parallel resistors

Dunnsept

I'm just finishing up WAM and have gone through some of the·other materials,
·and have a general question. I hope this is an appropriate place for it.

Every time I see a transistor used as an amp, or trigger or whatever, the schematic shows 2 resistors in parallel on the base.
like:


··········· ---R1---··································/ C
·········· |·········· |······························ /
input ---············

---

Base of trans--|
·········· |·········· |······························ \
··········· ---R2----·································\ E

(oooh, ascii art circuits)

if each of R1 and R2 are say 100k ohms, why put them in parallel instead of just a 50kohm
and call it good?· Do the parallel resistors have some other function in this circuit and if
so, could somebody please be so kind as to clue me in?· ;-)

It's been a long time since I've done anything with circuits and I guess I never truly learned it,
since I have now forgotten it all. (an old teacher of mine said that if we truly learned the material,
we would not forget it)

thanks!
paul...

·

Shawn Lowe

As far as I know, there is no other purpose other than making the resistance you get with the resistors in parallel.

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Shawn Lowe


Remember - No matter where you go
There you are.

Dunnsept

That's what I thought. my boss was an AT in the navy, I was an EM, neither of us could think of any reason for
having them in parallel. But there must be SOME reason, why else would it be done? except maybe to increase
resistor sales

Bruce Bates

Paul -

I'll give you a (rather specious) reason, but I'm not any more pleased with it than you may be. Simply put, it may just be a matter of economics, as it reduces the total number of resistors that need to be supplied to perform all the various experiments, when they can do "triple duty" (singly, in series or in parallel) <sigh>.

I, for one, would be happy to contribute to the "More Resistors Fund" by sending along a few thousand resistors, if this proves to be the actual case, but I can't in all honestly believe that it is, however <shrug>. Just my best guess, in any case.

Regards,

Bruce Bates

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Dunnsept

thanks. I've been searching the web for an answer, and it's difficult to sort through the 100's of 1000's of pages that come up.
I can only think that there must be some reason for it, biasing or who knows what. otherwise, you would just put a (in the above example)
50k resistor in place and be done.
now, if you want to donate parts to my learning effort, I'm all for it
I could really use a new computer at home AND a Pro development board, wouldn't mind moving to SX chips either

Chris Savage

Paul,

·· In my over 25 years in electronics I myself have never seen two resistors on the base of a transistor, but I couldn't help wonder if it had to do with current consumption.· Maybe instead of using a 1/2W 50K they used two 1/4W 100K?· Sounds weird but...I guess knowing how much current/voltage we were talking might help decide that.

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Chris Savage
Parallax Tech Support
csavage@parallax.com

Robert Kubichek

Dunnsept said...
I'm just finishing up WAM and have gone through some of the other materials,
and have a general question. I hope this is an appropriate place for it.



Every time I see a transistor used as an amp, or trigger or whatever, the schematic shows 2 resistors in parallel on the base.

like:





---R1--- / C

| | /

input ---

---

Base of trans--|

| | \

---R2---- \ E



(oooh, ascii art circuits)



if each of R1 and R2 are say 100k ohms, why put them in parallel instead of just a 50kohm

and call it good? Do the parallel resistors have some other function in this circuit and if

so, could somebody please be so kind as to clue me in?



It's been a long time since I've done anything with circuits and I guess I never truly learned it,

since I have now forgotten it all. (an old teacher of mine said that if we truly learned the material,

we would not forget it)



thanks!

paul...

It also could be that using 2 resistors in parallel the final resistance is closer to the target resistance, due to the
accuracy of the resistors 1% 5% 10% ect.

A 100K resistor is a common value, and is used more than a 50k resistor.


Bob N9LVU

Chris Savage

Bob,

·· That is true, but then, depending on the circuit, I am surprised to see such a high value on the base of a transistor in the first place.· I guess·we may never know...Unless I can find a setup like this in some circuit and see what's going on.· Reminds me of the good ol' days of reverse engineering.

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Chris Savage
Parallax Tech Support
csavage@parallax.com

Bruce Bates

Chris -

Perhaps you missed part of the point here. This set-up is used in the ACTAUL WAM text EXAMPLES! I checked for myself, and indeed it appears just as presented by Paul!

Who wrote WAM, he should have a reasonable answer?

Regards,

Bruce Bates

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Martin Hebel

A 50K Ohm resistor does not commonly exist. They come in really odd sizes, 47K, 56K, etc.

By putting 2 common 100K's in Parallel, we can get a nice round value needed of 50K.

-Martin

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Martin Hebel
Perform an Employer's Survey of Electronic Technologies Graduates· - Click here!
Personal Links with plenty of BASIC Stamp info
and SelmaWare Solutions - StampPlot - Graphical Data Acquisition and Control

Dunnsept

thanks guys, I guess since no 50k exists, but a 47 does, it would make sense then to have it that way. But with the overlap because
of the tolerance, is it that critical? with 5% tolerance a 100k could be from 95 to 105, and assuming we're at the extremes would put our
total resistance at like 49.8, whilst a 47k could be 49.3, altho it could also be 44.65. I dunno, are things _that_ critical?

sorry if I seem too questioning, just trying to learn this stuff.

thanks again
Paul...
(take my calculations with a HUGE grain of salt, no accuracy implied or suggested)

Martin Hebel

I think it was simply a matter of making the math easy for the end user. I'm sure Andy will shime in soon.

