Misc Electronic Questions

[Billy Mayo]

My Manager said that the input capacitance on TTL and CMOS gates need to be discharged, because they they aren't discharged using a pull down resistor , it will delay the output or create add a XXX nano seconds delay

 

[audioguru]

My Manager said that the input capacitance on TTL and CMOS gates need to be discharged, because they they aren't discharged using a pull down resistor , it will delay the output or create add a XXX nano seconds delay

Not true. Usually the input of a logic gate is driven by another logic gate.
The output current of a logic gate is high and the input current of a logic gate is low so they switch very quickly without a resistor. If an un-needed pull down resistor is added then the gates might switch slower.

 

[KeepItSimpleStupid]

So is PF= 0.7 is it leading or lagging?

Can't tell.

PF= 0.7 is not sinusoidal?

Can't tell.

Only PF=1 is sinusoidal right?

With power line power, the answer is yes because the voltage is forced to be sinusoidal, but is PF=1 always sinusoidal in a general sense - No

 

[Billy Mayo]

Normally a transistor switch has no hysteresis. It simply switches on or switches off.
You can add hysteresis to a transistor switch if you want.

When do you need to add hysteresis to a transistor switch? when the input on the base is what kind of signal?

Why would a designer add hysteresis to a transistor switch? for what reason?

Hysteresis is simply some positive feedback

Why would positive feedback give you hysteresis?

 

[audioguru]

When do you need to add hysteresis to a transistor switch? when the input on the base is what kind of signal?

When you want a slowly changing input voltage to switch something on and off.
My solar garden lights, my street lights.

Why would a designer add hysteresis to a transistor switch? for what reason?
Why would positive feedback give you hysteresis?

The input voltage changes slowly then passes the threshold voltage. Then the output increases the input voltage with positive feedback to make the output switch very quickly.

 

[Billy Mayo]

Thanks for the info

So it's all about a slow input signal and creating a fast switching output

 

[KeepItSimpleStupid]

Or, putting it in these terms: Say you had a (signal) that activated a relay. You might want to add hysteresis so the relay didn't chatter.

Or

Say you had a controller like your UPS that needs to make 120 VAC. 120 exactly might cause problems, but making it 120-124 V makes the system more stable.

Filling a tub is a large amount of hysteresis. You would have an empty switch, a full switch and say a 1/2 full switch.
When the tub is emptied below 1/2 if fills to full. Your saving wear and tear on the pump. the pump might have to constantly start and stop.

Just some examples.

 

[Billy Mayo]

Thanks for the examples

Why would a relay chatter with a switch with no hysteresis?

Does adding hysteresis change the on and off levels? Off is at zero volts but hysteresis can make off and On have different voltage points

To me when a transistor switch has hysteresis it becomes a comparator switch?

 

[audioguru]

To me when a transistor switch has hysteresis it becomes a comparator switch?

a comparator is a good example of a circuit that needs hysteresis. Since its gain is 100,000 to a few million then it oscillates at a high frequency when an input gets close to its input threshold voltage. Adding hysteresis makes the output switch suddenly high or low.

 

[Billy Mayo]

So when using a transistor as a switch with hysteresis, its a comparator?

Why do designers use a transistor as a switch instead of using a FET was a switch? what does the transistor switch differ from a FET switch?

 

[KeepItSimpleStupid]

Why would a relay chatter with a switch with no hysteresis?

Depends on the signal. A gross example is say a solar tracking system that needed to not move because of clouds.

Does adding hysteresis change the on and off levels? Off is at zero volts but hysteresis can make off and On have different voltage points

Precisely.

To me when a transistor switch has hysteresis it becomes a comparator switch?

It's tough to make a single transistor have hysteresis, BUT since they trap charge they don't switch at ON and OFF at exactly the same times.

A comparator is a good way to implement hysteresis.

I kinda worry about all of these questions in this thread, because usually you answer at the level of understanding, not at some "atomic level". I had arguments with teachers because of what I knew. I won. I was told though, to answer the questions with what "I'm SUPPOSED" to know or were taught in the class.

I put together systems that could measure the resistance of a piece of paper and that resistance changes with moisture content. Paper, you know as an insulator. I know that moving wires in the earth's magnetic field generates currents. Very small ones, but they can be measured. I've worked on electronic circuits where "fingerprints" matter and high voltage high currents (13 kVDC @ 1.5A) and (100 kVDC @ 0.1A) where a mistake could kill. I've also worked on circuits where the detector had to be cooled at Liquid Nitrogen to minimize noise.

Just beware, that my answers may not be the norm.

 

[Billy Mayo]

Are these transistor switches? how do they work in this circuit?

 

[kubeek]

what is the difference between those four pictures?

 

[Billy Mayo]

they are all the same, i just don't know how those transistors work, are they switches and how do they work?

 

[kubeek]

Q1 is definitely a switch of some sorts, Q2 is probably an indication that +15V is on.

 

[Billy Mayo]

I'm confused on how the transistors switch works

How does the transistor switch turn on? its from the collector

Mostly I have seen transistor switch turn on from the base

 

[kubeek]

I dont really get what youre asking. The transistor needs current flowing into the base to get it on, that is what R172 and R71 achieve.

 

[Billy Mayo]

The transistor needs current flowing into the base to get it on, that is what R172 and R71 achieve.

So the transistor switches are Always Turned ON than? the collector will turn the transistors OFF?

I'm confused on how the transistors Switch ON and OFF, what turns them on and what turns them off

 

[kubeek]

the input to U6A and U6b turns Q1 on. The +15V rail turns Q2 on.

Why don´t you just go apply to some electronics courses? Your abillity to keep a job and yet know nothing about it still amazes me..

 

[Billy Mayo]

the input to U6A and U6b turns Q1 on. The +15V rail turns Q2 on.

What Turns OFF Q1?

What Turns OFF Q2?