About C. J. Abate

C. J. Abate is Circuit Cellar's Editor in Chief. You can reach him at cabate@circuitcellar.com and @editor_cc.

Testing Power Supplies (EE Tip #112)

How can you determine the stability of your lab or bench-top supply? You can get a good impression of the stability of a power supply under various conditions by loading the output dynamically. This can be implemented using just a handful of components.

Power supply testing

Power supply testing

Apart from obvious factors such as output voltage and current, noise, hum and output resistance, it is also important that a power supply has a good regulation under varying load conditions. A standard test for this uses a resistor array across the output that can be switched between two values. Manufacturers typically use resistor values that correspond to 10% and 90% of the rated power output of the supply.

The switching frequency between the values is normally several tens of hertz (e.g. 40 Hz). The behavior of the output can then be inspected with an oscilloscope, from which you can deduce how stable the power supply is. At the rising edge of the square wave you will usually find an overshoot, which is caused by the way the regulator functions, the inductance of the internal and external wiring and any output filter.

This dynamic behavior is normally tested at a single frequency, but the designers in the Elektor Lab have tested numerous lab supplies over the years and it seemed interesting to check what happens at higher switching frequencies. The only items required for this are an ordinary signal generator with a square wave output and the circuit shown in Figure 1.Fig1-pwrsupply

You can then take measurements up to several megahertz, which should give you a really good insight for which applications the power supply is suitable. More often than not you will come across a resonance frequency at which the supply no longer remains stable and it’s interesting to note at which frequency that occurs.

The circuit really is very simple. The power MOSFET used in the circuit is a type that is rated at 80 V/75 A and has an on-resistance of only 10 mΩ (VGS = 10 V).

The output of the supply is continuously loaded by R2, which has a value such that 1/10th of the maximum output current flows through it (R2 = Vmax/0.1/max). The value of R1 is chosen such that 8/10th of the maximum current flows through it (R1 = Vmax/0.8/max). Together this makes 0.9/max when the MOSFET conducts. You should round the calculated values to the nearest E12 value and make sure that the resistors are able to dissipate the heat generated (using forced cooling, if required).

At larger output currents the MOSFET should also be provided with a small heatsink. The gate of the FET is connected to ground via two 100-Ω resistors, providing a neat 50-Ω impedance to the output of the signal generator. The output voltage of the signal generator should be set to a level between 5 V and 10 V, and you’re ready to test. Start with a low switching frequency and slowly increase it, whilst keeping an eye on the square wave on the oscilloscope. And then keep increasing the frequency… Who knows what surprises you may come across? Bear in mind though that the editorial team can’t be held responsible for any damage that may occur to the tested power supply. Use this circuit at your own risk!

— Harry Baggen and Ton Giesberts (Elektor, February 210)

Electronics Grounding (EE Tip #107)

Whether you are professional electrical engineer or part-time DIYer, before you start your next project, read through this primer on grounding. This short survey covers one of the most fundamental topics in electronics: grounding.

Electronics Signal Ground or Circuit Common

Signal ground is the current return to the power supply. Current leaves the power supply, passes through the various electronic components, and then returns to the supply. The typical symbol for signal ground is shown in Figure 1.EE107-F1-2

 Chassis Ground or Earth Ground

Chassis ground is an electrical safety requirement to prevent an electrical or electronic device’s chassis from delivering an electrical shock. A long copper rod is driven into the ground outside of the building, and a wire connects the metal chassis to the rod which is at the approximate 0 V potential of the earth. The symbol for earth ground is shown in Figure 2.

Ground Details

Consider the following two details about ground. First, ground is not exactly 0 V. And second, two physically different ground points will not be at the same voltage potential.

Ground Loop

By definition, current will flow in an electrical conductor connected to a difference in voltage potential between two points. Because two physically different ground points are not at the same potential, current will flow through an electrical conductor connected between those two points. This is a ground loop.

Notice this current flowing between these two different ground points is not related to or correlated to any electronic data or message signal. This is noise or garbage that will interfere and distort any information contained in the electronic system.

Note: While “noise” can be added to systems on occasion, it is specifically controlled and the exact quantity is regulated.

Example

Given: A ground loop producing 610 μV of ground noise. It’s a very small quantity. You have a 16-bit A/D converter with a 0- to 10-V input. The smallest voltage it can resolve is:

= 10 V/16 exp 2

= 10 V/65,536

= 152.5ìV

Note that the ground loop noise is four times greater than the actual data, so that A/D converter loses two bits of resolution, and it is now a 14-bit converter.

