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Transistor function confusion

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skmdmasud

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Hello..
i am a new member and very new to circuits. I built a timer based on CD4060 following this design 24Hr Timer {2} **broken link removed** **broken link removed**

as you can see there is a transistor BC557 which is a PNP type. What I dont understand is when there is no voltage in the Base of the tranaistor it keep on passing the +ve through emitter to collector and when there is voltage in base it stops switching??why?

As we know that transistors without any V in Base is like totally off.

Regards.
 
Hi and Welcome to the ETO forums. The following statements were taken from here. Note how the transistor is used in the circuit.

NPN vs PNP

NPN
1) NPN. If the base is at a higher voltage than the emitter, current flows from collector to emitter.
2) NPN. Small amount of current also flows from base to emitter.
3) NPN. Voltage at base controls amount of current flow through transistor (collector to emitter).

PNP
4) PNP. If the base is at a lower voltage than the emitter, current flows from emitter to collector.
5) PNP. Small amount of current also flows from emitter to base.
6) PNP. Voltage at base controls amount of current flow through transistor (emitter to collector).

Hope that helps
Ron
 
I don't believe that circuit. Why isn't the transistor base drive, via R7 and D1, taken from a Q output of the IC (as shown in the Range table) ?? As shown, it's coupled to gate input pin 11 !
 
I don't believe that circuit. Why isn't the transistor base drive, via R7 and D1, taken from a Q output of the IC (as shown in the Range table) ?? As shown, it's coupled to gate input pin 11 !
It is taken from the desired Q output per the Range table. You select the option you want when you build the circuit.

The connection through the diode to pin 11 stops the oscillator when the selected time is reached.

To expand on Ron's comments, it's the base-emitter voltage that determines whether the transistor is on, not the base voltage to circuit common.
 
Thank you Carl.

The posted circuit was taken from here. They show two versions using a NPN and a PNP. Then as Carl points out the 4K7 base resistor goes to the Q (Range Output) in the table.

Ron
 
Yes, The D1 stops the oscillation which De-energizes the relay.

In a PNP transistor like the diagram shown in the post does it always passes form emitter to collector and stops the flow when there is voltage at the Base.

Every tutorial about transistors says the opposite??
 
I finally found out that its the PNP, NPN that makes the difference.
For those who wants see a simulation
https://www.indiabix.com/electronics-circuits/npn-transistor-bipolar/
https://www.indiabix.com/electronics-circuits/pnp-transistor-bipolar/

and as explainted by Reloadron
PN vs PNP

NPN
1) NPN. If the base is at a higher voltage than the emitter, current flows from collector to emitter.
2) NPN. Small amount of current also flows from base to emitter.
3) NPN. Voltage at base controls amount of current flow through transistor (collector to emitter).

PNP
4) PNP. If the base is at a lower voltage than the emitter, current flows from emitter to collector.
5) PNP. Small amount of current also flows from emitter to base.
6) PNP. Voltage at base controls amount of current flow through transistor (emitter to collector).

Thanks.
Masud.
 
Am i correct
NPN = Negative Positive Negative (base = positive, emitter & collector = negative)
PNP = Positive Negative Positive (base = negative, emitter & collector = positive)

Can i use a PNP transistor with base = positive, emitter & collector = negative, will it burn my transistor and the the other way round for NPN
 
NPN = Negative Positive Negative (base = positive, emitter & collector = negative)
PNP = Positive Negative Positive (base = negative, emitter & collector = positive)
The N and P denote the type of doping of the semiconductor material inside the transistor: not whether you should apply a negative or positive voltage. The transistor should be connected externally as Reloadron indicated.

Can i use a PNP transistor with base = positive, emitter & collector = negative,
Not in most circuits. An exception would be if you were using a junction in a Zener-like way.
will it burn my transistor
Yes, if the applied voltages cause junction breakdown and reverse currents aren't limited.
 
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