# calculating resistor values

Discussion in 'The Projects Forum' started by Gadersd, Jan 17, 2013.

Dec 8, 2012
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I am planning on creating some complex circuits with many transistors and other semiconductors. My problem is calculating the resistor values. It there an easy way to do this without solving countless equations?

2. ### MrChips Moderator

Oct 2, 2009
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The only method I know of for calculating resistance is derived from Ohm's Law, R = V/I.

3. ### GRNDPNDR Well-Known Member

Mar 1, 2012
520
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Those aren't countless calculations.

Design an ohmmeter using a darsenvaux(SP?) movement.

2 pages worth of calculations just to find resistances and the proper scale.

Ohms law is simple and if you're designing the circuit then you likely know every other value needed to find a resistance.

4. ### hondabones Senior Member

Sep 29, 2009
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I don't think there are any kind of shortcuts. If you are trying to bias transistors and create amplifiers, you are stuck using the long list of equations to get through it.

5. ### Tahmid Active Member

Jul 2, 2008
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There are certain situations where each resistor need not be accurately calculated, since such detailed accuracy is not required. You may simply use values you know work - you've previously tested okay and know that the current range is ok.

For example, if you're driving LEDs from a 5V source, without calculating you may select 330Ω resistor. This is a common value.

If you're driving, say, a BC547 from a PIC, you may use a resistance between 1k and 10k.

That being said, however, I would still advise you to make the required calculations for each stage.

6. ### WBahn Moderator

Mar 31, 2012
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The key is to understand the circuit you are designing well enough to identify what factors are important and which are affected by which resistor. Frequently you can size many/most of the resistors by just mentally considering one or two things. Others will require a bit of computation, and others will require a lot. Each circuit is different.

7. ### wayneh Expert

Sep 9, 2010
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The OP's original question is nonsensical.

8. ### WBahn Moderator

Mar 31, 2012
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Yes and no. It's not too unreasonable for someone just starting out to suspect that there are some generic rules and techniques that will let them determine the appropriate compoment values in pretty much all situations. It takes time, experience, practice, and education to learn the degree to which there are and the degree to which there are not such grand techniques.

9. ### gootee Senior Member

Apr 24, 2007
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Unless your circuits are infinitely complex, the number of equations won't be countless.

Seriously, though, although engineering can be thought of in many ways, one way is that it involves taking a large or complex problem that we don't know how to solve and breaking it into a set of smaller problems that we do know how to solve. It is possible and maybe likely that your circuits will have functional blocks that are are only related to each other by one or two variables. That already makes the equations much more managable. Each functional block might be the same way. Eventually you might have a large set of relatively-simple equations.

A simulator, such as the free LT-Spice, from linear.com, might also be very helpful. If you get a sub-circuit's topology and desired behaviors defined and there are one to three resistors for which you need values, you can have LT-Spice automatically sweep their values over chosen ranges (even three at once) and evaluate the resulting plots of circuit values of interest, and then pick the set of resistor values that make the circuit do what you want.