# Teaching an old dog new tricks.

#### AnalogKid

Joined Aug 1, 2013
11,190
You picked a good time to jump in.

1. Stay here. I'm on 7 forums, and this one is the best for beginners. The innergoogle is awash in instructional sites and materials, from grade-school science class through PhD level. Of course some of it is garbage, but this site is an excellent curator of those other people.

2. It's fine to have six different questions at the same time, but put them in six different threads.

ak

#### dl324

Joined Mar 30, 2015
17,093
From the section on transistor saturation. Need I say more...

#### MrChips

Joined Oct 2, 2009
31,055
Let's get a few things straight. The water analogy is old school. Don't go there. You don't need it and it will only get you into deeper water.

Make sure you can master Ohm's Law.
Ohm's Law states that the current flowing through a conductor is directly proportional to the voltage across the conductor and inversely proportional to the resistance of the conductor.

Stated mathematically,
I = V / R
where,
I = current in amps
V = voltage difference in volts
R = resistance in ohms

Note that I is the dependent variable. V and R are the independent variables. If you know V and R, you can determine I.
Also note that I said V = voltage difference. There is no such thing as a voltage at a single point. It is always a voltage with reference to another point.

There are two corollaries of Ohm's Law.
V = I x R
R = V / I

While we're at it, let's define power P in watts.
P = I x I x R
By applying all known substitutions of Ohm's Law as stated above, we can calculate power as:
P = V x V / R
P = V x I

#### dl324

Joined Mar 30, 2015
17,093
I just tried to create a "Self Made" Power supply out of foam board and hot glue and 5v ac to dc power plug.
Get yourself some plexiglass and solvent for making boxes. If you have a Tap Plastics nearby, they sell cut-offs by the pound.

It's fairly easy to score and snap. If you're trying to remove smaller portions, I use a metal bending tools:

You can also use copper clad board.

A "box":

It takes a fair amount of heat to make copper clad boxes. It also helps if you have a metal shear to cut it. My 8" shear cost me \$125 at Harbor Freight. Unfortunately, they don't sell them anymore.

#### BobTPH

Joined Jun 5, 2013
9,238
Get yourself some plexiglass and solvent for making boxes. If you have a Tap Plastics nearby, they sell cut-offs by the pound.
Oh, come on. Get into the current millennium.

3D printing is the way to make hobby project enclosures.

#### WBahn

Joined Mar 31, 2012
30,286
MOD NOTE: As already noted, threads tend to meander, just like conversations at a party do. As the Thread Starter, our general policy is that you own the thread. So if you don't mind the direction that the conversation is going, you can just let it meander and we will generally assume that it's okay with you. If it's not, then simply use the Report button on posts that you think are too far afield and we will generally either delete them or move them to another thread (at our discretion).

#### RexR35630

Joined Dec 9, 2023
17
You picked a good time to jump in.

1. Stay here. I'm on 7 forums, and this one is the best for beginners. The innergoogle is awash in instructional sites and materials, from grade-school science class through PhD level. Of course some of it is garbage, but this site is an excellent curator of those other people.

2. It's fine to have six different questions at the same time, but put them in six different threads.

ak
Thank you. I feel confident that AAC is the best to learn from. Glad I'm here.

#### RexR35630

Joined Dec 9, 2023
17
Yes, almost. Because it is so old too.
Wow!!!!!
Thank you so much Danko!!!
All you guys are great and so kind to me.
Really love's AAC!!!

#### RexR35630

Joined Dec 9, 2023
17

#### AnalogKid

Joined Aug 1, 2013
11,190
Back on topic, check out the World Radio History archive. It is gigantic. Here is their bookshelf for the electronic hobbyist. In it are several layers of introductory books on electronic circuits, some heavy on math and theory, some not. Browse around in there to find some at your level.

ak

#### Tonyr1084

Joined Sep 24, 2015
7,986
Using Ohm's Law
R = V x I
R = 3 ÷ 0.01
R = 300K Ohms
Oops. You made a common mistake. Though you typed it correctly I think you may have used 10mA instead of 0.01A. That's where I always got confused when studying as a student in high school. It's an easy mistake; at least it was for me.

#### Ya’akov

Joined Jan 27, 2019
9,263
I'm not a fan of AOE. You can probably find free PDFs. I have a couple versions. Don't remember where I got them and they're too large to attach to a post.
For the record, there are no legitimate free PDFs of the Art of Electronics and only one relatively recent e-book version. Any PDF you find on the net is bootlegged and simple copyright infringement.

