# Newbie with Questions!!

#### Dexer

Joined Jul 30, 2007
5
Hi guys,

I'm newbie to this forum but not to the site. I have couple questions

about electronics ( As a beginner of course ) .

Q1) How can capacitor protects circuits from DC source?? I need a scientist

Q2) what is the difference in application between BJT and MOSFET? can you

give me your comment for this circuit?!

#### Papabravo

Joined Feb 24, 2006
21,225
A1) DC current cannot flow across an open circuit. A capacitor is often modeled as a pair of parallel plates that do not touch each other. I'm not sure that capacitors are actually used as protection devices. I think a fuse or a polyswitch is a more likely candidate for that application.

A2) A BJT is a current controlled current device, while a FET is a voltage controlled current device. For your circuit you can use either device with appropriate battery voltages(which may not exist in practice) to good effect.

#### Dexer

Joined Jul 30, 2007
5

but to be honest with you you didn't convince me with the first question.

You said DC current can't flow through an open circuit, is it the same case

with AC?

if not why is that?

anyone has an idea?

Thank you again

#### Dave

Joined Nov 17, 2003
6,969

but to be honest with you you didn't convince me with the first question.

You said DC current can't flow through an open circuit, is it the same case

with AC?

if not why is that?

anyone has an idea?

Thank you again
AC current doesn't technically flow across the open circuit. Because the AC current is oscillating it charges and discharges the capacitor plates therefore current is flowing to and from the component. For DC current the current charges only one plate, and once the plate is fully charged all current to the component ceases.

Dave

#### recca02

Joined Apr 2, 2007
1,212
in case of ac the charges (e-) first get stored on one of the plate like in dc for first half of the cycle then the direction of applied voltage polarity changes and the stored charges on the capacitor start flowing in opposite direction to the initial one. this continues for each cycle and in this way current flows.

#### Dexer

Joined Jul 30, 2007
5
Thank you dave and recca

can you give me what is timer 555 really is used for? I know it is for oscillation but I don't khow how and why we use it?!

thank you again

#### recca02

Joined Apr 2, 2007
1,212
555 ic is a timer ic and u can generate pulses of desired frequency and desired on/off time (imagine a dc pulse wave).
using these pulses u can control many electronic components and cicuits by using it as trigger and enabling and disabling them.like a clock pulse.

#### techroomt

Joined May 19, 2004
198
#1 - no current flows through a capacitor (other than small leakage current). it flows to and from the capacitor
#2 - the two plates, when charged with a voltage will take on a negative charge (the plate that accepts additional electrons) and positive charge (the plate that loses electrons), until the potential across the plates equals the supply voltage (appx 99% charged after 7 time constants), then current basically ceases. so, other then an initial charging current, dc will not continue to flow to or through a capacitor.

it is said the reactance of a capacitor (opposition to electron flow) is infinity at 0 Hz. (DC). as input signal frequency increases, a capacitor's reactance will decrease and will allow an increased current flow.

#### Dexer

Joined Jul 30, 2007
5
Thank you all

2 more questions and I will stop

Q1) Sometimes when someone deals with BJT's and MOSFET, they use

something called SMALL SIGNAL, can anybody expalins what is this used for?

Q2) Is the cpacitor behaves like an open circuit or short in High Frequency?

Thank you again for all of you

#### recca02

Joined Apr 2, 2007
1,212
the reactance (opposition to current) of capacitor is given by Xc= 1/(2.pi.F.C)
where f is frequency hence for higher frequency the reactance is lesser.for infinite frequency it wud be a short

#### techroomt

Joined May 19, 2004
198
since transistors have the ability for both voltage and current gain, the amplifier configurations (circuits) they are placed into are suited for different needs and applications. a small signal amplifier, designed for more voltage gain, provides amplification for small amplitude signals, often in millivolts - from microphones, antennaes, playback heads, etc.. often a second amplifier stage is required for additional voltage gain.

btw a large signal amplifier then takes that signal and provides power gain (in both voltage and current).

#### n9352527

Joined Oct 14, 2005
1,198
Small signal models are used to analyse or simulate non-linear devices operations in terms of linear equations. The main reason to use these models is to make analysis and simulations easier to perform.

The electrical behaviours of the device are first calculated around a fixed bias point, and then the small variations of signals around that bias point in response of input are then obtained. Usually, this is obtained either through differentials of the original response equations or iterative process in the case of complex circuits.

These small signal models are approximately true for small signal variations around the bias points, but could be inaccurate for large signal variations or different bias points.

#### Dexer

Joined Jul 30, 2007
5
Well

All I have to say is Thank you guys very very much

Regards