July 31, 2012
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Resistance is Futile!
A continuation in our Component Selection
series
Many times when we see plans for a good project we think
"Hey, I'd like to make this project but ..."
we don't have the exact parts called for. But if we dig around in the junk
box for parts we do have
we can often use them just fine by altering the project specs or schematic
accordingly.
This week's program focuses on ... The Ubiquitous Resistor!
Overview
So you think you know all about resistors, eh? Well, join us here to see if we've covered all the situations for selecting the correct resistor for RF, DC, audio , power, size, SMT or leaded, package, vendors and more. Also, we dissect the two circuits presented last time (oscillator and pi attenuator pad) to see what would be the selection criteria for various components.
73, George N2APB & Joe N2CX
Audio Recording ... (Listen to the MP3 podcast)
Discussion Notes:
<20:01:38> "Joe N2CX": whiteboard link
http://dl.dropbox.com/u/43021514/CWTD/Jul%2031.html
<20:25:24> "Alan W2AEW": for 5%, and even 1%, trimming usually isn't done - but
it is common for tighter values
<20:26:35> "Alan W2AEW": Are you going to mention the size designations for SMT?
<20:29:48> "Alan W2AEW": it still depends on the manufacturer, but some produce
1% w/o trimming, but cost more than 5%
<20:31:19> "George - N2APB": Even though I like homebrewing with SMT components,
I prefer not to go any lower than 0805 packages.
<20:31:56> "Alan W2AEW": Agreed - 0603 is OK, but hand soldering og 0402 is
tricky w/o proper equipment.
<20:33:12> "Alan W2AEW": tee-pee
<20:33:22> "Terry WB4JFI": Yes, 0603 are OK for me too. Just don't sneeze.
<20:33:37> "Joe N2CX": I thought that was a medical condition...
<20:34:00> "Charles WC5MC": oh yeh and put them upside down on tape to hold them
and solder in middle to join is another technique
<20:34:16> "Alan W2AEW": esp easy to fracture terminations on multilayer ceramic
caps
<20:38:55> "Alan W2AEW": For SMT - the copper traces on the PCB are the heat
sink
<20:40:36> "George - N2APB": Yep, and as a layout designer, you can allocate
more copper pad area and trace width to handle more heat, if necessary
<20:43:03> "Pete - WB2QLL": I have one of those IR thermometers, have for years,
works great for troubleshooting.
<20:43:27> "Alan W2AEW": Harbor Freight has one for about $25
<20:44:03> "Pete - WB2QLL": Yes, works great to check heating of regulators,
power transistors, etc.
<20:44:32> "Pete - WB2QLL": You can see if the things are matched, gotta try it
with power tubes, too.
<20:47:24> "Charles WC5MC": Kees Talon
<21:04:20> "Frank N3PUU": a negative resistor? is that slang for a battery?
<21:04:39> "Alan W2AEW": tunnel diode! (gm really isn't negative resistance...
but you already know that)
<21:10:56> "Alan W2AEW": Sounds like someone designed this thing with a 1%
resistor kit on hand, and didn't put much thought into dialing back to 5%
standard values
<21:11:13> "Ken - VA3KMD": I think the standard list of values is a logarithmic
series.
<21:11:45> "George - N2APB": Hmmm ... Never really thought about it befopre Ken.
Perhaps you're right.
<21:16:50> "Alan W2AEW": extra bands for tight tolerance, and sometimes for
tempco
<21:17:03> "Ken - VA3KMD": Re: colour codes: 10% of males are colourblind - like
me!
<21:17:56> "Alan W2AEW": I like the status
<21:18:25> "Terry WB4JFI": The project status is a great add!
<21:18:33> "Alan W2AEW": hmmm - 63.2%?
<21:18:37> "Charles WC5MC": 63.2% ! :D
<21:19:02> "Joe N2CX": 63.2 is a standard value...
<21:21:15> "Alan W2AEW": yeah - I'm looking forward to the 10MHz ref unit!
<21:22:42> "Frank N3PUU": thanks guys, great as always!
<21:23:27> "Ray K2ULR": 73
<21:26:13> "Charles WC5MC": another good ESR meter circuit:
http://ludens.cl/Electron/esr/esr.html
<21:26:32> "Alan W2AEW":
http://www.youtube.com/watch?v=bmYAgat-sOQ
<21:27:06> "Alan W2AEW": could be dissipation factor (DF)
<21:27:29> "Alan W2AEW": and it would be a good measure of ESR
<21:30:05> "Alan W2AEW": he called it "mandatory listening"
SESSION NOTES
So many to choose from ... which is right for my application? ...
