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9th January 2013, 11:06 AM  #31 
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Join Date: May 2012
Location: Denmark

I just noticed a Vandal Resistant Push Button Switches on the page, dont use it as power switch, it is only rated to 48VDC
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Regards Max 
9th January 2013, 05:19 PM  #32 
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Join Date: Dec 2012
Location: SF Bay Area

Oh, a pair of resistors ... to do the work of "one" ... has a lot of utility! It is certainly the cheapest way to get higherwattage dissipation tolerance, if space and layout is of little concern. Seriesconnected resistors are also quite a bit more tolerant to overvoltages (which for most amplifiers isn't really a concern). Did I mention "cheaper"? Its usually true: a pair of 1/2W resistors is usually less expensive than a 1W. Oh yes ... there's also increaseinprecision: the variance from stampedon value tends to average by the number of resistors placed in parallel (or series). The increase in precision beats speciallyvetted highprecision resistors any day (at least when manufacturing the things). Its also possible to use DIFFERENT resistors  with different temperature coefficients, to cancel each other's drift. Did that all the time up at LBL. Hmm... and cheaper.
Of course, the mostusual reason of all for using a pair or trio of resistors is to obtain a hardertofind resistance value with offtheshelfinyourlab candidates. That too saves money. And time. And frustration when the wrong part arrives. GoatGuy 
9th January 2013, 06:06 PM  #33  
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Join Date: May 2007

Quote:


9th January 2013, 10:48 PM  #34 
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Join Date: May 2010
Location: was Chicago IL, now Long Beach CA

Ah, but it can be very important for consistency rather than accuracy of stated value, for instance in push/pull or balanced circuits. That's why the Jensen discrete op amp uses an integrated circuit with massively paralleled transistors...the resulting characteristics are statistically averaged and achieves precision consistency so any two production items are perfectly matched. In theory...

9th January 2013, 11:43 PM  #35 
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Join Date: Oct 2012

Just been doing a bit of maths, combining normal distributions with the paralleling resistor formula.
I make it two 1% sd paralleled resistors of equal value become a 1.6% sd resistor, so paralleling is bad, not good. The error gets smaller, but not by as much as the resistance gets smaller, so comparatively the error is bigger. However, combining series resistors does improve things. Two 1% resistors become a 0.7% resistor. Of course I could have royally messed up here. Last edited by Robert Kesh; 9th January 2013 at 11:55 PM. 
10th January 2013, 10:16 AM  #36 
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Join Date: May 2007

Massively parallel transistors are almost always for low noise, not consistency. BJTs are very consistent in following the EbersMoll exponential model.
Paralleling resistors should have the same effect on error as putting them in series, as you are just adding the conductances instead of the resistances. 
10th January 2013, 10:49 AM  #37  
diyAudio Member
Join Date: Oct 2012

Quote:
for adding, multiplying and dividing standard deviations see. http://www.cartage.org.lb/en/themes/...ubtraction.htm Multiplication and Division of Values with Standard Deviation Last edited by Robert Kesh; 10th January 2013 at 10:54 AM. 

10th January 2013, 11:55 AM  #38 
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Join Date: May 2007

If you have 10% resistors then it is better to buy better resistors.
If you have 1% resistors then +1% gives +1.01/0.99% which is close enough for the Central Limit Theorem to work its magic. 
10th January 2013, 12:20 PM  #39  
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Join Date: Oct 2012

Quote:
CLT is for the arithmetic mean of many samples, and so works for their sum too, which is why it works for series resistors, hence the 1/sqrt(2) in my first post on this, sd will reduced by a factor of 1/sqrt(n) for n similar resistors. Paralleling resistors is related to the harmonic mean. I am not convinced it gives a similar result to the CLT. Though I confess I did the numbers when I was very tired. 

10th January 2013, 12:43 PM  #40 
diyAudio Member
Join Date: Mar 2004
Location: Budapest, Hungary

I see only one reason to connect resistors parallel in opposite direction: the stamped value (or the color code) can be read from either side...

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