Originally Posted by Richard Crowley
If you MUST calculate the current, you can approximate it from some of the specifications for the active components (integrated circuit op-amps, etc.) But it is practically an upper-level course in electronics just to learn to read your typical spec sheet. For example, you could use the maximum power dissipation from an op-amp IC, and that would give you a worst-case figure with a large margin for error, more than Vitalis and doulos30 are recommending. But when you start getting into circuits with discrete components (transistors, etc.) then you need to really do some complex circuit analysis and run some non-trivial equations. Which is why I recommended just measuring it vs. trying to predict it.
Sorry, man, but I still disagree with you for the following reasons. First you say;
"if you MUST calculate the current...".
What I'm saying is that you MUST (LoL) calculate the current because there is no other way to do it. You can't build the thing first and then measure it because how will you know how to build it if you haven't calculated it first? Measurement is for a circuit that already exists. As I understand, the OP desires to build the circuit from scratch and not simply measure an existing one - or maybe I misunderstood him.
You also say;
"For example, you could use the maximum power dissipation from an op-amp IC, and that would give you a worst-case figure with a large margin for error..."
The data sheet does not leave anything to guess work, but gives you the Minimum, Typical, and Maximum values for all specifications. Understanding these values will tell you whether that part is actually correct for the application or not.
You also say;
"But when you start getting into circuits with discrete components (transistors, etc.) then you need to really do some complex circuit analysis and run some non-trivial equations."
On a DIY level, what you would do is search the web, or where ever, for a schematic that you believe can do the job that you need to be done. At that point you can began to collect the data sheets for all the semiconductors and look them over, paying special attention to max voltages, but also to those parameters which are important to your project, for example, audio performance, heat dissipation, part size, cost, and so on. Your advantage is that the engineer who designed the circuit will have done all the complex circuit analysis, so all you have to do is copy it and package it. The question is if you have the skills to copy it and package it.
And lastly you say;
"Which is why I recommended just measuring it vs. trying to predict it."
Here again I say that your biggest asset is your ability to predict how a circuit will perform before you actually build it; you have to plan it from top to bottom. After you plan it, you draw up a schematic and build the circuit according to your - or someone else's - schematic.
Hope that clears up my side of things.