Switching circuit explanation?

[du.chci1990]

Hi all,

Just looking for an explanation of this circuit.. i know this is a switching circuit, but what's th use of diode?
Also can anyone tell me how to design this circuit.. i mean, why are these values of resistors used? why can't any other resistor be used?

I'm an amateur in this field, so please help me!!

Thanks..

 

[btbass]

The diode is the wrong way around, this will not work, the transistor will go up in smoke!

Simply put, the base resistor value is chosen so that enough base current is available, times the transistors current gain, to produce the required collector current.
In a simple circuit like this, the values are not critical.

to drop 5 volts over 10k i = 5/10000 = 500uA
Typical transistor current gain is 100.
Required base current 0.0005 / 100 = 5uA
Base resistor = 3.2 / 0.000005 = 640K.
Times ten rule = 64K
So a preferred value of 62K would do the job.

Actual base current = 3.2 / 4700 = ~680uA

In this circuit, more base current is flowing than collector current?
It is an inverting / level translating switch circuit.

The diode should be the other way around.
The diode speeds up the turn off time of the transistor by allowing more current to flow in the negative direction. Discharging the base capacitance more rapidly.

This is over kill in this circuit unless it is very high frequency switching.

 

[thannara123]

what is your actual need ??

 

[du.chci1990]

Thanks btbass,

i was not sure which way the diode is connected, but there is a diode connected! In fact, it happens to be a zener diode.. leaving the diode for a moment..

I had done the calculations just the way you have explained... i realized that i was getting different resistor values. but the fact is, the circuit uses the very same values of the diagram i have shown, and it is working! i don't know how. (assuming the diode was connected in the correct way).

This circuit is trying to give a clear HIGH(5v) or LOW(0V) signal to a microcontroller from an Ir sensor whose voltage keeps varying from 1V to 5v. It's basically an obstacle detecting circuit..

I'm pretty confused about this myself, so please try and clear it for me??

---------- Post added at 21:19 ---------- Previous post was at 21:14 ----------

@thannara123

This circuit is trying to give a clear HIGH(5v) or LOW(0V) signal to a microcontroller from an Ir sensor whose voltage keeps varying from 1V to 5v. It's basically an obstacle detecting circuit..

So, the microcontroller needs upto1mA current input to know a HIGH signal.. That's why the 5V and 10k on the collector side. On the base side, a resistor is used so as to give enough base current to turn on the transistor, no matter what the input (Vin) voltage is, ranging from 1V to 5V..

But the problem is, these values of resistors don't satisfy the necessary design conditions, just the way #btbass has described it above.

is there a way to explain this circuit, or is it completely wrong?

---------- Post added at 21:33 ---------- Previous post was at 21:19 ----------

The diode is the wrong way around, this will not work, the transistor will go up in smoke!

Would the diode be correctly connected in this diagram, if the transistor used was PNP instead of NPN?