(Now a bi-weekly program)

November 12, 2013

The Art and Science of Evaluating Electrical Connections
From the simple cases, to the specialized ones, with a couple of simple-yet-effective instruments you can build for the bench.



So you think you know about making resistance measurements, eh?  Just take the ol' $2.99 "special" VOM you got from Harbor Freight or Best Buy, probe that connection, and voila! 

Well, sometimes it's as easy as that, but things can get tricky if you need to measure accurately at the very low end of the range ... say in the milliohm region (i.e., 10 milliohms = 0.010 ohm).  That inexpensive VOM probably won't hack it, and even some DMMs run out of gas down there will not be good enough for use.

So our topic this time is making resistance measurements in those hard to reach areas ... And we'll present a couple of simple-yet-effective instruments that you can build to do it with.

73, George N2APB & Joe N2CX

Audio Recording ... (Click to download or listen to podcast)

Discussion Notes:

<19:58:15> "Clint-ka7oei": http://www.sureelectronics.net/category.php?id=169
<19:59:21> "Clint-ka7oei": The aforementioned device goes both ways on BlueTooth...
<19:59:37> "Clint-ka7oei": This one only receives BT and sends out audio: http://www.sureelectronics.net/goods.php?id=2355
<20:04:23> "Joe N2CX": BTW Bluetooth module that Clint found is a small pcb tht only costs $13.90
<20:05:16> "Terry WB4JFI": Gabriel.
<20:35:01> "Al - N8WQ": lol
<20:39:13> "Al - N8WQ": Helper was there last name, lol
<20:41:10> "Terry WB4JFI": Bonnie Meadow Rd
<20:41:20> "Terry WB4JFI": meow
<20:41:57> "George - N2APB": awwww geez ... you must be a Dick Van Dyke afficionado Terry!!!
<20:42:14> "Terry WB4JFI": rings a bell for some reason
<20:45:37> "Ray K2ULR": Hi Gang: Can't stay in.... poor net connection here in Savannah. 73 de Ray
<20:45:49> "George - N2APB": 73 Ray
<20:48:58> "Terry WB4JFI": harbor freight just had a coupon for a free DMM, normally about $5.
<21:06:10> "Clint-ka7oei": Kelvin
<21:28:51> "Armand WA1UQO": Thanks Joe & George, as always, learn something each time.
<21:29:21> "Terry WB4JFI": another fun show. Resistance threw me for a loop, I had butterflies thinking about it.
<21:29:58> "Joe N2CX": Resistance is inevitable!

CWTD New Product Watch

Here are a few new products that have recently come to our attention ...

From Gabotronix ... The Oscilloscope Watch



Jambox from Jawbone
... A bluetooth-connected amplified audio speaker & speakerphone https://jawbone.com/speakers

Or how about building one yourself?  Here's a very cool project from DIYPerks

  Ohm, Ohm on the Range  

or ... "Ohm My Goodness this is low!"

A very wise man I know once said:

    ... "90% of all problems in electronics deal with connections"


Type of Connections

  • Connectors



  • Relays



  • Broken wires (cables, ...)



  • Terminal strips



  • Solder connections



  • PCB traces

  • Mechanical/physical joints


Flow Charting the Obvious


Continuity Testing with an Electro Mechanical Buzzer (or LED)

  • Standard old way

  • Generates higher voltages

  • Higher current needed

Audible Continuity Tester

  • Modern equivalent of the Buzz box

  • Joe's Quickie #30 ... Quick Tone Oscillator



  • Changing tone ==> Changing resistance

Digital Multi-Meter (DMM, Volt-Ohm Meter (VOM


  • Suitable for 1 to 1 Megohm

  • Harbor Freight, Radio Shack


  • For ranges 1 ohm to 1 milliohm (0.001 ohm = 1/1000th ohm)

  • R = E / I  ==>  Use a Voltmeter sensitive enough to measure low voltages

  • So ... try using 100 ma through device/connection/etc, the calculate ohms by R = E / I

  • With 100 ma ...

    .001 ohms becomes .0001V

    .01 ohms becomes .001 V

    1 ohm  becomes 0.1 V

    10 ohms becomes 1 V

  • Could use higher current to ease the voltage measurement, but ....

When do you need to measure milliohms?

  • Switch & relay contacts -- Rated in terms of 10's or 100's of milliohms

  • Multi-layer rotary switches used in boatanchors

  • Coax switches & relays

  • Pressing mechanical connections to pcb -- like MFJ SO-239 ... high resistance ground

  • Mag loop antennas

        cap connections -- butterfly caps, vacuum variable caps




  •     Conduction compound

Why is Low R needed with Mag Loops?

  • Rr, Rloss

  • Physical connections R

  • Efficiency = Rr / (Rr + Rloss)

  • Rr is usually pretty low, so need to reduce losses ... e.g., increase dia (more surface area) to reduce Rloss


So ... Use a Milliohmmeter!!

  • What is an "okay" Rloss for a loop?

& spreadsheet ... http://www.google.com/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=1&ved=0CCkQFjAA&url=http%3A%2F%2Fwww.aa5tb.com%2Faa5tb_loop_v1.22a.xls&ei=eMeCUvPZJLTZsAS3pYK4Bg&usg=AFQjCNHUoQi8KDzG1Qrpa4XPRQlEu15jhA&bvm=bv.56343320,bs.1,d.cWc


  • Connections to Connection-Under-Test (connection-wise!)

Measure I, Measure V, Computer R

Could Use a Constant Current Source

  • Set to 100 ma (for example)

    .001 ohms becomes .0001V

    .01 ohms becomes .001 V

    1 ohm  becomes 0.1 V

    10 ohms becomes 1 V

Let's Make a Practical Milliohmmeter ... With readily available parts

Let's also build the Audible Continuity Tester

  • piezo buzzer with battery ==> go / no-go


  • Audio Oscillator with varying tone