Experimenting with Dielectric Absorption

Dielectric absorption occurs when a capacitor that has been charged for a long time briefly retains a small amount of voltage after a discharge.

“The capacitor will have this small amount of voltage even if an attempt was made to fully discharge it,” according to the website wiseGEEK. “This effect usually lasts a few seconds to a few minutes.”

While it’s certainly best for capacitors to have zero voltage after discharge, they often retain a small amount through dielectric absorption—a phenomenon caused by polarization of the capacitor’s insulating material, according to the website. This voltage (also called soakage) is totally independent of capacity.

At the very least, soakage can impair the function of a circuit. In large capacitor systems, it can be a serious safety hazard.

But soakage has been around a long time, at least since the invention of the first simple capacitor, the Leyden jar, in 1775. So columnist Robert Lacoste decided to have some “fun” with it in Circuit Cellar’s February issue, where he writes about several of his experiments in detecting and measuring dielectric absorption.

Curious? Then consider following his instructions for a basic experiment:

Go down to your cellar, or your electronic playing area, and find the following: one large electrolytic capacitor (e.g., 2,200 µF or anything close, the less expensive the better), one low-value discharge resistor (100 Ω or so), one DC power supply (around 10 V, but this is not critical), one basic oscilloscope, two switches, and a couple of wires. If you don’t have an oscilloscope on hand, don’t panic, you could also use a hand-held digital multimeter with a pencil and paper, since the phenomenon I am showing is quite slow. The only requirement is that your multimeter must have a high-input impedance (1 MΩ would be minimum, 10 MΩ is better).

Figure 1: The setup for experimenting with dielectric absorption doesn’t require more than a capacitor, a resistor, some wires and switches, and a voltage measuring instrument.

Figure 1: The setup for experimenting with dielectric absorption doesn’t require more than a capacitor, a resistor, some wires and switches, and a voltage measuring instrument.

Figure 1 shows the setup. Connect the oscilloscope (or multimeter) to the capacitor. Connect the power supply to the capacitor through the first switch (S1) and then connect the discharge resistor to the capacitor through the second switch (S2). Both switches should be initially open. Photo 1 shows you my simple test configuration.

Now turn on S1. The voltage across the capacitor quickly reaches the power supply voltage. There is nothing fancy here. Start the oscilloscope’s voltage recording using a slow time base of 10 s or so. If you are using a multimeter, use a pen and paper to note the measured voltage. Then, after 10 s, disconnect the power supply by opening S1. The voltage across the capacitor should stay roughly constant as the capacitor is loaded and the losses are reasonably low.

Photo 1: My test bench includes an Agilent Technologies DSO-X-3024A oscilloscope, which is oversized for such an experiment.

Photo 1: My test bench includes an Agilent Technologies DSO-X-3024A oscilloscope, which is oversized for such an experiment.

Now switch on S2 long enough to fully discharge the capacitor through the 100-Ω resistor. As a result of the discharge, the voltage across the capacitor’s terminals will quickly become very low. The required duration for a full discharge is a function of the capacitor and resistor values, but with the proposed values of 2,200 µF and 100 Ω, the calculation shows that it will be lower than 1 mV after 2 s. If you leave S2 closed for 10 s, you will ensure the capacitor is fully discharged, right?

Now the fun part. After those 10 s, switch off S2, open your eyes, and wait. The capacitor is now open circuited, at least if the voltmeter or oscilloscope input current can be neglected, so the capacitor voltage should stay close to zero. But you will soon discover that this voltage slowly increases over time with an exponential shape.

Photo 2 shows the plot I got using my Agilent Technologies DSO-X 3024A digital oscilloscope. With the capacitor I used, the voltage went up to about 120 mV in 2 min, as if the capacitor was reloaded through another voltage source. What is going on here? There aren’t any aliens involved. You have just discovered a phenomenon called dielectric absorption!

Photo 2: I used a 2,200-µF capacitor, a 100-Ω discharge resistor, and a 10-s discharge duration to obtain this oscilloscope plot. After 2 min the voltage reached 119 mV due to the dielectric absorption effect.

Photo 2: I used a 2,200-µF capacitor, a 100-Ω discharge resistor, and a 10-s discharge duration to obtain this oscilloscope plot. After 2 min the voltage reached 119 mV due to the dielectric absorption effect.

