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!


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"



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




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



Let's go through these schematics here with a "component checklist" in mind ...

Component Selection Factors



     RF or low frequency?

     Precision  values needed?

     Temperature stability?

     Power dissipation?



     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)?


     Operating frequency – DC, audio, RF, VHF or higher?

     Operating power level?

     Low noise?

     Operating impedance?

     Operating DC voltage?

     Special use?




     RF detector?

     DC bias or switch?

     Operating voltage?

     Operating current?

     Special purpose?

         PIN for RF switch

         Voltage variable capacitor




     Operating frequency – DC, audio, RF, VHF or higher?

     Operating power level?

     Iron core or Ferrite?

     Core diameter?


     Inductor or Transformer?


Voltage Regulator


     Power/current capability?



     Location, location, location?


     Low drop out?




This will soon be a CWTD Project!
(See the NA5N pdf file on "Step Attenuator" in the References section

Power 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%


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



Digi-Key Midnight Design Solutions 
Mouser Electronics JUMA Kits 
Newark Electronics Kanga US 
Farnell Small Wonder Labs
Arrow Electronics Kits and Parts 
Future Electronics QRP Kits
Allied Electronics Dan's Small Parts
Jameco SDR-Kits
FAR Circuits QRPme
Radio Shack Flex-Radio Systems
ByteMark CW Touch Keyer
Texas Instruments Ye Olde Disk Shoppe
Philips Semiconductors EleCraft
Fair Radio Sales EMTECH
Barker & Williamson American Morse
International Crystal MFJ Enterprises
NTE Electronics S&S Engineering
Radio Adventures Oak Hills Research
Vector Electronics Whiterook Products Co.
Harbor Freight Tools Ten-Tec
Lashen Electronics Wilderness Radio
Surplus Sales of Nebraska Milestone Technologies
Radio Daze DIY Electronics
Antique Radio Supply Almost All Digital Electronics
Nationwide Radio & Eq. Vectronics
Parts Express LDG Electronics
Goldmine Electronics Ramsey Electronics
All Electronics Hy Power Antennas
B.G.Micro Amateur Electronic Supply


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