Hi all.

Please forgive me if this is a simple answer but I am still learning about transistors.


My question is, "What would be a simple way to create a PNP Base/Emitter voltage of 5 volts, whilst using a 12 volt battery?"

With 12v across an NPN, an LM7805 could do the job, but I was wondering how about when a PNP Collector/Emitter voltage is 12v?


James

 

Hi all.

Please forgive me if this is a simple answer but I am still learning about transistors.


My question is, "What would be a simple way to create a PNP Base/Emitter voltage of 5 volts, whilst using a 12 volt battery?"

With 12v across an NPN, an LM7805 could do the job, but I was wondering how about when a PNP Collector/Emitter voltage is 12v?


James

There are many possibilities but we’d need more info to make a choice. Let’s start with why you need that voltage. The transistor will turn on at 11.3V and you’ll need to limit the base current to avoid burning up the transistor.

A schematic would save a lot of questions.

 

Hi all.

Please forgive me if this is a simple answer but I am still learning about transistors.


My question is, "What would be a simple way to create a PNP Base/Emitter voltage of 5 volts, whilst using a 12 volt battery?"

With 12v across an NPN, an LM7805 could do the job, but I was wondering how about when a PNP Collector/Emitter voltage is 12v?


James

I am not sure what you mean. In a normal configuration of a transistor in the conducting mode the base to emitter voltage is very close to 0.6V. The polarity depends on which type in use NPN or PNP.

 

With 12v across an NPN, an LM7805 could do the job

The 7805 generates 5V all by itself.
What's the purpose of the NPN?

 

PNP or NPN, it doesn't matter.

What are you trying to do?

 

You will need a 6.2V zener, and you should in series with the PNP as below:
Vin E(P) → C(P) Vout
Vin B(N) → (-)6.2V Zd(+) → C(P) Vout
0.7V = Vbe
Vo = 12V - (0.7V+6.2V) = 5.1V

The output voltage of this kind of simple PNP circuit will be have a little changing when the draw current of load is changing.

 

Thanks for the replies.

It was more just a theoretical question, however I can't seem to upload a schematic.

If you imagine a 12 volt, 3A fan and perhaps a MJ11015 transistor (with base resistor), which is controlled by a manual switch coming from the 12 volt battery, then you will see that the circuit is missing a component to reduce/limit the voltage, as there is a 5 volt maximum across the emitter/base junction.


I had been wondering about a zener.

 

Is this 5 volts you are talking about the base emitter reverse voltage rating ?

Les.

 

Thanks for the replies.

It was more just a theoretical question, however I can't seem to upload a schematic.

If you imagine a 12 volt, 3A fan and perhaps a MJ11015 transistor (with base resistor), which is controlled by a manual switch coming from the 12 volt battery, then you will see that the circuit is missing a component to reduce/limit the voltage, as there is a 5 volt maximum across the emitter/base junction.

I had been wondering about a zener.

Have you got any message?
You can compress it as .zip file and upload it, I can decompress(unzip) it to the original circuit.

 

Is this 5 volts you are talking about the base emitter reverse voltage rating ?

Les.

Yes

 

Have you got any message?
You can compress it as .zip file and upload it, I can decompress(unzip) it to the original circuit.

Can't seem to upload anything.

The window appears with the 'browse' option, but with only a 'cancel' button... ?

 

Did you click on the browse button ? It should let you navigate to the file you want to upload.

Les.

 

Can't seem to upload anything.

The window appears with the 'browse' option, but with only a 'cancel' button... ?

Did you see the button shown as "Upload a File" and it is at the right bottom of the message box.
Or you want to use the FireFox to upload the file.

 

Yes. When I select the 'Upload a File', a box appears and I can browse to my selected archive.

When selected, it shows 3 movings bars in the corner of the window however nothing uploads.

Have tried different formats and various files.

It is not so important. I believe the question was answered with the Zener option.


Many thanks.

 

I just tried to uploaded the file and it is ok.

 

If you imagine a 12 volt, 3A fan and perhaps a MJ11015 transistor (with base resistor), which is controlled by a manual switch coming from the 12 volt battery, then you will see that the circuit is missing a component to reduce/limit the voltage, as there is a 5 volt maximum across the emitter/base junction.


I had been wondering about a zener.

You don't need anything as fancy as a zener, a simple current-limiting resistor will do. The base-emitter resistance is very low once you overcome the two diode-drops of ~0.6V each (two because your transistor is a darlington). With no resistor and unlimited available current, >5V applied to the base would indeed roast the transistor due to excessive base-emitter current. But suppose you place a 100Ω resistor in series with the base, and connect the other end of that resistor to ground. Now the maximum current that could flow is limited to (12 - 1.2)/100 = 0.108A. Almost all the voltage drop occurs across the resistor and the base remains at a safe 1.2V and a current well below the 1A rating.

So just put your manual switch and a resistor in series between the base and ground, and you'll have manual control over the transistor. Use a resistor that allows about 1% of the load requirement to flow on base-emitter. That will ensure that the transistor is turned fully on.

 

This is what you expect to see when you click on "Browse"

You would then click on the file you want to upload.


Is this the configuration you are talking about to use a PNP transistor as a switch ?


If so the base emitter junction is never reverse biased. If you are using a normal transistor (Not a darlington.) then you would choose R1 to give a base current about one tenth of the load current. R2 is not really required it just reduces any leakage current. I would choose about 10 times the value of R1. It was more important in the days of germanium transistors as they had higher leakage currents than silicon transistors.

Les.

 

This is what you expect to see when you click on "Browse"
View attachment 169028
You would then click on the file you want to upload.

Is this the configuration you are talking about to use a PNP transistor as a switch ?
View attachment 169029

If so the base emitter junction is never reverse biased. If you are using a normal transistor (Not a darlington.) then you would choose R1 to give a base current about one tenth of the load current. R2 is not really required it just reduces any leakage current. I would choose about 10 times the value of R1. It was more important in the days of germanium transistors as they had higher leakage currents than silicon transistors.

Les.

What he want is from 12V to 5V.

 

What he want is from 12V to 5V.

Maybe. I though he was concerned about the absolute maximum rating of the transistor per the datasheet, which lists the max base-emitter voltage at 5V.

 

Is this the configuration you are talking about to use a PNP transistor as a switch ?

This is exactly what I was envisioning in my post #16. Nothing more than what you have drawn is needed to protect the base of the transistor from the 5V max base-emitter voltage.