MOTM

--- In motm@yahoogroups.com, "object88" <object88@y...> wrote:
> OK, I need to finally get off my fattening arse and order up some
> parts from Mouser / Digikey to complete some work.  But there are
> questions.
> 
> 1) Resisters.  Seems like a no-brainer, but the someone with
> half-a-brain comes along and mucks things up.  What's the difference
> between carbon comp, carbon film, metal film, and metal oxide, aside
> from price?  What's supplied with the kits?  And should I ever need
> 1/2W, or is 1/4W sufficient?  I plan on getting a kit w/ a variety 
of
> values-- I assume something along the lines of 1.1K-100K
>        <snip snip>

Q: What's the difference between carbon comp, carbon film, metal 
film, and metal oxide, aside from price?
A: <wordy response follows>

Carbon composition resistors are comprised of a carbon-based solid 
core into which "end caps" (with the wire leads) are molded into the 
resistor body.  The exact chemical composition will determine the 
exact value of the resistor.  These are the older resistors you would 
normally find in vacuum tube applications and other "vintage" 
equipment.  They can be had most commonly in tolerances of 5%, 10% 
and 20%.  They are more expensive than other type of resistors and 
are not commonly used for most applications these days.  However, 
they are used in applications where there is a momentary power 
overload.  Other resistor types are more fragile in that respect, so 
some applications demand a carbon composition resistor.  They are not 
terribly temperature stable and they do not age gracefully... their 
values can drift wildly over many years.  I cannot think of a single 
reason to use a carbon composition resistor in a synth.  Resistors 
with power dissipation capacities from 1/32 up to 5 watts were 
available.  (Others should feel free to point out any application I 
might have not considered!)

Carbon film resistors are comprised of a "ceramic" (non-conducting) 
core, onto which a resistance film is deposited on the surface of the 
core.  End caps are crimped onto the core much like a carbon 
composition resistor.  The whole resistor is then coated with a 
protective covering.  Various chemical mixes determine the final 
resistance value, when the resistance mixture is deposited onto the 
core in a film coating.  Most carbon film resistors are 5% tolerance 
these days although one can still find 10% tolerance units.  These 
are the most common resistors found in synth module kits where 
somewhat imprecise values are suitable.  They are reasonably stable 
over temperature and time.  One can commonly get values up to 22 
megohms.  Resistors with power dissipation capacities from 1/16 up to 
2 watts are commonly available.

Metal film resistors are comprised of a "ceramic" (non-conducting) 
core, onto which a resistance film is deposited on the surface of the 
core.  End caps are crimped onto the core much like carbon 
composition and carbon film resistors.  The whole resistor is then 
coated with a protective covering.  Various chemical mixes determine 
the final resistance value, when the resistance mixture is deposited 
onto the core in a film coating.  Most metal film resistors are 1% 
tolerance these days and tolerances to 0.01% (and beyond) are 
possible.  These are the most common resistors found in synth module 
kits where fairly precise values are required or temperature 
stability is wanted.  Many 1% metal film resistors have a temperature 
coefficient of 100 PPM.  That is, their value will only vary 100 
parts-per-million per 1 degree Centigrade change in temperature.  
Mouser stocks 1% resistors with only 50 PPM temperature drift at the 
same price that Digi-Key charges for 5% carbon film resistors!  
Needless to say, I don't buy anything except 1% resistors from Mouser 
(these days).  In many cases that is overkill spec-wise, but at only 
2 cents apiece, why buy anything else?  For any DIY undertaking, buy 
Xicon 1% resistors from Mouser and call it a day.  The only "kicker" 
is that metal film resistor values poop out at much above 2.2 
megohms.  Sure, you can get higher values through commercial 
distribution channels (like Arrow) but minimum buys usually apply.  
But if you really need a 0.1% 25 PPM resistor, metal film is the way 
to go.  Resistors with power dissipation capacities from 1/16 up to 
225 watts are commonly available.

Metal oxide resistors... don't recall having seen them in a long 
time.  I think they were a poor man's metal film of sorts.  I seem to 
recall that Corning Glass Works used to make them in like 2% 
tolerance, but they weren't that much cheaper than metal film.  Sorry 
but I don't recall much else.  Resistors with power dissipation 
capacities from 1/8 up to 2 watts were available once upon a time.  
Today, your guess is as good as mine.

Now, the above ramblings refer mostly to leaded, through-hole 
resistors.  Surface-mount components are a whole other ballgame.  
Since most synth modules do not use surface mount components 
extensively, I'll defer to anyone else who'd like to tackle that.  
And, the above is generalized information believed to be reasonably 
accurate but not the whole story.  For specifics, I suggest you 
download the component data sheets for the specific choices you would 
consider.  You'll get much more information like how hard you pull on 
the leads before they separate from the resistor body, and so forth.


Q: What's supplied with the kits?  (I assume you mean MOTM kits, 
right?)
A: Paul supplies exactly what is needed for the specific circuit at 
that point.  Many times, a 1/4W 5% resistor is just fine and dandy.  
Then, you will find 5% carbon film resistors in your kit for those 
specific resistor locations.  But sometimes the circuit requires 
tighter tolerances and Paul will supply 1% metal film resistors for 
those certain locations.  Still, some requirements (like in VCOs) the 
need is for those really temperature stable, high precision, tight 
tolerance, and "don't ask much they cost, you don't want to know" 
resistors.  In those cases, that is what Paul supplies!  That's one 
of the many cool aspects of MOTM... you'll get exactly what is needed.


Q: And should I ever need 1/2W, or is 1/4W sufficient?
A: That depends entirely on what you plan to do!  For 99+% of what I 
do, 1/4W is fine.  (But I did just buy some 25W 1% resistors for a 
load box, so go figure.)


Q: I plan on getting a kit w/ a variety of values-- I assume 
something along the lines of 1.1K-100K
A: The values you need will always be the ones you didn't buy!  
Personally, I'd go from 22 ohms to 2.2 megs.  But I must confess I 
did pop for the 5% carbon film assortment from Digi-Key.  That was 
200 of *every* 5% value of 1/4W resistor up to something like 4.7 
megs.  All 32,000+ resistors took a bit of time to bin, but now I can 
simply grab any value needed.  That shows what kind of parts junkie I 
am.  Did the same for 1/8W resistors too.  (A hard-core parts 
junkie!)  That was either a very smart move or something totally 
moronic.  I still haven't decided which it was.  I would advise that 
if you plan to mostly build kits, just buy the resistors you want 
when you need them.  Most people don't do what I did.  But, Jim 
Williams, a rather talented designer for Linear Technology advises 
doing exactly what I did... and doing it for all 1% resistor values, 
too!  Now THAT is a lot of money to spend on resistors.  The ultimate 
answer depends entirely on how much DIY stuff you plan to build and 
how many digits there are in your gross annual income.

Cheers!
Tom Farrand