What kinds of circuits need precise resistors and how is their precision guaranteed?

Answer 1

Precision resistors are used in measurement , callibration ,and other precise and critical instruments .
A resistor has a tolerance around marked quantity , which must be concidered to be better than errors respected in the system .
But an other important aspect is that the precision resistor is also more stable in environmental changes , like temperature and humidity .
The precision is guaranteed by material , construction and technology involved .
1- carbon resistors & polymer resistors 5%
2- metal film 1%
3-metal film & wire wound 0.1%
4-wire wound beter than 0.1%

Answer 2

Let's say you have a 1K and a 3K resistor. If you use them to build a 4:1 divider all is kewl, right?

Wrong!! If the 1K resistor is 10% over (1100Ω) and the 3K resistor is 10% under (2700Ω) then the *exact* division ratio will be
1100/(1100+2700) = .289 which is *not* .25 (as you wanted)

Likewise, if they happen to fall the other way
900/(900+3300) = .214 Again *not* .25 as you wanted.

Is this 'accurate enough'? It depends on the application. Some circuits are so critical that the temperature coefficient (the change in resistance with temperature change) has to be 'matched' for each resistor.

If you want the ultimate in precision and tempco matching, you go to Vishay and buy their very best .001%, 'zero tempco' resistors. And you pay around $70 to $100 each for them ☺


Doug

Answer 3

Precise resisters are needed in testing equipment, critical safety circuits, and anywhere precision consistency between circuit boards are needed. They are also needed in research and development to determine the validity of circuit diagrams and the associated math used to determine the resister values needed in a particular circuit.

The resister is considered precision when it's value is is within +/- 1% of the stated value.

Often times, the precision resisters all come off the same production line as more conventional resisters where special equipment separates them into 1% - 2% - 5% - 10% and so on depending on the testing results.

Answer 4

Ones that do comparisons often need matched sets.

In reality nowadays the manufacturer measures each resistor automatically. If it is within 1% of a number that is wanted the machine marks it as a precision resistor for that number. If it is only within 20% of a standard number it is marked as being a standard non-precision resistor for that value.