Electronics > Beginners
PNP transistor switching voltage
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danners430:

--- Quote from: Hero999 on August 13, 2018, 09:55:18 pm ---
--- Quote from: rx8pilot on August 13, 2018, 08:17:18 pm ---It may help to see a visual representation of the current flowing in a transistor.

PS: The important detail is to see that a BJT transistor is controlled by the flow of electrons (current). A MOSFET transistor is controlled by the buildup of electrons (voltage).

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For the sake of simplicity, in a saturated switch yes, but the reality is much more complex. To illustrate this, try building the circuit I posted, using a 470R resistor for the load. Apply a 20kHz signal to the base of Q1 and look at the voltage across the load with an oscilloscope. Now try removing R2, since it's theoretically not needed and merely interrupting the base current should stop the collector current, but notice how much longer it now takes for Q2 to turn off? The base-emitter junction holds a considerable charge, which is responsible for keeping the transistor on.


--- Quote from: danners430 on August 13, 2018, 07:16:23 pm ---Hey guys, sorry to have neglected this - very, very busy afternoon all of a sudden!!

I've already had a look in terms of transistor specs, but here are the requirements since you asked so nicely [emoji13]

The system is to switch a model railway signal, which can only be done on the high side, as they use a common negative. Each transistor will be switching a maximum of 2 LEDs, usually just 1, from a 12V supply, and switched using a microcontroller.

VCC = 12V
Vmcu = 5V
Il = 20mA
Rl = 600r
Ib = 5mA

Hope this helps :-)

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Then in the circuit I posted, R1 can be 3k3.

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That seems ideal - I'll get KiCAD out tomorrow and work out some diagrams :-
Cheers guys!!

Sent from my ONEPLUS A3003 using Tapatalk

Audioguru:
Every little transistor saturates very well when the base current is 1/10th the collector current so 2mA of base current is fine.
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