure below, the oscilloscope and the DMM are measuring the output voltage of the power supply. Both instruments can be used at the same time because they both present a high impedance load to the supply and do not load the output. In other words, the oscilloscope and DMM look like 10 MOhm resistors to the power supply, and since no current deviates from the load path, they are insignificant to the output of the power supply.
Characterize Maximum Current Supply
To evaluate the maximum current capability of the different supply rails the resistance of the load gradually decreases until the power supply goes into current limit, or crunches. In each iteration of this experiment the peak-to-peak ripple of the output voltage is measured with Oscium’s iMSO-104 oscilloscope and the power dissipated in the load resistor calculate is calculated using the familiar equation P = V^2 / R.
Enercell rates the power supply at 300 mA. The power supply is probably capable of sourcing an amp under some loads or off some rails, but not quite as much off other rails or heavier loads. The point of this experiment is to test that hypothesis.
Next Time: Voltage Characterization
Tutorial: The next blog of the series steps through making accurate DC voltage measurements using Oscium’s oscilloscope and it compares the reading from the DMM and the oscilloscope and make sense of where they differ. The blog also touches on Oscium’s new single trigger feature with iMSO-104, and shows you how to properly configure it to capture just the signals you want.
Results: The next installment in the series will also be loaded with the collected data (raw and uncensored) for your own analysis.
Oscium's handheld oscilloscope is now available with universal platform support! So, if you're interested in using the scope on iOS, Android, PC or Mac, Oscium supports you. Please go to iMSO-204x for more information.