# logic gate foundation concepts

#### Don23

Joined Feb 22, 2011
10
When I reviewing the first logic gate topic for the NOT gate, two "foundation concepts" were alluded to but not explicitly stated. Because I'm a newby in electronics I found myself struggling to understand this topic until I sussed out these underlying assumptions.

1) The terminus of an open (non-conducting) circuit always adopts the full voltage at the source or ground, whichever it leads back to.

2) Two parallel branches of a closed circuit MUST support the same voltage drop, else one or the other will not conduct. It's central to the logic of the NOT gate that the voltage drops of the diodes on both parallel branches are uneven. The resistor above the junction cannot have one voltage drop for one branch and a different voltage drop for the other branch at the same time.

Moreover the voltage drops of diodes are "fixed", unlike a resistor. Adding a resistor to the single diode branch would flexibly even out the voltage drop so that both branches would conduct, so a resistor cannot be added to this branch.

Perhaps these assumptions are inaccurate. If so, please clarify. By the way, if accurate, I could not find any previous All About Circuits topic that explicitly discusses these concepts, including the ones about diodes or parallel circuits.

Don

#### Wendy

Joined Mar 24, 2008
22,471
Welcome to AAC!

Feedback and Suggestions Forum for providing feedback and suggestions about All About Circuits, including corrections to the e-book. This forum is not for getting help with technical questions.
A moderator will move it shortly. If there had been a error your were pointing out in the book this would be the place to put it.

1> Correct.

2> Not sure what you mean, two separate voltage dividers will obey Ohm's Law, and each will drop its own voltage. The voltages feeding each divider, the power supply or other, will be the same.

In general schematics are a better way to describe electronics, the odds for misunderstandings go way down. Schematics are the language of electronics. I use M/S Paint to draw with, I've come up with a set of templates called PaintCAD.

A diode drops a fixed voltage, this is true. However, the remaining voltage left over determines the current. If there is no ohmage then the current will be extremely high, and the diode will shortly burn out. LEDs and diodes absolutely require resistance somewhere to control the current.

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#### Don23

Joined Feb 22, 2011
10
Bill,

Speaking of question 2, see the attached image, captured from the "NOT Gate" topic. I tested this on a breadboard with 1 diode on one branch vs two diodes on the other branch. A LED was on either branch. I could toggle back and forth between both lights on or off by opening then closing the branch to the single diode. When I closed the single diode branch, the two diode LED would not light. When I opened the single branch diode, the two diode LED would light.

By the way, I had no idea that " the remaining voltage left over determines the current". I'll reread the diode section again.

Thanks, Don

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#### Wendy

Joined Mar 24, 2008
22,471
I recognize the drawing, it is from the simplified TTL gate drawing in the book.

The first diode sets an absolute voltage that is very stable. The resistor above it sets the current. It will be (Vcc-0.6)/R . The rest of the schematic is covered up by your notes, but it will follow from that assumption.

Diodes and resistors will make good AND and OR gates, transistors provide the invert functions. Their biggest flaw is they have elements of analog functions, you need an amplifier now and again to convert the signal to proper digital levels.

My advice is to get a protoboard and start wring some examples that you don't have a firm grasp on. Practical experiments tend to stick better than simple reading alone.