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# ELECTRONICS STUDENTS QUESTIONS

If you are a student in electronics or a professional in electronics, we encourage you to be a part of our FORUM.  Ask questions.  Get help from others or just see what is going on.  Let's have some fun while we are learning.

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### Replies to This Discussion

Diode Tutorial & How to build an AC to DC power supply

Voltage regulator tutorial & USB gadget charger circuit

I'm trying to get into electronics and I'm starting with a simple 555 timer circuit. I'm reading up on general circuit design but I find most websites tend to just give circuit diagrams with no explanation of the purpose of the components. I've not got any intuition for the various components yet so things are a bit confusing.

The circuit I'm using is a 555 astable one from here:
http://www.kpsec.freeuk.com/555timer.htm#astable

I want a 50% duty cycle. I can see examples with a diode across R2 and presumably with R1 and R2 equal it would be 50% (more or less). With my limited knowledge though I wonder why two resistors are required in that case (ie. why not remove R1 and the diode). The only reason I can think of is that R1 is doing something else in the circuit other than affecting the charge/discharge time. I've also read that it's not accurate to use a single resistor. Could anyone enlighten me as to why I need R1? Or is there a good resource that explains the 555 astable circuit thoroughly?
flush pin 7 and wire a resistor from pin 3 to pin 2 and 6, will give u nice 50% duty cycle.

A student asked about the effect of temperature on their BJT transistor circuit.  Temperature is the enemy to transistors.  It affects how they work.  Also, we would have to be considering two different temperatures, the temperature internal to the transistor and the ambient temperature outside the transistor.  Circuit design can have some control over the internal temperature to keep it from burning up, but what about the external ambient temperature.

You could consider the extremes of circuits located inside a freezer where the ambient temperature is typically around zero degrees.  Then the opposite of that would be a circuit working in a hot environment such as under the hood of your car.  What happens?

Your BJT transistors have something called beta and we use the symbol

to represent it.  There is a DC beta and an AC beta.  On a datasheet the DC beta is shown as hFE and the AC beta is shown as hfe.

As the ambient temperature increases, both the DC beta and the AC beta increase.  The DC beta moves the operating point in the direction of saturation.  This could introduce distortion depending on the amplitude of your AC signal.  The AC beta increase causes your circuit to have greater gain which could push your signal into saturation or cutoff, either way it introduces distortion.

Student having problem with voltage regulator, see my diagram with response below.  If you want to know more about this circuit please let me know.  I can help.

Students, find more electronic information on www.MoreDat.com

i am given assignment by my teacher to design class A power amplifier that gives 3W output....
the circuit must be implemented both by series fed and transformer coupled techniques!!!!!

i have to:
1)implement the design problem on papers.......then find the efficiency also for both cases

2)design the circuits on multisim

Actually i dont know much about which transistor will be the best suited for this problem and will give output close to 3W....please suggest a transistor and also help me on how to start solving this problem as i am weak in designing !!!!

I would use something like the BD239 NPN Silicon Power Transistor.  It’s more than adequate to achieve your 3 watts of output and it’s cheap.  Since you are using it for power you will setup your circuit as a Common Collector (CC) arrangement with the input on the Base and the output on the Emitter.  Connect your Collector to Vcc.  Voltage divider Base biasing is the easiest way to setup your operating Q point.

Student Question:

I am v.new to electronics and helping my kid science school project.
We are trying to build a v.simple water level indicator. We have to use one of the 74xxx series I.C. So we decided to use NAND gate I.C (74LS00).

Circuit diagram is shown below. It works fine in the breadboard when input of one gate is connected to (-ve) of the circuit the LED lights on. The problem is when we dip the wires in the water the light doesn't turn on.

I searched internet and it seems that in those cases one should put pull-up or pull-down resistors. Is that correct?

Can anyone please help us in this. His project is due next week and we can't figure out why it doesn't work in the water. Water supposed to be good conductor of electricity.

Much appreciated.

thanks,

Your water will conduct but it is not a good conductor.  What you are essentially asking your circuit to do is for the water to short the two wires together to produce the ground or near ground at the input of your NAND gate.  This is not going to happen.  The tiny change made by putting the two wires in the water would have to be greatly amplified with something like an operational amplifier (op amp).

Student Inquiry

Zener Diode Pointless Fun

I am trying to understand Zener Diodes, so I decided to make a simple circuit, with several different diodes in place.

I never really understood the zener voltage and zener diodes: whenever someone tries to explain it, they always get in too deep for me, so this was why I decided to do a simple hands on experiment.

Does this experiment look to be correct: am I using the zener diode the way was was meant to be used. I kept with one resistor to keep things simple.

Any other help with understanding zener diodes, there use, and etc. would be greatly appreciated. Even some simple experiments would be beneficial too.

The reason for this experiment is just to see how a zener voltage affects in this case the brightness of the LED.

Thanks for the help and suggestions.

Here are the circuits

Response:

All of your circuits look fine.  Here are some observations.  First off what is meant by your labeling of the LED ideal measurement is that if you have approximately 2 volts across it and 20mA running through it then it will glow brightly.  Notice how in your circuits the voltage across the LED only changes slightly but the current changes significantly.  I know we are not really talking about the LED here but the change in current will affect how brightly the LED glows.

Now let's talk about the reason the current is different in the various circuits.  Your zener diode is like the poor man’s voltage regulator.  It attempts to maintain a constant voltage across it.  By the way, notice that the voltage across your LED is fairly constant.  So we basically have two constant voltages in the circuits.  Whatever voltage is left over from the 9 volt supply is available and measured across the resistor.  The current then can be calculated by using Ohm’s law with the voltage across the resistor and the resistor value.  As we move from one circuit to another, keeping the resistor value the same and keeping the LED voltage approximately the same, then let’s look at what changes.  The zener voltage changes with a different zener and the voltage across the resistor changes.  Since the voltage across the resistor changes then the current must change.

I hope this helps you to understand what is going on in your circuits.

Student Question:

[BJT] Changing bandwidth without changing gain

Hi everybody!

If I have a 2 stage Bjt circuit amplifier with a common emitter and then a emitter follower how can i make the bandwidth to grow without changing the gain of the circuit?

Maybe a common base between the 2 stages (like a cascode configuration)?

Odd electronics test question

I could really use some help. I am taking an electronics test, and this is one of the questions:

Direct current would NOT ordinarily be used in which of the following?
A. Ships
B. Airplanes
C. Battery Charging
D. Arc Welding
E. Electric clocks

Now I may be totally crazy, but don't all on the list use or are capeable of using both AC & DC? I've pretty much narrowed this down to either battery charging or arc welding, but both of these have AC & DC versions... Battery chargers are typically run on AC, but output DC, though there are the 'quick charge' packs that use entirely dc...

Any help appreciated.

My NOT DC choice

Electric Clocks is my choice. An electric clock would be one that is plugged into AC and runs an AC motor in the clock turning the gears that move the hands. No DC is required.

A portable battery operated clock would be different. It would require DC. Also a digital electric clock would require DC.

The key is in the wording.