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please help a newbie


terrakota
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hi,
i just start to learn electronics from a book, i already studied the parallel, series and parallel-series circuits,
i can solve all the book problems, now a know how to "make" a certain resistor with series-parallel circuits but i cant find a way to create "custom" resistors using standard value resistors, ie:
suppose i need a 61 ohm resistor that is not standard, so i create a parallel, etc circuit to find this resistor, if i use homs law i can find the resistors a need to create this one but the other resistos values are are not standard too, so in paper a can create the circuit but in real life how can i find the resistors i need to "create" the one i need?

i already know that i cant find the exact values i need but at least very close.

my question:
is there a way , tip, trick, etc, to find fast the standad value resistors i need?

i think im confused in something

please excuse my poor english, and thanks for any help

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Hi Terrakota,
Welcome to our forum.
Your designs shouldn't need resistors with extreme accuracy. "Normal" resistors aren't very accurate anyway, a 5% tolerance is common so your 61 ohms example could be from 58 ohms to 64 ohms. The standard 5% resistance range includes a 62 ohms resistor which will work in your circuit just fine.

You need to know how to calculate series and parallel resistors not to make an odd value, but to understand for example the effect on the gain of a transistor of the resistance of a following stage's input biasing resistors, which are in parallel with themselves and also in parallel with the transistor's collector resistor. The total parallel combination determines the gain.

Frequently you will need resistors with values in a certain ratio, for example one resistor must be 2.35 times another one. I just start with a useable value like maybe 10K, multiply it by 2.35 and know that a value of 23.5K doesn't exist. I continue multiplying standard 5% resistor values by the ratio until I end up with a 20K resistor and a 47K resistor, which are exactly the correct ratio. Of course each one will have a 5% tolerance, so the actual ratio could be from 2.13 to 2.60.

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terrakota,
It is hard to come up with any kind of accuracy with series parallel resistors that have a 5% or 10% tolerance. All of the resistors have the plus and minus inefficiency, so the error is multiplied.

If you are making something that needs high accuracy, there are 1% tolerance resistors and even 0.1% tolerance resistors. You will find they can be found in different resistance sizes than the standard 5 or 10 % tolerance ones. They can usually give you what you need. In cases where you still cannot come up with the size needed or close enough to the size needed, you can use an industrial grade 10 or 20 turn pot to achieve what you need.

I do not even stock the 5% or 10% tolerance resistors. The 1% resistors are a little more expensive but allow for much better result in the type of work that I do. If you are just working with blinky light toys or transistor switches, then you might want to use the 5% and just get somewhat close. In many types of circuits there is a large tolerance and you do not need to be exact.

MP

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