Connect with Single-Ended/Unbalanced Amps

In Figure 3 the two grounds exist at different potentials, so some current will flow between the grounds. EE107-F3

This ground current has nothing to do with any signals being amplified, and it is noise decreasing the accuracy of the system. Figure 4 is a complete schematic.EE107-F4

Connect with Transformers

When connecting with transformers, keep the following in mind:

  • There is no ground connection, so there can be no Ground Loop.
  • Common-mode rejection of RF interference.
  • Signals are AC coupled, so of limited use for circuits with DC data such as accelerator focus and bend magnets (see Figure 5).EE107-F5

Connect with Differential Amps

Refer to Figure 6 for connecting two systems with differential amplifiers.

  • There is no ground connection, so there can be no Ground Loop.
  • Common-mode rejection of RF interference (see Figure 7).
  • Signals are DC coupled, so this is the perfect solution for circuits with DC data.EE107-F6EE107-F7

—Dennis Hoffman

Note: This article first appeared in audioXpress  (June 2011). It is from a class that Dennis Hoffman teaches at the SLAC National Accelerator Laboratory (Menlo Park, CA). Like Circuit Cellar, audioXpress is Elektor International Media Publication.

Programmable Logic Video Lessons

Interested in learning more about programmable logic? You’re in luck. Colin O’Flynn’s first article in his “Programmable Logic in Practice” column appears in Circuit Cellar’s October 2013 issue. To accompany his work, Colin is producing informative videos for you to view after reading his articles.

In the first video, Colin covers the topic of adding the Xilinx ChipScope ILA/VIO core using automatic and manual insertion with ISE.

 

Since 2002, Circuit Cellar has published several of O’Flynn’s articles. O’Flynn  is an engineer and lecturer at Dalhousie University in Halifax, Nova Scotia. He earned a Master’s in applied science from Dalhousie and pursued further graduate studies in cryptographic systems. Over the years, he has developed a wide variety of skills ranging from electronic assembly (including SMDs) to FPGA design in Verilog and VHDL to high-speed PCB design.

Embedded Sensor Innovation at MIT

During his June 5 keynote address at they 2013 Sensors Expo in Chicago, Joseph Paradiso presented details about some of the innovative embedded sensor-related projects at the MIT Media Lab, where he is the  Director of the Responsive Environments Group. The projects he described ranged from innovative ubiquitous computing installations for monitoring building utilities to a small sensor network that transmits real-time data from a peat bog in rural Massachusetts. Below I detail a few of the projects Paradiso covered in his speech.

DoppleLab

Managed by the Responsive Enviroments group, the DoppelLab is a virtual environment that uses Unity 3D to present real-time data from numerous sensors in MIT Media Lab complex.

The MIT Responsive Environments Group’s DoppleLab

Paradiso explained that the system gathers real-time information and presents it via an interactive browser. Users can monitor room temperature, humidity data, RFID badge movement, and even someone’s Tweets has he moves throughout the complex.

Living Observatory

Paradiso demoed the Living Observatory project, which comprises numerous sensor nodes installed in a peat bog near Plymouth, MA. In addition to transmitting audio from the bog, the installation also logs data such as temperature, humidity, light, barometric pressure, and radio signal strength. The data logs are posted on the project site, where you can also listen to the audio transmission.

The Living Observatory (Source: http://tidmarsh.media.mit.edu/)

GesturesEverywhere

The GesturesEverywhere project provides a real-time data stream about human activity levels within the MIT Media Lab. It provides the following data and more:

  • Activity Level: you can see the Media Labs activity level over a seven-day period.
  • Presence Data: you can see the location of ID tags as people move in the building

The following video is a tracking demo posted on the project site.

The aforementioned projects are just a few of the many cutting-edge developments at the MIT Media Lab. Paradiso said the projects show how far ubiquitous computing technology has come. And they provide a glimpse into the future. For instance, these technologies lend themselves to a variety of building-, environment-, and comfort-related applications.

“In the early days of ubiquitous computing, it was all healthcare,” Paradiso said. “The next frontier is obviously energy.”

Embedded Wireless Made Simple

Last week at the 2013 Sensors Expo in Chicago, Anaren had interesting wireless embedded control systems on display. The message was straightforward: add an Anaren Integrated Radio (AIR) module to an embedded system and you’re ready to go wireless.

Bob Frankel demos embedded mobile control

Bob Frankel of Emmoco provided a embedded mobile control demonstration. By adding an AIR module to a light control system, he was able to use a tablet as a user interface.

The Anaren 2530 module in a light control system (Source: Anaren)

In a separate demonstration, Anaren electrical engineer Mihir Dani showed me how to achieve effective light control with an Anaren 2530 module and TI technology. The module is embedded within the light and compact remote enables him to manipulate variables such as light color and saturation.

Visit Anaren’s website for more information.