Carbon composition
OT's remember these as Allen Bradley resistors used in boatanchors
Power ratings 1/10 watt thru 2 Watt
General purpose up to 10% tolerance
Good for RF, audio, DC
Drift with temperature
Resistance increases with age up to 100%+
Carbon film
Modern replacement for carbon composition
Often brown or gray body color
Power ratings 1/10 watt thru 1 Watt
General purpose up to 10% tolerance
Good for RF, audio, DC (can be electrically noisy)
Reasonable temperature drift
Inexpensive
Metal film
Power ratings 1/10 watt thru 2 Watt
General purpose 0.1 to 5% tolerance
Good for RF, audio, DC (some manufacturers use spiral trim adding inductance)
Low temperature drift
More expensive than carbon film but not excessive
Surface mount and specialty
Power ratings from 0.1W to 1+ wartts (with heat sinking)
Generally metal film or bulk materials
General purpose 0.1 to 5% tolerance
RF, Audio DC ... Very low stray inductance
Inexpensive but smaller sizes tough for hand assembly
20W surface mount resistors!
Resistor network in an IC-like package
SMT resistor packaging
SMT sizes
Wirewound power resistors
Power ratings above a watt
Looser tolerances
Some adjustable
AC and DC applications
Too much inductance for RF
Though some are touted as ”non-inductive”
Bulk power resistors
Conductive ceramic tubes
Power ratings up to 100's of watts
Relatively low inductance
Used for dummy loads and high power applications
Specialty power resistors
Convenient way to mount a TO-220 packaged resistor for good heat dissipation
These resistors are RF Power Components (now Anaren) pn 400-50R resistors that are rated 50 ohms @ 400W up to 500MHz
ANALYZE THIS!!!
Let's go through these schematics here with a "component checklist" in mind ...
Component Selection Factors
Resistors
RF or low frequency?
Precision values needed?
Temperature stability?
Power dissipation?
Size?
Capacitors
Operating frequency?
VFO or tuned circuit? (temp. stability)
Bypass or coupling?
Operating voltage?
DC power filter?
High RF current (as in xmtr or xmit filter)?
Transistors
Operating frequency – DC, audio, RF, VHF or higher?
Operating power level?
Low noise?
Operating impedance?
Operating DC voltage?
Special use?
Phototransistor
Other?
Diodes
RF detector?
DC bias or switch?
Operating voltage?
Operating current?
Special purpose?
PIN for RF switch
Voltage variable capacitor
Photodiode
LED
Toroids
Operating frequency – DC, audio, RF, VHF or higher?
Operating power level?
Iron core or Ferrite?
Core diameter?
Permeability?
Inductor or Transformer?
Bi-filar?
Voltage Regulator
Packaging?
Power/current capability?
Adjustable?
Bypassing?
Location, location, location?
Grounding
Low drop out?
ANALYZE THIS TOO!
This will soon be a CWTD Project!
(See the NA5N pdf file on "Step Attenuator" in the References sectionPower Dissipation in a Pi pad Resistive Attenuator
Atten. R1 R2 R3 1 dB 5.8% 10.2% 4.6% 3 dB 17.1% 24.2% 8.6% 6 dB 33.2% 33.2% 8.3%
10 dB 51.8% 32.9% 5.2% 20 dB 81.8% 16.4% 0.8%
AQRP Group's "Damn Reliable Attenuator" ... http://www.qsl.net/k/k5bcq/Kits/Kits.html
REFERENCES
-
Kits and Parts toroid info: http://www.kitsandparts.com/toroids.php ... Click on top page links for toroid turns calculator, toroid core spec page etc.
-
GQRP Technical pages: http://www.gqrp.com/tech.htm
-
NA5N Handyman's Guide to Capacitors ... http://www.gqrp.com/na5ndatasheet.pdf
-
W7ZOI 8640Jr oscillator, http://w7zoi.net/testgear/8640jr.pdf “A Step Attenuator You Can Build”, Shriner WA0UZO, Page N1FB, QST Sept, 1982 pp. 11-13
-
“ Building VHF Power Attenuators”, Paul Wade N1BWT, QEX April, 1994 pp 29-29
-
NA5N step attenuator good to 1 GHz <www.qrpedia.com/sites/default/files/STEP_ATTEN_1.pdf>
-
QRP Pops Attenuators <http://www.qrp.pops.net/RF-workbench-2.asp>
-
Handy collection of links for collecting parts of every size, shape and age! ...