Nothing in Lacoste’s column about experimenting with dielectric absorption is shocking (and that’s a good thing when you’re dealing with “hidden” voltage). But the column is certainly informative.

To learn more about dielectric absorption, what causes it, how to detect it, and its potential effects on electrical systems, check out Lacoste’s column in the February issue. The issue is now available for download by members or single-issue purchase.

Lacoste highly recommends another resource for readers interested in the topic.

“Bob Pease’s Electronic Design article ‘What’s All This Soakage Stuff Anyhow?’ provides a complete analysis of this phenomenon,” Lacoste says. “In particular, Pease reminds us that the model for a capacitor with dielectric absorption effect is a big capacitor in parallel with several small capacitors in series with various large resistors.”

Next-Generation Wi-Fi Modules

eConaisThe EC19D family is small, easily integrated, low-standby power single chip 802.11b/g/n Wi-Fi System In Package (SiP) modules for the Internet of Things (IoT).

The SiP modules help designers quickly and easily connect their devices to 802.11b/g/n Wi-Fi networks. At 8-mm × 8-mm, the EC19D modules can be embedded in almost any product or application. The EC19D will also include FCC, IC, and EC certifications to further simplify and speed up product design and production for use with Wi-Fi networks.

The EC19D incorporates the newest Wi-Fi 802.11b/g/n standards and features to provide designers with many options for embedding the module in their designs. The EC19D’s features include Wi-Fi Direct, ProbMeTM configuration, full TCP/IP stack, HTTPS/SSL, DHCP Client/Server, WPS, legacy Wi-Fi Client, and SoftAP modes with WPA/WPA2 support, serial to Wi-Fi, and Cloud service support.

Contact eConais for pricing.

eConais Inc.
www.econais.com

RS-422/-485 Serial Interface for PCI Express

SealevelThe 7802e is a PCI Express serial interface adapter that provides eight serial ports individually configurable for RS-422 or RS-485 communications. The adapter is well suited for applications including test and measurement, security systems, and broadcast.

The board’s high-performance 16C950 UART includes 128-byte FIFOs for error-free operation in high-speed serial applications. The 16C950 UART also supports 9-bit framing and is software compatible with legacy UART applications.

A PCI Express link supplies the 7802e’s 62.5-MHz clock. This ultra-high speed clock is divided by a flexible 8-bit clock prescalar. In RS-485 mode, the transmitter is automatically enabled in hardware, eliminating the need for application software control. This enables the 7802e to be used with standard serial applications without the risk of bus contention and data corruption.

All Sealevel PCI Express serial adapters include SeaCOM software for Windows and Linux OSes. The adapters also include WinSSD, a full-featured application for testing and diagnostics including bit error rate testing (BERT), throughput monitoring, loopback tests, and test pattern message transmissions.

The 7802e costs $469.

Sealevel Systems, Inc.
www.sealevel.com

Remote Control and Monitoring of Household Devices

Raul Alvarez, a freelance electronic engineer from Bolivia, has long been interested in wireless device-to-device communication.

“So when the idea of the Internet of Things (IoT) came around, it was like rediscovering the Internet,” he says.

I’m guessing that his dual fascinations with wireless and the IoT inspired his Home Energy Gateway project, which won second place in the 2012 DesignSpark chipKIT challenge administered by Circuit Cellar.

“The system enables users to remotely monitor their home’s power consumption and control household devices (e.g., fans, lights, coffee machines, etc.),” Alvarez says. “The main system consists of an embedded gateway/web server that, aside from its ability to communicate over the Internet, is also capable of local communications over a home area wireless network.”

Alvarez catered to his interests by creating his own wireless communication protocol for the system.

“As a learning exercise, I specifically developed the communication protocol I used in the home area wireless network from scratch,” he says. “I used low-cost RF transceivers to implement the protocol. It is simple and provides just the core functionality necessary for the application.”

Figure1: The Home Energy Gateway includes a Hope Microelectronics RFM12B transceiver, a Digilent chipKIT Max32 board, and a Microchip Technology ENC28J60 Ethernet controller chip.

