Applying mutiple resistors to create an 18k

1. Jun 17, 2011

Megasundato

Hello. my robot requires an 18k resistor at one point. my problem is that I don't have one and my radioshack has never heard of it.

anyway, I was wondering on how to create one. I know that adding in a series circuit it's just R1+R2... and I think that parallel circuits add with the reciprocal.

so how can one get an 18k. Someone told me that I could make a parallel circuit with a 22k and some 1ks, but I don't think that would work. I know that I could create one in a serial circuit, but that might take a lot of resistors.

thanks for the help.

2. Jun 17, 2011

uart

18k is very much a standard value. In higher tolerance parts many more values may be available, but even the most basic 10% parts should be available in decade multiples of the following.

1, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2

3. Jun 17, 2011

MATLABdude

5% resistors follow the E24 series: there are 24 resistor values in a decade. That's fancy speak for there being 24 logarithmically-spaced values from 10 to 91 ohms (and 24 from 100 to 910 and so forth).

You can calculate the value of two parallel resistances as follows:
$$\frac{1}{R_{parallel}}=\left(\frac{1}{R_{1}}+\frac{1}{R_{2}}\right)$$ or
$$R_{parallel}=\left(\frac{1}{R_{1}}+\frac{1}{R_{2}}\right)^{-1}$$

Three resistors in parallel add the same way:
$$\frac{1}{R_{parallel}}=\left(\frac{1}{R_{1}}+\frac{1}{R_{2}}+\frac{1}{R_{3}}\right)$$

A handy thing to remember is that two equal resistors in parallel have an equivalent resistance of half their value (three in parallel having a third, and so on). So if you don't have an 18k, you can put two 36k resistors in parallel (which should be more plentiful--hit search to do the calculation):

4. Jun 17, 2011

uart

Yep, and both the E12 and the E24 series include 18k. So it's pretty hard to believe that radioshack has never heard of it.

5. Jun 17, 2011

MATLABdude

Maybe the sales guy was just trying to sell some batteries or an extended warranty?

In any case, to the OP, I recommend finding a real electronics supply / hobby shop in your area (assuming one still exists).

6. Jun 17, 2011

Megasundato

thanks for your help. I see how I can do this now.