Discussion in "General help Guidance and Discussion" started by    prajwalsnh    Apr 30, 2013.
Tue Apr 30 2013, 05:39 PM
HI frenz,

Iv completed my final year project. Though i have finished it successfully there are some

questions for which i don have an answer. I need help from people who r willing to

make me understand.

My first question is the transistor shown in below circuit to control relay

is working as an amplifier. am i right?? If so its in common emitter mode. why do they

connect 1k resistor in series with base?? Y only 1k why not other value??

[ Edited Tue Apr 30 2013, 05:41 PM ]
Tue Apr 30 2013, 05:42 PM
sorry the circuit is this not the one shown above
Wed May 01 2013, 05:33 AM
Transistor is not used as amplifier but as a switch. We operate transistor in saturated (on state) and cut-off (off state) mode only.

And why 10K or 1K it actually depends on type of transistor used and collector current you want. Following are the exact parameters that defines your resistor value:

1. hfe/gain
2. Collector current
3. Drive voltage (voltage at pin)
4. Max drive current controller Pin can provide.

resistance value = V - 0.7 / C / hfe
where V is Supply voltage at pin (~5v)
C is collector current you want
hfe is gain of transistor (see datasheet for this value)
Tags Transistor Base Resistor CalculatorTransistor as Switchbase resistor for transistor switch
Thu May 02 2013, 05:44 AM
@Ajay Bhargav

I saw the datasheet of BC547B. I have used that transistor in my ckt... In data sheet its given as dc current gain as 100mA. If i substitute that value then i will get the answer as 43 which means 43 ohms... i guess im wrong in my calculation.
Sat May 04 2013, 05:09 AM
well gain is never in mA as gain is a ratio of current. Pleas make sure you have seen the right value. It should be mentioned with hfe. If you have a multimeter you can place your transistor in DMM and test its hfe.

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