Simple PNP switching problem

[audioguru]

What is low level transistor?

It is a transistor with a very low input current like the 1st transistor in a darlington.

Resistors should be added in between the base and the collector?

No, that would turn it on. I said a resistor is needed to turn off the transistor.

I've neer seen this type of circuit before. How about NPN type?

They are called darlington transistors and have a very high current gain. Yes, NPN types are also available. Some are power darlington transistors and others are little darlington transistors.

 

[Russlk]

All bipolar transistors have leakage from collector to base (Icb). Without a resistor from base to emitter, that current becomes Ibe which is multiplied by beta in the collector. This heats the transistor causing the leakage to increase and so on until the transistor is fried.

 

[bananasiong]

I've seen a few datasheet, most of the have this circuit. It means the resistors are built in?

All bipolar transistors have leakage from collector to base (Icb). Without a resistor from base to emitter, that current becomes Ibe which is multiplied by beta in the collector. This heats the transistor causing the leakage to increase and so on until the transistor is fried.

They are called darlington transistors and have a very high current gain. Yes, NPN types are also available. Some are power darlington transistors and others are little darlington transistors.

If I make my own Darlington pair, should I include the resistors by following the value of the manufacturer? Is the diode used for protecting the transistors?

The datasheet shows that the Base-Emitter on voltage is 2.5Vdc, but how come you said it is 11V?

I want to know that, usually the datasheet from which company is more preferable? I see some of them don't have much information.

Thanks!

 

[audioguru]

I've seen a few datasheet, most of the have this circuit. It means the resistors are built in?

Of course.

If I make my own Darlington pair, should I include the resistors by following the value of the manufacturer?

If your circuit doesn't have resistors to turn off the transistors then either they won't turn off or they will turn off very slowly. I said that already.

Is the diode used for protecting the transistors?

A protection diode should be across a coil, not the transistor.

The datasheet shows that the Base-Emitter on voltage is 2.5Vdc, but how come you said it is 11V?

The MAX base voltage is 2.5V at a collector current of 3A. The typical voltage is much less and is shown on curves in the datasheet. It is almost as low as 1V with a collector current of 0.1A. The emitter connects to +12V so the base is +11V when it is almost turned off.

I want to know that, usually the datasheet from which company is more preferable? I see some of them don't have much information.

I try to get datasheets from the company that invents the part. The TIP series were invented by Texas Instruments. The LM series were invented by National Semiconductor.

 

[bananasiong]

The typical voltage is much less and is shown on curves in the datasheet.

I'm not sure whether the graph I refer to is correct or not: https://www.onsemi.com/pub/Collateral/TIP120-D.PDF#search="tip125 datasheet"
Is it in Figure 10 from this datasheet? If not, can you provide one with that curve?

It is almost as low as 1V with a collector current of 0.1A. The emitter connects to +12V so the base is +11V when it is almost turned off.

What voltage? collector? emitter?

Thanks!

 

[audioguru]

I'm not sure whether the graph I refer to is correct or not: https://www.onsemi.com/pub/Collateral/TIP120-D.PDF#search="tip125 datasheet"
Is it in Figure 10 from this datasheet?

Yes, figure 10 shows the typical base-emitter voltage is 1.25V at a collector current of 100mA, and it is a little less with less collector current.

What voltage? collector? emitter?

I said that the emitter connects to +12V, so the base is about +11V when it is almost turned off.

 

[DirtyLude]

The result:

The button on the front of the case cycles through the different lighting modes. The bottom CCFL's only come on during special occasions, though, or they bug me.

**broken link removed**

 

[bananasiong]

Yes, figure 10 shows the typical base-emitter voltage is 1.25V at a collector current of 100mA, and it is a little less with less collector current.
I said that the emitter connects to +12V, so the base is about +11V when it is almost turned off.

Let me digest.... When the collector current is around 100mA the Vbe is 1.25V (assumed to be 1V). 12V is supplied to the emitter, drop 1Vbe, the rest of 11V is flowing out from the base. So 11V is needed to supply in to the base to against it.???

