BJT as electronics switch

[bananasiong]

No. Look at the datasheet. At a collector current of 3A and a base current of 300mA, the base-emitter voltage is typically about 1.0V and could be as high as 1.5V. At 6A it is higher.

Where is it? Under 'ON characteristics > Base-emitter On viltage' ?? I see that it is 2Vdc.. Or under the graph? I still don't know which part of the datasheet to read..

Wrong. Of course the base current is extremely important. If it is more than 1/10th of the collector current then the base-emitter junction might fail. If it is less then the transistor might not turn on hard enough then it will have a voltage across it and a high current through it which makes a lot of heat in it.

So, you mean the base current must be 1/10th of the collector current, not more than or less than that?

I don't see why you're messing around with BJTs, a MOSFET is probably more suited to your application.

I want to learn more about BJT first, since I've learned some about BJT before. I will learn MOSFET after this.

 

[audioguru]

Where is it?

The written specs are its worst but are still are guaranteed to be the maximum.
The graphs show the typical spec's but each transistor could be more or be less.

So, you mean the base current must be 1/10th of the collector current, not more than or less than that?

That is what all manufacturers recommend for best performance and good reliability.

 

[bananasiong]

The written specs are its worst but are still are guaranteed to be the maximum.
The graphs show the typical spec's but each transistor could be more or be less.

What do you mean by its worst?


I couldn't find the graph from the datasheet that i found here:
https://www.electro-tech-online.com/custompdfs/2006/09/TIP41A-DPDF-2.pdf

but there is another graph that i think carry the same information:
the Vbe when Ic is 3A, Vce is 4V is 1V. right? (the red one is my edition).

So, to turn the load on at 3A for tip41, the base current has to be 0.3A, and 1V, right?
and.. is the base current always 1/10th of the collector current?

 

[audioguru]

Each spec for the same kind of a transistor is a range of numbers, some of the transistors will be very good and others not very good.
The saturation voltage of a transistor is important because if it is high then the transistor gets hot and needs a bigger and more expensive heatsink to survive.
For a TIP41 transistor with 6A of collector current and 600mA of base current, a "good" transistor has a collector-emitter saturation voltage of 450mV or less and therefore needs just a small heatsink, and the "worst" one has 1.5V for its saturation voltage and will melt without a pretty big heatsink.

EDIT: Max saturation voltage corrected

 

[bananasiong]

For a TIP41 transistor with 6A of collector current and 600mA of base current, a "good" transistor has a collector-emitter saturation voltage of 450mV or less and therefore needs just a small heatsink, and the "worst" one has 2.0V for its saturation voltage and will melt without a pretty big heatsink.

But the datasheet won't show the 'good', then how are we going to know it? As mentioned before, the datasheet only shows the 'worst', but I see that the collector-emitter saturation voltage is only 1.5V but not 2.0V (red circled of the attachment). I'm not sure whether my reading skill in datasheet is correct or not, please correct me if I'm wrong.

And, note the blue circled part, what is DC current gain? The Collector-Emitter voltage I can see is 4V. I don't understand this part.

 

[audioguru]

But the datasheet won't show the 'good', then how are we going to know it?

It doesn't matter how good a transistor saturates. It is nice to know the typical saturation voltage as shown with a curve on a chart so you can calculate how well a circuit will work. The minimum ratings (worst) are given so that you can design the circuit to always work with any transistor of that part number.

As mentioned before, the datasheet only shows the 'worst', but I see that the collector-emitter saturation voltage is only 1.5V but not 2.0V (red circled of the attachment).

With a 6A collector current and a 600mA base current, the max c-e saturation voltage is 1.5V. I was wrong before when I said 2V.

And, note the blue circled part, what is DC current gain? The Collector-Emitter voltage I can see is 4V. I don't understand this part.

The current gain changes a little when the collector-emitter voltage is changed, so they say its range with Vce= 4.0V. The current gain with a collector current of 300mA is guaranteed to be a minimum of 30. The current gain with a collector current of 3A is somewhere from 15 to 75. They can't make them exactly the same, some are better than others.

 

[bananasiong]

With a 6A collector current and a 600mA base current, the max c-e saturation voltage is 1.5V. I was wrong before when I said 2V.

Is the Vce controllable? I mean, so that it doesn't go over its max. Maybe using resistor or what??

The current gain changes a little when the collector-emitter voltage is changed, so they say its range with Vce= 4.0V. The current gain with a collector current of 300mA is guaranteed to be a minimum of 30. The current gain with a collector current of 3A is somewhere from 15 to 75. They can't make them exactly the same, some are better than others.

When the Vce=4.0V and Ic=3A, the current gain will be between 15 and 75, but i thought the Vce max is only 1.5V? Or they are different things?

 

[audioguru]

Is the Vce controllable?

Of course Vce is controllable. It is controlled by the amount of base current your circuit is giving the transistor.

Vce is the lowest when a high base current turns on the transistor as hard as it can. It is typically a very low voltage for most transistors but some are not so good and have the max saturated Vce which is still pretty low.

A transistor's amount of base current times its current gain determines its collector current. When your circuit gives the transistor a low base current then the collector current becomes low and then the voltage drop across the collector resistor is low so the collector voltage is high. When your circuit gives the transistor a high base current then the collector current becomes high and then the voltage drop across the collector resistor is high so the collector voltage is low.

