ESR Test Harness: Part 1

Introduction

Because I recover electrolytic capacitors from junked electronics for hobby use, and because I restore vintage audio equipment with old electrolytics, I wanted a way to test these capacitors as they are known to have a short lifespan compared to other components.

Bad capacitors, if they aren't shorted outright, will show a high equivalent series resistance (ESR) so I wanted to measure ESR by building Stephen M. Powell's ESR test harness.

I figure this meter will save me money by letting me salvage caps and replacing only those that have started to go bad.

One can measure ESR of a capacitor by sending a small, high frequency square wave through it and looking at the wave form on an oscilloscope. Mr. Powell's circuit does just that, so I thought I'd share my build experience.

The Circuit

The circuit consists of two parts. On the left is the signal generator built around the good old 555 chip, designed to deliver a milli-Volt 100kHz square wave. It's design to be adjustable by way of the two variable resistors which control frequency and duty cycle as described on this handy 555 calculator webpage.
The right side is the test harness which simply drops the peak voltage seen by the capacitor and oscilloscope. The back to back diodes bleed off any DC charge on the capacitor to avoid overvoltage reaching the scope or 555 IC.

Prototype

I prototyped a very simple version of the circuit on a breadboard and tested a few of my recovered capacitors (below middle). I did find a shorted capacitor (lower left), interestingly enough. I also saw the effect of a small capacitor integrating the square wave, resulting in a sawtooth wave (lower right).

Board Layout and Enclosure

Laying out the board without knowing where the board will be install is kind of a time waster. Eventually I may learn this lesson and, next time ...

1. Rough out the layout enough to get a sense of the board dimensions,
2. Select the battery source to add to the required enclosure dimensions, and
3. Find an appropriate box that can fit the board and battery.

In my case I did most of the board layout, then selected a Hammond 1593P enclosure (the one I used for my Mini Function Generator project and pictured above right. Source: Hammond website).

The datasheet for the enclosure shows the board dimensions and mounting hole placement and with a little massaging the board layout was done.

From the function generator project, I learned to leave room inside the box for switches hanging down from the top of the case.

So I left a large section at the front of the enclosure unpopulated by components so I could mount the on/off toggle and an toggle to select testing of high or low ESR caps at the front top of the case. The BNC connector and the capacitor probes will exit the front panel of the box.

Then I thought it might be helpful to see if I could render the board in 3d and ran across Eagle 3d, an Eagle ULP script, which generates a POV-Ray file, pictured at the beginning of the article.

Installation was easy for both Eagle 3d as well as POV-Ray. To get the rendering to work I had to copy the include files (etc) out of the Eagle 3d's Eagle\ulp\Eagle3D\povray directory into the My Documents\POV-ray\v3.6\includes directory so POV-ray could properly include the files. It probably wouldn't hurt to copy the Eagle 3d ulp script into Eagle's ulp directory.

Rendering only took a few seconds even on this old 1GHz P4 machine.

Parts Purchase

Lately I've found it is a giant pain in the butt to buy components for these projects, especially the oddball ones. While I have a stash of most of the components required for this project, a few things need purchasing. I thought I'd play around with the Bill Of Materials script in Eagle. Simply run the bom.ulp script and follow the dialog prompts to save a txt file.

I imported this file into OpenOffice Calc using fixed width separation and added a left column to indicate the items I needed to buy: a couple of switches that will fit in the 17mm vertical space between the top of the board on the bottom of the enclosure, the enclosure itself, some power resistors, and probe wires (possibly also banana jacks if I use removable probe wires, and a trim potentiometer in a size I don't have.

Been a bit busy lately so will continue this project at some later date...

Edward Isaac Bot: Sketchbook 5

The weird third eye thing on this alternate sketch of EIB is supposed to be a Polaroid sonar transducer. But much has changed in the last 15 years in the way of robotic rangefinders.

Pokey uses Sharp GPD12 infrared rangers (right) which provide surprisingly accurate results. They determine distance based on the angle of reflection. They're cheap, only $12 each, and they're small, maybe 1.5" x 1/2".

There is a whole family of IR rangefinders, each with a unique (but somewhat limited) distance range. The GPD12's 4"-30" range seemed ideal for measuring distances within the firefighting maze in which Pokey competed.

Modern sonar systems are significantly smaller and also inexpensive, about $30 for something like the Devantech SRF04 Ranger (right). The device is about 2" x 3/4" and includes the driver circuitry on the back of the board.

This particular model uses a separate transducer and receiver. However, you can also get single transducer/receiver models. This device can measure distances from 3cm to 3m!

Of course in the 1990's, tiny cameras with any kind of image processing fell into the realm of impossible or unaffordable.

The CMUcam2+ (left) is able to do high contrast color tracking, all in a 2.25" square package. How times have changed!

There are a number of sources for this gear. I got a sonar ranger from Radio Shack when they were having a sale. I got my Sharp rangers from Acroname (the source of the pictures and info above; no advertising affiliation with the company, I just like 'em).

This flying robot from MIT is just ... wow.

I wonder what laser scanner they use??

Slowing Down

I have a lot of irons in the fire and up to various stuff that I can squeeze in between the time when baby girl goes to sleep and when I do. Or occasionally during a lunch hour.

I'm in the midst of several electronics projects: IR repeater, oscilloscope calibrator, ESR test harness, ideas for a mini robot made of audio parts. You'll see posts on these in the ensuing weeks and months.

Meanwhile I've been fixing a vintage receiver with some help. I'm helping out someone in South Africa diagnose and repair an oscilloscope. And I've got a stack of non-functioning audio gear I want to work on, like an all-tube Fisher 400, a Yamaha CR-2020, and much more.

I've also got some prep work to do for hunting season, some work to do on the Jeep (both stories for my other blog). And my hobby fund needs to fatten up a bit...

So I need to slow the pace of posting here to give myself a break. Most likely you'll see articles every other week instead of weekly, at least for awhile.