(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.
Overview
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 N2CXAudio 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
http://www.kickstarter.com/projects/920064946/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
http://www.youtube.com/watch?annotation_id=annotation_2267343187&feature=iv&src_vid=Ob_E91VUQZU&v=-frBfOsSOR0
♫ 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
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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)
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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
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Suitable for 1 to 1 Megohm
Harbor Freight, Radio Shack
Milliohmmeter
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
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http://www.monarchcapacitors.com/
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?
Notice f(delta-f) (in spreadsheet)
Link to AA5TB site ... http://www.aa5tb.com/loop.html
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
Parts List:
Rheostat: Radio Shack p/n 271-265 ... 25-ohms, 3W
15-ohm, 1/2W
LM317 ... Digi-Key p/n LM317TFS-ND ... http://www.digikey.com/product-detail/en/LM317T/LM317TFS-ND/458678
5-way binding posts
4-cell AA batteries & holder
Let's also build the Audible Continuity Tester
piezo buzzer with battery ==> go / no-go
or
Audio Oscillator with varying tone