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Power Dissipation Across A Transistor In Cutoff

Akshatha Venkatesh

Jan 14, 2017
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I'm not going to do all your homework for you.

You know how the circuit works now, you should be able to do it yourself.
Can you please help me with one more thing , how does the transistor T201(PNP) fully turn on ? I mean what are the conditions for the T201(PNP,left side) to fully turn on ?
 

Audioguru

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The datasheet for every transistor shows how much base current is needed to cause it to fully turn on (saturation).
Since it is PNP then the base voltage must be a base-emitter voltage drop more negative than the emitter voltage for it to turn on.
 

Akshatha Venkatesh

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Audioguru

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It is simple to calculate the base current. Don't you know that the base-emitter voltage for a modern silicon transistor is about 0.7V? Don't you know Ohm's Law? The datasheet says the range of the resistance in series with the base and the range of input voltage needed for the transistor to turn on but not be fully saturated.
 

Akshatha Venkatesh

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It is simple to calculate the base current. Don't you know that the base-emitter voltage for a modern silicon transistor is about 0.7V? Don't you know Ohm's Law? The datasheet says the range of the resistance in series with the base and the range of input voltage needed for the transistor to turn on but not be fully saturated.
http://assets.nexperia.com/documents/data-sheet/PEMD2_PIMD2_PUMD2.pdf

Ok , I think I know the resistance in series with the base , but where is the range of input voltage needed for the transistor to turn on but not be fully saturated given in the datasheet , what is it called ?
 

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Is it the On state input voltage ?

See page 7 of the datasheet.

what does it mean exactly?

See figure 6 on page 8.

can you please explain ? so how do i find out the base current and hence the collector current

Even if it's a transistor, the base current doesn't determine the collector current.

These devices are designed for use with digital inputs. The behavior is defined for input voltage above some value and below another.

The presence of the resistors means that you need to allow for the voltage which appears at the input, and the loading of the voltage divider by the Vbe of the transistor which will affect the voltage across the Rbe. Any current through Rbe will be totally wasted.

You don't seem to understand how transistors work. What sort of homework is this? What do you actually need to do?
 

Akshatha Venkatesh

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See page 7 of the datasheet.



See figure 6 on page 8.



Even if it's a transistor, the base current doesn't determine the collector current.

These devices are designed for use with digital inputs. The behavior is defined for input voltage above some value and below another.

The presence of the resistors means that you need to allow for the voltage which appears at the input, and the loading of the voltage divider by the Vbe of the transistor which will affect the voltage across the Rbe. Any current through Rbe will be totally wasted.

You don't seem to understand how transistors work. What sort of homework is this? What do you actually need to do?
I know that the on state input voltage is on page 7 of the datasheet. but what does it mean ? what does "on state input voltage" mean ? and what does "off-state input voltage" mean ?
 

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The on state voltage is the minimum voltage which must be applied to the input terminal (the "base") with respect to the emitter terminal to cause the on-state as defined in the datasheet to be achieved.

The off state voltage is the maximum voltage which can be applied to the input terminal (the "base") with respect to the emitter terminal to allow the off-state as defined in the datasheet to be maintained.
 

Akshatha Venkatesh

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The on state voltage is the minimum voltage which must be applied to the input terminal (the "base") with respect to the emitter terminal to cause the on-state as defined in the datasheet to be achieved.

The off state voltage is the maximum voltage which can be applied to the input terminal (the "base") with respect to the emitter terminal to allow the off-state as defined in the datasheet to be maintained.
Thank You.
My homework is that, I'm trying to find the range of voltage during which the transistors are on(saturated) and the range of currents also. What I understand is that the collector current Ic of T201(PNP) flows through the base of the other two transistors(T201-npn & T200) , so I'm trying to determine the range of the forward current that flows through the diode, then the transistor and then through the other two transistors and finally to the output.
 

