# calculating minimum and maximum resistor resistance

Discussion in 'Homework Help' started by axedrytouch3, May 18, 2011.

1. ### axedrytouch3 Thread Starter New Member

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hi, i'm new to DC circuits, i'm currently taking DC circuit class in college. i know nothing about DC circuits, as of now i'm having trouble solving these stuff on my homework

on my homework they gave an example

22 | Red Red Black Gold | 2 2 0 5%

and it says (5%)(22) = (0.05)(22) = 1.1

max value = 22 +1.1 = 23.1
min value = 22 - 1.1 = 20.9

but it doesn't tell alot since there's not steps or info on what to do first...

so what do i do with these then?
6.8 | blue gray red gold | 6 8 2 5%
100 | red black red gold | 2 0 2 5%
220 | red red brown gold | 2 2 1 5%

if anyone can tell me step by step on what to do i can try to figure out what to do and solve the rest of the problems by myself. thanks in advance =[

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3. ### Georacer Moderator

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The last band tells you about the tolerance of the resistor value. A 5% resistor will have a value that is within a 5% limit of its nominal value.

4. ### t_n_k AAC Fanatic!

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I think axedrytouch3 wants to know how to work out the range of values a resistor might have given its nominal value and tolerance.

The reasoning used is as follows

1. Say you have a nominal 22Ω resistor with 5% tolerance.
2. The actual resistor value could be as low as 22Ω - 5% of 22Ω
3. Or the actual resistor value could be as high as 22Ω + 5% of 22Ω
4. Do the calculation to find that 5% of 22Ω is 1.1Ω
5. The actual resistor value could be as low as 22Ω - 1.1Ω = 20.9Ω
6. The actual resistor value could be as high as 22Ω + 1.1Ω = 23.1Ω

A slightly different way to calculate the range is to state that the possible range is from 95% of 22Ω (or 20.9Ω) to 105% of 22Ω (or 23.1Ω)

5. ### t_n_k AAC Fanatic!

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This question reminded me of a few subtleties associated with the notion of tolerance.

I've never worked in the component manufacturing area but I presume resistors a made in a process whereby every resistor isn't actually checked to make sure it falls within a certain range of values. I guess samples are taken from any particular batch run and routinely tested for compliance.

I assume also that the manufacturing process makes nominal resistor values which are "Normally" distributed. The Normal distribution is assigned a standard deviation σ. In a truly Normally distributed population, 99.7% of all samples would fall within 3 standard deviations of the population mean μ.

Suppose a manufacturer produces 100Ω ±5% resistors. Suppose also that the 100Ω production process is Normally distributed with a currently known σ of 2 ohms. What proportion of manufactured 100Ω resistors produced would fall outside of the stated tolerance? So ±5% is therefore ±2.5σ and 98.76% of produced items would fall within the tolerance range. So the manufacturer knows that 1.24% of all produced 100Ω resistors will likely fall outside of the stated tolerance. Would the manufacturer be satisfied with that situation? If not what could be done?

6. ### axedrytouch3 Thread Starter New Member

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yes i need to find out the min and max for each of the following

resistor(nominal value)| color | 1 2 3 4
6.8 | blue gray red gold | 6 8 2 5%

100 | red black red gold | 2 0 2 5%

220 | red red brown gold | 2 2 1 5%

7. ### t_n_k AAC Fanatic!

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Well I outlined the method in post #4. You should now make an attempt. Don't just expect to get the answers. Post your working and see how things progress from there.

8. ### bigBangTheory New Member

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EXPLANATION
Ok so here how it goes,
You have to look at the resistor color codes on google because every color has a value. But a good way to remember the colors and its value in order is by learning this sentence "Bad Boys Run Over Yellow Grass But Valley Girls Walk"
Boys - Brown - 1
Run - Red - 2 (2 % reliability)
Over - Orange - 3
Yellow - Yellow - 4
Grass - Green - 5
But - Blue - 6
Valley - Violet - 7
Girls - Grey - 8
Walk - White - 9
N/A - Gold - 0.1 (5% reliability)
N/A - Silver - 0.01 (10% reliability)
EXERCISE
1) 22 ohms = red red black gold. Gold has a tolerance of 5%. So 22x(5/100) = 1.1. Max Resistance = 22+1.1 and Min Resistance = 22-1.1
2) 6.8 kilo ohms = blue grey red gold. Gold has a tolerance of 5%. So 6.8k x (5/100) = 340. Max Resistance = 6.8k + 340.
and so on...

9. ### WBahn Moderator

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Actually, resistor manufacturers DO measure every resistor coming off the line. This was especially the case back when production tolerances weren't so good. They measured the resistor and that determined how it got marked. As a result, if you took a bunch of 20% resistors (the least costly and most common resistor back in the day) and measured them what you found was that none of them were outside 20%, but also that none of them were within 10% (because THOSE were marked as either 10% or 5% resistors). There were exceptions, but this was primarily the result of the production quotas for each resistor value and tolerance combination. If they had too many resistors that were between 5% and 10% but were short on 20% resistors, then some were marked as 20% to meet the production quotas.

A different line was usually used for 2% or better because these were "precision" resistors and had other specs that were tighter, too.

It might seem amazing that every resistor was measured, but it was all automated. Consider integrated circuits -- on most lines every single die is tested, even of the simplest SSI gates. Binning is also done. For instance, memory and FPGA and other parts often come in different speed grades. Well, they usually are simply parts coming off the same line and as they are tested if they pass spec for the tighter quality part that is how they are marked but if they don't they are marked as the lower quality part. It wasn't uncommon, particularly as quality control got better, for them to not have enough parts that failed to meet the tighter spec, so they would mark those parts as lower grade parts in order to meet their sales demands. As the production tolerances got better and better, many of these parts stopped being binned altogether.