EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: almamater on August 10, 2021, 12:30:36 pm
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Sorry for the basic question... but I want to control a circuit (on/off) with a transistor. I need to know two things, the base resistor and if it is necessary a resistor at the collector or not..
The circuit is very simple, i have one 555 that activates a transistor that turns on another circuit with some LEDs and a buzzer (total ~40mA).
(https://i.postimg.cc/K1NsVLNL/Transistor.png) (https://postimg.cc/K1NsVLNL)
My question is about to find the correct value of Rb, I would use one BC547B or BC548C.
In the case of BC547B, and according to the datasheet (https://pt.mouser.com/datasheet/2/308/1/BC550_D-2310266.pdf), this calculations are corret?
Rb= hFE x (VI - VBE) / IC
Rb=200? x (10-0.9) / 0.04 (40mA)
Rb=41850
A resistance of about 40K?
and should i put another resistor between the collector and the circuit\load?
Thank you!!
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It depends on whether "the 40mA circuit" simply needs to be powered on with 12V or whatever its power supply is or needs to have exactly 40mA regulated through it by the BC547.
If it's just switching then you are right, although lower resistance and more base current wouldn't harm unless minimizing power consumption is a priority (then a FET would be better anyway).
If it's regulation then there are probably better ways, because β is quite unreliable (varies from part to part and with internal transistor temperature).
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Is the "40 mA circuit" one that contains an actual current limiter (like a resistor in series with an LED), or a circuit that requires 40 mA but does not limit it?
As stated above, you cannot trust beta of an individual BJT, but the minimum value in the manufacturer's data sheet can be trusted. This makes the base resistor easy to calculate, using the worst-case (lower limit) of beta.
However, the "short circuit" collector current will vary with beta, and may destroy the device (with high Vce and high Ic simultaneously).
If your 40 mA circuit is approximately a constant-voltage device (such as a naked LED), then you must supply the current limit (like a resistor) in the collector circuit.
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Your design is not so good. Why see photo.
Don‘t believe me but Musaomi Suzuki. Who is he?
Boss of accuphase, writing a book regarded as a bible
of hifi. He is my teacher. :)
Musaomi Suzuki鈴木雅臣
If you need to drive a load at high current, Rc
no needed and a darington structure required.
To low current, led, needed as a limiter.
photo tells you how to convert an amp into a switch.
Rb:R= Rb/(Rb+R)×Vcc = 1.5~2v