Transistor Switching, urgent help required

[muashr]

Hi,

I have two circuits shown in the figures. In the first circuit (with npn) the lamp must glow but in the other circuit (with pnp) it must not glow. However the lamp glows in both cases which is not understandable. Can anyone explain what actually going on and why the lamp glows when it should not glow?

 

[Ratchit]

muashr,

In figure 2, the transistor is certainly biased off. What is the max voltage rating of the collector to base with that transistor? Is it possible that the breakdown voltage is exceeded, and the charge is flowing from the collector to the base when you don't want it to do so? Reduce the collector voltage from 12 volts to a lower voltage and see what happens.

Ratch

 

[alec_t]

Double-check the pinout of the pnp transistor; it may be wired incorrectly.
And what makes your request 'urgent'? Is this overdue homework?

 

[muashr]

muashr,

In figure 2, the transistor is certainly biased off. What is the max voltage rating of the collector to base with that transistor? Is it possible that the breakdown voltage is exceeded, and the charge is flowing from the collector to the base when you don't want it to do so? Reduce the collector voltage from 12 volts to a lower voltage and see what happens.

Ratch

Hi,

The PNP transistor used is BC 160.

 

[muashr]

Double-check the pinout of the pnp transistor; it may be wired incorrectly.
And what makes your request 'urgent'? Is this overdue homework?

Pinout is correct. This is not the homework.

 

[Ratchit]

Hi,

The PNP transistor used is BC 160.

muashr,

OK, so what is the specification I asked about?

Edit:
Never mind, I looked it up and it is 40 volts. Perhaps the transistor is defective. If you have a power supply, see if you can get diode action between the base and collector. Limit your current to about 5 ma and see if the current exists from the collect to the base. Then reverse the leads and see if the current is close to zero. That should check if your collector-base junction is OK or not.

Ratch

 

[crutschow]

You do know that the circuit in Figure 2. when operating correctly, can never be turned on. For that you would need to reverse polarity on the +5V supply on the left.

 

[Ratchit]

You do know that the circuit in Figure 2. when operating correctly, can never be turned on. For that you would need to reverse polarity on the +5V supply on the left.

Yes, and also reverse the collector voltage on the right.

Ratch

 

[crosslakeguy]

Its one of those PPN transistors,

 

[Ratchit]

Yes, and also reverse the collector voltage on the right.

Edit:
Whoa! All of us are fools. That transistor and circuit is operating correctly. In figure 2. the collector is more positive than the base. That means the collector-base junction is forward biased, so naturally the light will illuminate. I sure feel stupid for not seeing that earlier.

Ratch

 

[muashr]

Edit:
Whoa! All of us are fools. That transistor and circuit is operating correctly. In figure 2. the collector is more positive than the base. That means the collector-base junction is forward biased, so naturally the light will illuminate. I sure feel stupid for not seeing that earlier.

Ratch

Hi,

The task has to do with the difference in NPN and PNP transistors operation with respect to biasing. That is, the voltage biasing for NPN transistor must not work for the PNP transistor and therefore the lamp must not be on for the PNP trasistor with the biasing of NPN transistor.
Therefore, the question remains the same as to what biasing be used for NPN which doesnot allow PNP transistor to let lamp switch on?

 

[crosslakeguy]

sounds like a school thing?? so I am going to give an Idea that may be what you are looking for? you have 12v and 5v available. are you looking to perhaps use the difference between the 5v and 12v to run your light at around 1/2 (7v) of the voltage, I which the 5v becomes the ground and the difference between (bias) 5v and 12v (7v) is your positive supply? ground not used at all 5v becomes ground... what is allowed to change in the circuit? anything? kinda leaving out an important fact. the circuit must remain the same only changing voltage? or can the whole circuit be changed using the same components? when is the light supposed to light? ever? or is the end of the story......... why does it not go off...... the end.

 

[audioguru]

Figure 2 uses a PNP transistor connected backwards so its collector-base junction is forward-biased like a diode and its reverse-biased base-emitter has avalanche breakdown at 5V to 7V like a zener diode. When the pot is turned down then the lamp gets brighter.

 

[Ratchit]

Hi,

The task has to do with the difference in NPN and PNP transistors operation with respect to biasing. That is, the voltage biasing for NPN transistor must not work for the PNP transistor and therefore the lamp must not be on for the PNP trasistor with the biasing of NPN transistor.
Therefore, the question remains the same as to what biasing be used for NPN which doesnot allow PNP transistor to let lamp switch on?

muashr,

You previously asked a question, specifically why the lamp in figure 2 was lit when you thought it should not be. I believe I answered that question. The voltage biasing for figure 1 does not work for figure 2, because the lamp stays even though the PNP transistor is not biased into its active region. So the lamp is on for both configurations for different reasons. I am not sure what you are trying to do here, but now you have an explanation of what is happening in each case. It is up to you to know what you want and design a circuit accordingly.

Ratch

 

[crosslakeguy]

gonna have to answer 5v positive ground...... Is this a test?

 

[muashr]

muashr,

You previously asked a question, specifically why the lamp in figure 2 was lit when you thought it should not be. I believe I answered that question. The voltage biasing for figure 1 does not work for figure 2, because the lamp stays even though the PNP transistor is not biased into its active region. So the lamp is on for both configurations for different reasons. I am not sure what you are trying to do here, but now you have an explanation of what is happening in each case. It is up to you to know what you want and design a circuit accordingly.

Ratch

I got confused because according to the manual of the kit I have the lamp in the circuit with PNP must not glow. And since I discovered the situation otherwise I thought that I might have done something wrong. However, thank you for your comments.

 

[crosslakeguy]

I got confused because according to the manual of the kit I have the lamp in the circuit with PNP must not glow. And since I discovered the situation otherwise I thought that I might have done something wrong. However, thank you for your comments.

its great learning! my favorite quote by "me" is, the more I learn, the more I realize I don't understand. ...... never be afraid to ask questions.

 

[tvtech]

Edit:
Whoa! All of us are fools. That transistor and circuit is operating correctly. In figure 2. the collector is more positive than the base. That means the collector-base junction is forward biased, so naturally the light will illuminate. I sure feel stupid for not seeing that earlier.

Ratch

Hi Ratch

So good to see you being just a normal guy who sometimes screws up. You have changed recently for the better.

I remember your long winded posts and making simple things complicated a while back. Definitely a "like" for this post of yours from me
Please keep it up as you have taught me something too by keeping things simple.

All the best,
tvtech

 

[Ratchit]

Hi Ratch

So good to see you being just a normal guy who sometimes screws up. You have changed recently for the better.

I remember your long winded posts and making simple things complicated a while back. Definitely a "like" for this post of yours from me
Please keep it up as you have taught me something too by keeping things simple.

All the best,
tvtech

I sometimes make mistakes, which I either admit or correct. I have not changed at all. Long windedness is a preception. I always post for correctness, not brevity. Some things in this world are complicated, and have to be explained. Look for more of the same if necessary.

Ratch

 

[tvtech]

I sometimes make mistakes, which I either admit or correct. I have not changed at all. Long windedness is a preception. I always post for correctness, not brevity. Some things in this world are complicated, and have to be explained. Look for more of the same if necessary.

Ratch

I love you Ratch....and have huge respect for you.

But you know what.....you have changed from the Ratch I first saw here. The Ratch that knew everything. And made simple things complicated just to impress. You have become a really nice clued up person.

And that is wonderful. You have a wealth of knowledge to share. And I am all ears to hear it.

Hows that?

Regards,
tvtech