Figure 1: The Home Energy Gateway includes a Hope Microelectronics RFM12B transceiver, a Digilent chipKIT Max32 board, and a Microchip Technology ENC28J60 Ethernet controller chip.

Alvarez writes about his project in the February issue of Circuit Cellar. His article concentrates on the project’s TCI/IP communications aspects and explains how they interface.

Here is his article’s overview of how the system functions and its primary hardware components:

Figure 1 shows the system’s block diagram and functional configuration. The smart meter collects the entire house’s power consumption information and sends that data every time it is requested by the gateway. In turn, the smart plugs receive commands from the gateway to turn on/off the household devices attached to them. This happens every time the user turns on/off the controls in the web control panel.

Photo 1: These are the three smart node hardware prototypes: upper left,  smart plug;  upper right, a second smart plug in a breadboard; and at bottom,  the smart meter.

Photo 1: These are the three smart node hardware prototypes: upper left, smart plug; upper right, a second smart plug in a breadboard; and at bottom, the smart meter.

I used the simple wireless protocol (SWP) I developed for this project for all of the home area wireless network’s wireless communications. I used low-cost Hope Microelectronics 433-/868-/915-MHz RFM12B transceivers to implement the smart nodes. (see Photo 1)
The wireless network is configured to work in a star topology. The gateway assumes the role of a central coordinator or master node and the smart devices act as end devices or slave nodes that react to requests sent by the master node.

The gateway/server is implemented in hardware around a Digilent chipKIT Max32 board (see Photo 2). It uses an RFM12B transceiver to connect to the home area wireless network and a Microchip Technology ENC28J60 chip module to connect to the LAN using Ethernet.

As the name implies, the gateway makes it possible to access the home area wireless network over the LAN or even remotely over the Internet. So, the smart devices are easily accessible from a PC, tablet, or smartphone using just a web browser. To achieve this, the gateway implements the SWP for wireless communications and simultaneously uses Microchip Technology’s TCP/IP Stack to work as a web server.

Photo 2: The Home Energy Gateway’s hardware includes a Digilent chipKIT Max32 board and a custom shield board.

Photo 2: The Home Energy Gateway’s hardware includes a Digilent chipKIT Max32 board and a custom shield board.

Thus, the Home Energy Gateway generates and serves the control panel web page over HTTP (this page contains the individual controls to turn on/off each smart plug and at the same time shows the power consumption in the house in real-time). It also uses the wireless network to pass control data from the user to the smart plugs and to read power consumption data from the smart meter.

The hardware module includes three main submodules: The chipKIT Max 32 board, the RFM12B wireless transceiver, and the ENC28J60 Ethernet module. The smart meter hardware module has an RFM12B transceiver for wireless communications and uses an 8-bit Microchip Technology PIC16F628A microcontroller as a main processor. The smart plug hardware module shows the smart plugs’ main hardware components and has the same microcontroller and radio transceiver as the smart meter. But the smart plugs also have a Sharp Microelectronics S212S01F solid-state relay to turn on/off the household devices.

On the software side, the gateway firmware is written in C for the Microchip Technology C32 Compiler. The smart meter’s PIC16F628A code is written in C for the Hi-TECH C compiler. The smart plug software is very similar.

Alvarez says DIY home-automation enthusiasts will find his prototype inexpensive and capable. He would like to add several features to the system, including the ability to e-mail notifications and reports to users.

For more details, check out the February issue now available for download by members or single-issue purchase.

Desoldering Components (EE Tip #118)

Every engineer and technician sooner or later faces the challenge of having to desolder a component. Sometimes the component can be a large transformer with 10 pins or a power chip with many connections, and desoldering tools are typically around the $1,000 mark and above.

Chip Quik is a solder-based alloy that stays molten for up to 30 seconds and makes desoldering any component very easy. The only drawback is that the cost of a 2´ length of Chip Quik is around $20. But a little experience can make this go a long way. Having some Chip Quik lying around in the workshop is reassuring for when that urgent job comes in.

Editor’s Note: This EE Tip was written by Fergus Dixon of Sydney, Australia. Dixon, who has written two articles and an essay for Circuit Cellar, runs Electronic System Design, a website set up to promote easy to use and inexpensive development kits. Click here to read his essay “The Future of Open-Source Hardware for Medical Devices.”