#### WBahn

Joined Mar 31, 2012
30,286
OK, Dealing with the current Schmatic you gave in your example ( I got confused )

5V supply
LED has 2Vf
With 10 mA of Current.

(5V - 2Vf ) = 3V

Using Ohm's Law
R = V x I
R = 3 ÷ 0.01
R = 300K Ohms
Herein lies one of the most valuable lessons you can learn: Always, always, ALWAYS track your units throughout your work.

3 is not a voltage, it is just a number. Similarly 0.01 is not a current, it is just a number. The numerical part of a quantity is only part of the value -- the units are critical. It is meaningless to say that my height is 72. It is 72 inches. This is the same as 183 cm (sans rounding). But this does not mean that, somehow, 72 is the same as 183.

R = 3 V / 0.01 A
R = 300 V/A

1 V/A = 1 Ω

So

R = 300 Ω

As you start out, be as deliberate and explicit as you need to be in doing units conversions in your work. With a bit of practice, you will be able to spot and perform the "usual suspects" so that you can do something like

R = 3 V / 10 mA = 0.3 kΩ = 300 Ω

You are going to make mistakes. We all do. Most of the mistakes we make will mess up the units, allowing us to catch them almost immediately instead of wasting a bunch of time on work that is guaranteed to be wrong from that point on. But this only works if the units are there to get messed up by those mistakes.

#### Ian0

Joined Aug 7, 2020
10,214
ALWAYS work in base units (Volt, Amp, Ohm, Farad, Henry etc.) never in mV, uA, pF,kΩ. Every scientific calculator has a EEX key or similar that makes it easy to put in x10^6 whatever.
The only exception is deciBels.

#### WBahn

Joined Mar 31, 2012
30,286
ALWAYS work in base units (Volt, Amp, Ohm, Farad, Henry etc.) never in mV, uA, pF,kΩ. Every scientific calculator has a EEX key or similar that makes it easy to put in x10^6 whatever.
The only exception is deciBels.
Six to one, half dozen to the other. Sticking to base units helps avoid some scaling errors (though the TS tried to work in base units and ended up off by three orders of magnitude), and can be helpful when you are just learning things as it reduces the mental load of juggling multiple concepts at once, but I've seen many examples where trying to work in base units with scientific notation was the root cause of scaling errors. In essence, the problems in both cases were the same -- an inability, usually due to lack of sufficient practice, to be proficient at working with scaling factors. So they see 10 pF and they put 10x10^-9. I just had an assignment that had 1 ppm as a parameter and several people used 10^-7. Normally, that would raise red flags about cheating, but it was really apparent from the work that there was no collaboration amongst any of them.

I find working with scaling prefixes to be quite natural, especially since many of them cancel out so nicely. But, I've spent the time getting comfortable with them and have also developed the engrained habit of mentally double checking almost everything I do (which doesn't mean that I don't still let silly mistakes slip through from time to time).

#### BobTPH

Joined Jun 5, 2013
9,238
Consider:

V = I R

If I is in mA, then V is in mV.

#### Ian0

Joined Aug 7, 2020
10,214
Six to one, half dozen to the other. Sticking to base units helps avoid some scaling errors (though the TS tried to work in base units and ended up off by three orders of magnitude), and can be helpful when you are just learning things as it reduces the mental load of juggling multiple concepts at once, but I've seen many examples where trying to work in base units with scientific notation was the root cause of scaling errors. In essence, the problems in both cases were the same -- an inability, usually due to lack of sufficient practice, to be proficient at working with scaling factors. So they see 10 pF and they put 10x10^-9. I just had an assignment that had 1 ppm as a parameter and several people used 10^-7. Normally, that would raise red flags about cheating, but it was really apparent from the work that there was no collaboration amongst any of them.

I find working with scaling prefixes to be quite natural, especially since many of them cancel out so nicely. But, I've spent the time getting comfortable with them and have also developed the engrained habit of mentally double checking almost everything I do (which doesn't mean that I don't still let silly mistakes slip through from time to time).
Converting everything to standard form (which is a more extreme example) is something I learned when we had slide-rules at school, and I was glad to leave it behind when I got my first scientific calculator.
Admittedly, it does require a working knowledge of all the prefixes.

#### WBahn

Joined Mar 31, 2012
30,286
Consider:

V = I R

If I is in mA, then V is in mV.
Not if R is in kΩ or mΩ or anything other than unscaled Ω.

V = I·R

is dimension agnostic. It merely expresses the relationship between three quantities, just like the good old distance = rate * time equation.

D = R·T

We would not say that if D is in mm that T must be in ms.