 

[bananasiong]

Is this correct?
Vcc = Vce + IcRc


So, the amount of Ic is determined by Ib. Usually the base is connected to the output of the microcontroller, but how do I know how much current does it give?

From the attachment, If the Ib is 500mA, how much is the Ic? 2.5A or 5A? Why there are two at the same base current?

 

[akg]

Is this correct?
Vcc = Vce + IcRc

yes provided there is no Re

So, the amount of Ic is determined by Ib. Usually the base is connected to the output of the microcontroller, but how do I know how much current does it give?

don't forget that the micro will see only the EB input impedence , which would be very low , and can load the o/p of the micro . a darlington / or mosfet will be useful .

 

[audioguru]

Is this correct?
Vcc = Vce + IcRc

Yes, but you have it written like you think the current plus the transistor's saturation voltage creates the Vcc.

So, the amount of Ic is determined by Ib. Usually the base is connected to the output of the microcontroller, but how do I know how much current does it give?

A microcontroller cannot give a few hundred milliamps, so another transistor must be added. Then a resistor in between the microcontroller's output and the 1st transistor determines the base current.

From the attachment, If the Ib is 500mA, how much is the Ic? 2.5A or 5A? Why there are two at the same base current?

The Ic is determined by the load's resistance and the power supply voltage. This graph shows the transistor being turned on hard, with different Ic and different Ib.
They show typical values of saturation voltages, a low saturation voltage when the collector current is low, and a higher saturation voltage when the collector current is higher.

 

[bananasiong]

Yes, but you have it written like you think the current plus the transistor's saturation voltage creates the Vcc.

So, how do you write it?

A microcontroller cannot give a few hundred milliamps, so another transistor must be added. Then a resistor in between the microcontroller's output and the 1st transistor determines the base current.

...means, the output of the microcontroller connected to the base of the transistor A through a resistor, collector of A is Vcc, then the emitter of A is connected to the base of transistor B, which the load is connected to the collector, right?
But we still need to know the output current of the microcontroller isn't it? Then how to measure it? I don't think connect the ammeter from the output to GND is a good way..

don't forget that the micro will see only the EB input impedence , which would be very low , and can load the o/p of the micro . a darlington / or mosfet will be useful .

what is EB input inmedence?
what is darlington?

 

[akg]

...means, the output of the microcontroller connected to the base of the transistor A through a resistor, collector of A is Vcc, then the emitter of A is connected to the base of transistor B, which the load is connected to the collector, right?

that is darlington connection
https://en.wikipedia.org/wiki/Darlington_pair
it has high input impedence and high current amplification

But we still need to know the output current of the microcontroller isn't it? Then how to measure it? I don't think connect the ammeter from the output to GND is a good way..

simple .. refer the max o/p current of the micro in its datasheet .

 

[bananasiong]

simple .. refer the max o/p current of the micro in its datasheet .

How if there is no output current from the datasheet? How to measure it?



How about pnp type? I mean to switch pnp type transistor. The current flow through the load is determined by the current flowing out from the base right? Everything is same as npn type except only this?

 

[akg]

How if there is no output current from the datasheet? How to measure it?

that's one of the purpose of datasheet , it will be there.

How about pnp type? I mean to switch pnp type transistor. The current flow through the load is determined by the current flowing out from the base right? Everything is same as npn type except only this?

the current and voltage polarities will reverse .

 

[bananasiong]

the current and voltage polarities will reverse .

the base current right?
the current flow in from tthe emitter and the load connect from the collector to GND. Anything more different?

from the graph above, it shows that the collector current (max) is 6A, so the base current should be 600mA, and the voltage is 2V. but as i know, the microcontroller's output is always 5V right?

 

[akg]

from the graph above, it shows that the collector current (max) is 6A, so the base current should be 600mA, and the voltage is 2V. but as i know, the microcontroller's output is always 5V right?

micros o/p is either 5 or 0v , and we add a small resistor between the o/p pin and the base of transistor .

 

[audioguru]

Look at the datasheet for your microcontroller. Its max output current is about only 25mA. You need another transistor to drive 600mA into the power transistor. The base current of the new transistor will be about 20mA or less and is limited with a series resistor.

The output voltage of the microcontroller is 5V without any current, and drops when loaded. It might be only 2.5V with a 25mA load.

 

[bananasiong]

Look at the datasheet for your microcontroller. Its max output current is about only 25mA. You need another transistor to drive 600mA into the power transistor. The base current of the new transistor will be about 20mA or less and is limited with a series resistor.

The output voltage of the microcontroller is 5V without any current, and drops when loaded. It might be only 2.5V with a 25mA load.

The outout of the microcontroller, 25mA turns the 1st transistor on, 600mA. Its emitter is then connected to the base of 2nd transistor to turn it on with 6A.
You said that a series resistor should be added between the output of the microcontroller and the base of the 1st transistor, how much resistance should it be? 100Ohm? 1kOhm?

 

[Nigel Goodwin]

The outout of the microcontroller, 25mA turns the 1st transistor on, 600mA. Its emitter is then connected to the base of 2nd transistor to turn it on with 6A.
You said that a series resistor should be added between the output of the microcontroller and the base of the 1st transistor, how much resistance should it be? 100Ohm? 1kOhm?

I've highlighted your relevent value! - use ohms law!.