 

[audioguru]

It is a PNP darlington transistor. PNP transistors have the emitter at the positive supply and current in a resistor from the base to ground turns it on. Voltage doesn't flow out, it is called base current.

The darlington's typical base-emitter voltage is 1V at very low collector current, 1.25V at 100mA collector current and could be its max of 2.5V at a high 3A collector current. So the base voltage is +9.5V to +11V when the supply is +12V.
The darlington has resistors across the base-emitter junctions to turn it off and to turn it off quickly when there isn't base current.

Transistors are turned on with base current, not voltage. The base-emitter of a transistor has a diode voltage drop of 0.5V at a very low current and up to 1.5V with a very high current. A darlington has two base-emitter diodes in series. There just needs to be enough voltage available for current to flow in the base resistor.

 

[bananasiong]

The darlington's typical base-emitter voltage is 1V at very low collector current, 1.25V at 100mA collector current and could be its max of 2.5V at a high 3A collector current. So the base voltage is +9.5V to +11V when the supply is +12V.

The table says that the Vbe is 2.5V when the collector current is 3A, but it is different under the curve? At 3A of collector current, Vbe is around 1.8V?
Base current turn off the transistor right? But the voltage needs to be enough to perform this, it is 11V right? How if the output of the microcontroller is only 5V?

 

[audioguru]

The table says that the Vbe is 2.5V when the collector current is 3A, but it is different under the curve? At 3A of collector current, Vbe is around 1.8V?

They cannot make them all exactly the same. Some are very good and others are weak. The table says the 2.5V max that some darlingtons will have and the curve shows the 1.8V typical that most will have.

Base current turn off the transistor right?

No. Base current turns a transistor on. Zero base current turns it off. Heat can make base leakage current so a resistor is needed between the base and emitter to bypass the leakage current so that the transistor can turn off.

How if the output of the microcontroller is only 5V?

Then the microcontroller controls an NPN transistor and a resistor at its collector controls the PNP transistor at a higher voltage.

 

[bananasiong]

Then the microcontroller controls an NPN transistor and a resistor at its collector controls the PNP transistor at a higher voltage.

How if just control the whole thing with only a darlington NPN? Won't it be easier?

 

[audioguru]

How if just control the whole thing with only a darlington NPN? Won't it be easier?

Of course, if you can connect the other terminal of the load to a positive voltage.
But this circuit uses a PNP darlington transistor to switch the positive wire of the load, maybe because its other wire is grounded.

 

[bananasiong]

If NPN (same spec with the PNP tip1255) is used, the base need that much to be turned on?

 

[audioguru]

If NPN (same spec with the PNP tip1255) is used, the base need that much to be turned on?

What is a TIP1255? Is it a power transistor or is it a power darlington transistor?
Just look at its datasheet to see how much base current is needed for a certain amount of collector current.
Most transistors are spec'd to saturate well with a base current of 1/10th the collector current.
Most darlington transistors are spec'd to saturate well with a base current of 1/250th of the collector current.

 

[bananasiong]

What is a TIP1255? Is it a power transistor or is it a power darlington transistor?
Just look at its datasheet to see how much base current is needed for a certain amount of collector current.
Most transistors are spec'd to saturate well with a base current of 1/10th the collector current.
Most darlington transistors are spec'd to saturate well with a base current of 1/250th of the collector current.

Not 1255, is TIP125. I know about the current, but you said that there just need to be enough the base voltage for the current to flow.

Let's take an example from this datasheet:
https://www.electro-tech-online.com/custompdfs/2006/10/TIP120-DPDF.pdf
from the figure 10, when the collector current is around 100mA, the Vbe is around 1V for tip125. If the supply to emitter is 12V, the base need to have 11V to turn it off, right?
Let's say for tip120, NPN darlington. With the supply of 12V also, To turn it on, lets say 100mA at collector, the base current is 1/125th of it which is very small, then how about the voltage that is enough for it? Is it around 1.25V?

Thanks for your explaination..