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Well, you now know that there are two valid ranges of voltages which should appear between the emitter and the base(ish) terminal. First find out what range of input voltages cause these to happen.

Then ensure that the other transistors remain in the valid on and off state throughout these ranges, then determine which voltages are most important to determine min and max current. You almost certainly need to calculate this from right to left (i.e. current through the diode last).
 

Akshatha Venkatesh

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Well, you now know that there are two valid ranges of voltages which should appear between the emitter and the base(ish) terminal. First find out what range of input voltages cause these to happen.

Then ensure that the other transistors remain in the valid on and off state throughout these ranges, then determine which voltages are most important to determine min and max current. You almost certainly need to calculate this from right to left (i.e. current through the diode last).
what is this "two valid ranges of voltages" that you are talking abt ?
 

Akshatha Venkatesh

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analysis of the circuit given below. I've analysed it , can you guys check it once ?
Thank You.
 

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It's really a shame you couldn't have provided some of that information earlier. I'm not sure if it affects any answers I've given, but it certainly would have helped provide more certain answers.

I'm no sure you can say R200 and TM+ act as a voltage divider. It all depends on what's connected to TM+ -- is it any more complex than a thermistor to ground?

It begs the question of whether the voltage at TM+ can ever exceed Vdd5, but I'll let you sort that out.

The first sentence of the third paragraph doesn't really make sense. Perhaps you mean the input to the microcontroller? If this was the sole reason, there are far easier ways of protecting the input (the dual diode and a resistor would be sufficient)

You continue using T201 for two devices. You should use T201a and T201b.

The term "dummy load" in paragraph 3 is incorrect.

The 4th paragraph is written in a hard-to-understand manner. What happens when the voltage is in a normal range? 12V is not the magic voltage. It is Vdd + the turn on voltage of T201a.

The last sentence is correct.
 

Akshatha Venkatesh

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It's really a shame you couldn't have provided some of that information earlier. I'm not sure if it affects any answers I've given, but it certainly would have helped provide more certain answers.

I'm no sure you can say R200 and TM+ act as a voltage divider. It all depends on what's connected to TM+ -- is it any more complex than a thermistor to ground?

It begs the question of whether the voltage at TM+ can ever exceed Vdd5, but I'll let you sort that out.

The first sentence of the third paragraph doesn't really make sense. Perhaps you mean the input to the microcontroller? If this was the sole reason, there are far easier ways of protecting the input (the dual diode and a resistor would be sufficient)

You continue using T201 for two devices. You should use T201a and T201b.

The term "dummy load" in paragraph 3 is incorrect.

The 4th paragraph is written in a hard-to-understand manner. What happens when the voltage is in a normal range? 12V is not the magic voltage. It is Vdd + the turn on voltage of T201a.

The last sentence is correct.
12 V is just an example that I have taken. It could be anything ranging from 6 to 13.5 V.
What should I use instead of the word "dummy load" ?
In the normal condition, none of the transistors turn on. The voltage at point K1A2 will always be less than 5V and this voltage will be available at output 11G and the microcontroller pin will be safe.

Let me take TM+ as 8 V, so the voltage at point K1A2 will be 8V too, but at point P204 , it will be a little less say 7.3. , since the Veb= 7.3-5 = 2.3 >0.7 , the transistor saturates , is this analysis correct ?
suppose the transistor is saturated, it behaves like a closed switch but still there will be some voltage drop across it , say 0.2V (in general). so now what is the voltage that is available at point P203 ?
 

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What should I use instead of the word "dummy load" ?

I don't know, perhaps you should just say it pulls the output low.

is this analysis correct ?

Vbe of 0.7 is not what you need to worry about. Remember that there are not just transistors. You need to use the minimum on voltage from the datasheet.

, it behaves like a closed switch but still there will be some voltage drop across it , say 0.2V (in general). so now what is the voltage that is available at point P203 ?

Vcc5 - 0.2V from memory.
 
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