circuit design

[mani26]

hi i am very fresh in electronics ,i want to know that how circuit designer select a value of resistor or capacitors etc in a circuit ?

 

[Supercap2F]

It depends on the circuit. Try to be more specific.

Dan

 

[KrisBlueNZ]

Personally, I choose them based on the colour scheme.

You see, resistors have these pretty coloured bands on them, and you have to coordinate them, otherwise they look bad. For example, a 560k resistor is good because it's green, blue, black, orange. Really nice bright colours. But I would never connect it to a 330 ohm resistor, because that's orange, orange, black, black, and that's pretty plain, so they just don't work together.

Seriously, there's no simple answer. Once you understand what the components do, and you know the voltages and currents that should be present in the circuit, you can calculate the appropriate values using various simple formulas; most often, Ohm's Law, for resistors at least, and the capacitive reactance formula, Xc = 1/ (2 pi f C), for capacitors.

Often, a component can have quite a wide range of values and the circuit will still work fine, so I try to reduce the number of different resistor values in the bill of material to simplify sourcing the components.

For example, in a typical CMOS logic circuit, a "pullup" or "pulldown" resistor (resistor that is used to pull a circuit node to a particular state) could really be any value from 1k to 100k, and it's conventional to use a standard value such as 10k. But if the same circuit also needed an 8.2k resistor for a different part of the circuit, and didn't need any resistors of "exactly" 10k, I might use 8.2k resistors for my pullups and pulldowns, to reduce the number of lines in the BOM.

Resistors are manufactured over a wide range of values - ten orders of magnitude and wider - and very low and very high values are needed for specific types of functions. Exact values are also needed to set exact voltages and gains in circuits. But often, values are not critical, provided they're in the right ballpark.

If you have a specific circuit you're interested in, post it, and we can talk about how you choose suitable values for the resistors and capacitors.

 

[Supercap2F]

You see, resistors have these pretty coloured bands on them, and you have to coordinate them, otherwise they look bad. For example, a 560k resistor is good because it's green, blue, black, orange. Really nice bright colours. But I would never connect it to a 330 ohm resistor, because that's orange, orange, black, black, and that's pretty plain, so they just don't work together.

Thats HILARIOUS!!! I was feeling down until I read that!!

Dan

 

[Harald Kapp]

I'm considering putting this Feng Shui style of design into our CAE manual

 

[KrisBlueNZ]

Thanks guys. I'm not sure why I posted that. But you've got to admit, green blue black orange is a pretty sequence of colours to look at Especially on a light blue background.

Perhaps we should create a new paradigm for electronic design, based on aesthetics! No more boring brown, black, black, red. Greens and blues all the way, with perhaps a yellow or a violet (not together, mind you) and the occasional orange, grey, and white, for extra excitement!

Away with the dull drudgery of work-a-day browns, blacks and reds! And brown, black, black, black... we'd have to make a law against 100 ohms.

Have to stop now... this is just getting silly.

 

[BobK]

Well, I do try to balance my PC boards with the weight / shape of components to give an good aesthetic result. I even consult my wife, who majored in art. Had not really thought about color much though.

Bob

 

[KrisBlueNZ]

Well, I do try to balance my PC boards with the weight / shape of components to give an good aesthetic result. I even consult my wife, who majored in art. Had not really thought about color much though.

That's interesting Bob. I don't think I've heard of that before.

I don't think about the aesthetics of the big picture. I'm careful with things like keeping diodes and electrolytics oriented the same way where possible, and avoiding burying small components among tall components. Weight balance could be important. And of course you keep the hot components away from the electrolytics, and at the top of the board if it mounts vertically.

Mostly colours are dictated by available components. But I do think that some resistor colour codes look nicer than others

 

[davenn]

And of course you keep the hot components away from the electrolytics

that's one thing commercial designers don't seem to worry about hahaha

Dave