# COMMUNITY : FORUMS : TECH

## Circuit Design Edumacation

Can anyone direct me to free internet resources that will help me with circuit design? I can usually figure out what type of parts are needed for a given circuit, but am often completely stymied by what specific parts to select - what resistor value, which transistor, what voltage and uF cap? What's available? Thanks!

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westfw9 years ago
The key epiphany is that if you're dealing with modern digital electronics, the actual values in your circuit are a lot more flexible than you might think. About the only thing you have to keep an eye on is maximum allowed current going into and out of pins and through transistors and resistors.
homba (author)  westfw9 years ago
I've been fiddling with a joule thief circuit or two and kind of found that out - I changed out several different transistors and coil sizes just seeing what would work - they pretty much all worked fine. I guess I'm looking for rules on how to calculate the 'best' values for different components. For example, I've read that for caps you want double the highest voltage the circuit will give. I really don't know how to figure out the uf values, though. I'm guessing that for resistors, Ohm's law is your best bet for the value and make sure to have a reasonably high wattage rating. Transistors are a little dicey for me and coils seem to be dark magic. I used to fiddle with crystal radios and find coils fascinating, but mysterious. Anyway, just kind-of looking for places that give good rules of thumb and perhaps more of the "why you should choose these values". What is really irritating is that I lost my only electronics book now that I've found the time and interest. It's an older Radio Shack publication and I think my 4-year-old walked off with it ... hmm, maybe I should ask him for help - he's been reading since he was three! Thanks for the reply
9 years ago
Ah. The Joule thief circuit is far from "digital" and is pretty much in the "black magic" category as far as I'm concerned as well. Given the basic circuit is working, you should think of it as a project ripe for qualitative or quantitative experimentation; I bet one could design a pretty good science fair project around the Joule Thief circuit (eh. Not much use for us old folk...)
homba (author)  westfw9 years ago
Well, I'm a good 20 years past my science fair days :) and the kids are not quite old enough for theirs. I'm trying to stay a step ahead of them - it used to be so much easier to learn new things! I think the kids conspire to keep me up at night to make me stupid ... I'm going to keep fooling around with this stuff and hit the library until I can find my books - thanks
9 years ago
Highly recommended: The Art of Electronics by Horowitz and Hill. Lots of detail without making you remember your advanced calculus.
homba (author)  westfw9 years ago
Here's one I just dug up - looks interesting:
9 years ago
that's one of my favorite electronic websites for learning. My favorite real book for electronics is practical electronics for inventors, it's pretty cheap on amazon (get the one with a blue cover with a light bulb on it)
9 years ago
oh and for calculating the resistor for a transistor as a switch:

Take the colector current (how much current you want to go through it), and divide it by the hfe of it. Then multiply that by 1.3. then use ohms law to figure out the resistor value.

For example: I want 100mA to go through and the hfe of the transistor is 100

100/100 = 1mA

1mA x 1.3 = 1.3 mA

You need 1.3mA of base current
now caluculating the resistor, lets say we're using 5 volts:
5/0.0013= 384 ohms
9 years ago
Um. Probably works ok as a rule of thumb, but it's not quite "right", because for switching you want to "saturate" the transistor, which means providing significantly more base current than then Hfe would imply you need, and then the actual Hfe is usually not quite known, and somewhat variable with collector current anyway. (a lot of the complexity of fancy amplifier circuits comes from designing them in such a way where the exact values of Hfe and other transistor parameters cancel out and become irrelevant.) And then you dropped a decimal place on your last calculation. 3.84k I sorta do: "for typical currents and typical transistors, use a 1k base resistor. For low power or high power, think more." :-)
homba (author)  westfw9 years ago
This is exactly the kind of stuff I'm looking for - looks like I have some reading to do - thanks
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