calculating resistor to use with transistor pins

[sairfan1]

Hi,

I'm building small circuit with BC557,58,447 type NPN and PNP transistors, for power I'm using 9v standard battery and some time 5v 1Am adaptor,

While building a basic circuit i burned some transistors, I referred to datasheet and came to know that voltage applied to transistor's base pin were high. By using some random resistors I was able to make it work.

I need an expert advise when we use transistors of different series like BC or BD how can we calculate resistor value for Base pin based on available volt and amp like 5v 1Am

 

[KlausST]

Hi,

We can't givd you a valid answer because there are a lot of informations missing.

One can use a transistir in maby different ways.
* it could be a voltage amplifier
* current amplifier
* temperature sensor
* power switch
* and many other things

Each of the above needs another circuit = connection to other electronic parts like resistors, capacitors, transistors....
Additionally the circuits need to be adjusted to: voltage, current, gain...

So please show us the complete circuit and give us all known informations about voltage and currents..

Klaus

 

[Audioguru]

Each transistor is manufactured by a few maufacturers and they all publish datasheets that are available on the internet. The datasheets specify the maximum allowed voltage, current and power dissipation. They also list minimum, typical and maximum specifications on things like voltage gain, current gain and noise level.

We use specifications on a datasheet and simple arithmetic to select resistor values, not randomly. We do not feed a voltage to the base of a transistor unless it is an emitter-follower, instead we feed a calculated current.

 

[sairfan1]

We use specifications on a datasheet and simple arithmetic to select resistor values, not randomly.

This is what I'm looking for, you guys can assume some common example to describe how to do arithmetic to select resistor or I'm currently trying to control brightness of 2/4/8 LEDs through PWM(some time 7 segments), so I believe in this case I'm using transistor as switch, I also need to use transistor with IR sensor to send/receive and drive small motor with DB series transistor. (At the moment you can give your own example as well. Thanks in advanced.)

 

[KlausST]

Hi,

I don´t know what to tell you .. as long as you give no useful informations.

The most common formula for us is Ohms´law: R = U / I.

It tells you how resistance, voltage and current depend on each other.

We use this formula to calculate base resistance, emitter resistance, load resistance ... with the values of voltage and current.
And we use the same formula to calculate voltage when we know current and resistance.. or to calculate current when we know voltage and resistance.

**
But my post#2 is still true:

So please show us the complete circuit and give us all known informations about voltage and currents..

Klaus

 

[Audioguru]

We cannot suggest an LED circuit unless we know the voltage or color of the LEDs and what current you want in each LED. LEDs can each have their own current-limiting resistor and use a low supply voltage of 3V to 5V depending on the voltage/color of the LEDs, or fours sets of two LEDs in series with a current-limiting resistor for each set of two LEDs, or two sets of four LEDs in series and a resistor for each set, or all 8 LEDs in series with a current-limiting resistor.

Most 5mm diameter LEDs are spec'd and are pretty bright at 20mA so if you want to switch 8 LEDs and use your 5V/1A power supply then the current will be 8 x 20mA= 160mA but a weak BC547 shows trouble above 50mA in its datasheet.

Take your pick and we will show you how easy it is to calculate the resistor values.

A motor has a suggested operating voltage and a maximum stalled current that we need to know to make a circuit to drive it.