 

[Hayato]

Not 1255, is TIP125. I know about the current, but you said that there just need to be enough the base voltage for the current to flow.

Let's take an example from this datasheet:
https://www.electro-tech-online.com/custompdfs/2006/10/TIP120-DPDF-1.pdf
from the figure 10, when the collector current is around 100mA, the Vbe is around 1V for tip125. If the supply to emitter is 12V, the base need to have 11V to turn it off, right?
Let's say for tip120, NPN darlington. With the supply of 12V also, To turn it on, lets say 100mA at collector, the base current is 1/125th of it which is very small, then how about the voltage that is enough for it? Is it around 1.25V?

Thanks for your explaination..

Sorry Guru, but I'll enter the conversation...

Banana, you are making a switching application, so it's interesting to make your TIP125 fully conduct. Just apply a voltage equal higher than the typical VBE on voltage (about 1.8V) and be happy.

Your :mu:Controller have an high output voltage >= 2.5V, good enough to drive the TIP125 and turn your led on. (As the TIP125 has a current gain of 1000, I believe that the :mu:C output current (arround tens of mA) is good enough too)

 

[audioguru]

from the figure 10, when the collector current is around 100mA, the Vbe is around 1V for tip125. If the supply to emitter is 12V, the base need to have 11V to turn it off, right?

The Vbe is about 1.25V for a 100mA collector current. Extend the graph and you will see that the collector current will be extremey low with a Vbe of only 1V. Therefore when the Vbe is less than 1V then the darlington transistor is off.
Never mind the voltage, just stop the base current and the darlington transistor will be off.

Let's say for tip120, NPN darlington. With the supply of 12V also, To turn it on, lets say 100mA at collector, the base current is 1/125th of it which is very small, then how about the voltage that is enough for it? Is it around 1.25V?

Never mind the voltage, at a collector current of 3A then the Vbe could be as high as 2.5V. Just provide enough base current to turn it on, then stop the base current to turn it off.

 

[bananasiong]

The Vbe is about 1.25V for a 100mA collector current. Extend the graph and you will see that the collector current will be extremey low with a Vbe of only 1V. Therefore when the Vbe is less than 1V then the darlington transistor is off.
Never mind the voltage, just stop the base current and the darlington transistor will be off.

For the TIP125 PNP transistor, if I want to turn it off, you said that just stop the base current, but the base is always connected to the microcontroller through a resistor, output 0 will turn it on and output 1 will turn it off.
I just wonder why base voltage is 11V but not according to the curve when the supply is 12V. The internal resistor will do nothing with this right? Just to fasterize the switching?
Thanks for willing to answer my questions patiently

 

[bananasiong]

Hi,
I've read the thread named Transistor Operation here:
https://www.electro-tech-online.com/threads/transistor-operation.33/#post167035

Rules for Transistor biasing

Rule 1. Base and Emitter of PN junction must be in forward bias and the voltage difference between these two pins must be within 0.1 to 1 volts.

Rule 2. Base and Collector of PN junction must be in reverse bias and the voltage difference between these two pins may be of any range.

Rule 3. For the transistor to be in biasing in PNP transistor, the highest positive voltage must be given to emitter and in base the voltage must be lower than emitter in the range 0.1 to 1 volts and the lowest voltage must be given to the collector.
For e.g.:if the emitter voltage is 12V then base must be in the range of 11.1V to 11.9V and then the lowest voltage say 0V in the collector.

Rule 4. For the transistor to be in biasing in NPN transistor, the highest voltage must be given to the collector,and the lowest voltage to the emitter then in base the voltage must be greater in the range of 0.1V to 1V than the emitter.
For e.g.: if the collector voltage is 12V then say 3V to the emitter and then the base voltage must be between 3.1v to 3.9V.

If these rules are right, then my questions are answered. Thank you audioguru, thank you everyone.

So, to switch a PNP with the supply of 12V, I need to add a NPN with its collector to the base of the PNP, its base to microcontroller through a resistor, and its emitter to GND. Am I right?