-Martin

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Martin Hebel
Perform an Employer's Survey of Electronic Technologies Graduates· - Click here!
Personal Links with plenty of BASIC Stamp info
and SelmaWare Solutions - StampPlot - Graphical Data Acquisition and Control

Beau Schwabe

Bob,

From an IC layout perspective, the technique you describe is used, more so to keep a certain level of consistency throughout any process variations.
With a unitary value resistor, you can either add more resistors in series or add more in parallel to get your desired resistance value. A typical resistor
structure looks like an array, with "dummy" resistor structures around the perimeter or edge. When trying to accurately match resistors, "dummy" structures
are important for a couple reasons. One, is so that ALL of the active resistors "see" the same structure as their neighbor. Another reason is for planarization.
purposes. The best way I can describe this is to imagine a drop of water on a waxed surface.... around the edges you have a curving effect which can
significantly alter the desired value of the component, so you place "dummy" devices in this region. ...across the middle and top, you still have a curving
effect, but it's not as severe as it is along the edges.

Martin Hebel said...

A 50K Ohm resistor does not commonly exist. They come in really odd sizes, 47K, 56K, etc.

The odd size I believe migrates from the manufacturing process... We as humans are analog
by nature, but when it comes to building something, a grid of some sort seems to make sense.

So... during the processing , the ohms/square unit is applied in a grid fashion. Unfortunately
the resultant ohms/square does not come out to be an exact figure as we would expect.
So the "odd-ball" sizes actually make sense from an ease of manufacturing point of view.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 3/6/2006 8:53:58 PM GMT

Shawn Lowe

Dunnsept-

Is this circuit used in the WAM coursework? If so, what resistor values does the text call for?



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Shawn Lowe


Remember - No matter where you go
There you are.

Dunnsept

grabbed the WAM.. v2.2·· page 266 for the introduction of transistors. it has exactly what I was talking about. the 10k pot between Vdd and Vss is connected to 2 100k resistors in parallel. Since this is 'showing' 50k ohm, my initial question still remains. why in parallel?
I can't imagine that it is so sensitive that it could not be replaced with a single 47k resistor since it is being fed by a pot in the first place.
Or am I really missing something on what the parallel resistors do?

Post Edited (Dunnsept) : 3/6/2006 7:16:19 PM GMT

Chris Savage

Bruce Bates said...(trimmed)
Perhaps you missed part of the point here. This set-up is used in the ACTAUL WAM text EXAMPLES! I checked for myself, and indeed it appears just as presented by Paul!
Who wrote WAM, he should have a reasonable answer?

Bruce,

·· You are correct...I have skimmed through WAM to get a handle on what is included for guidance to other students, etc. but am well past the content and as such have not read it through to completion.· Andy Lindsay is the author and I have e-mailed him and asked him to join this discussion.

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Chris Savage
Parallax Tech Support
csavage@parallax.com

RDL2004

Kind of like what Bruce said above is it possible the WaM kit comes with 100k resistors, but no 47k?


Edit: Well, the WaM component list I found didn't show any 47k resistors, but on the other hand, it didn't list any 100k either.

·- Rick

Post Edited (RDL2004) : 3/6/2006 8:46:09 PM GMT

Dunnsept

came with a whole mess of 100k resistors. But I've seen this is many other schematics online too. Like for a darlington transistor
amp that I built. It too had the 2x 100k resistors in parallel. So this must be some sort of design issue. One of the manuals I have for electronics from the navy shows the same thing, but does not explain why.

thanks to everybody for trying to help me out.

Andy Lindsay (Parallax)

Dunnsept said...
why in parallel?
I can't imagine that it is so sensitive that it could not be replaced with a single 47k resistor since it is being fed by a pot in the first place.
Or am I really missing something on what the parallel resistors do?<!-- Edit -->

The person who processes my new part requests had to fill more than just her own chair for a while right as I was putting the finishing touches on the What's a Microcontroller parts kit.· Delaying the product release for the 47 k was also not an option.· Fortunately, we did already have 100 k resistors on the shelf.·

Little did I know how that choice would come home to roost.· Next time What's a Microcontroller comes up for revision, I'll add a box about parallel resistor calculations along with 100 k || 100 k = 50 k calculations and a side note about a 47 k being close enough.·

Dunnsept

Andy, I wasn't trying to 'slam' the book or anything. I was just wondering if there was another reason for having it that way. If it truly is because you need 50k for the input of the transistor and this is how you get there, that's all I wanted to know. I would then assume that it's because of a design issue of the transistor, because every circuit I have seen uses the same setup, 2x100k in parallel. hence, my statement about the "some sort of design issue"
Your book is excellent and I learned a great deal very quickly, please do not take offense to my questioning.

thanks to everyone who posted too

Bruce Bates

Andy -

As far as answers go, that was the least expected, and MOST HONEST answer you could have ever offered. It now makes perfect sense, regardless of US noisy roosters!

Regards,

Bruce Bates

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Andy Lindsay (Parallax)

Dunnsept said...
I wasn't trying to 'slam' the book or anything...please do not take offense to my questioning.

Of course you weren't, and no offense taken.· Your line of questioning was perfectly reasonable, as was the way you presented it.· Also, the answer was not in the book, so the Stamps in Class forum is the next place we hoped you would use to get your questions answered.

Aside from being an·additional resource for people like yourself who are working through Stamps in Class texts, this forum is also·an important resource for us (the Parallax Education team).··Questions like yours help us improve future revisions of our books, so thanks for chiming-in.··We needed to know about that missing piece of information.

...Your book is excellent and I learned a great deal very quickly...

On the other hand, I might just have to take that one personally.Thanks for saying so.· Hopefully, it'll make a good launch pad into whatever you want to try next.

Best regards,

Andy