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Power Dissipation Across A Transistor In Cutoff

Akshatha Venkatesh

Jan 14, 2017
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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.
Is my analysis right in saying that the "if the voltage at TM+ is more than the fixed supply VDD5, the supply could get damaged and hence a load has been added-a transistor T201b , which pulls the output low when is in the saturation mode" ?
 

(*steve*)

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Is my analysis right in saying that the "if the voltage at TM+ is more than the fixed supply VDD5, the supply could get damaged and hence a load has been added-a transistor T201b , which pulls the output low when is in the saturation mode" ?

No, I don't think that is correct.
  1. I don't think the supply could be damaged
  2. "which pulls the output low when is in the saturation mode" is probably incorrect after the word "low"
You need to do more study before attempting this homework question. You are lacking a great deal of fundamental knowledge of things like resistors and transistors.
 

Akshatha Venkatesh

Jan 14, 2017
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No, I don't think that is correct.
  1. I don't think the supply could be damaged
  2. "which pulls the output low when is in the saturation mode" is probably incorrect after the word "low"
You need to do more study before attempting this homework question. You are lacking a great deal of fundamental knowledge of things like resistors and transistors.
Can you please tell me why the transistor T201b has been used ? It is used to indicate that there is a overvoltage condition. But can you please explain to me how?
It would be a great help.
 

Akshatha Venkatesh

Jan 14, 2017
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Can you please tell me why the transistor T201b has been used ? It is used to indicate that there is a overvoltage condition. But can you please explain to me how?
It would be a great help.

When the transistor T201(NPN) is fully on , the emitter pulls the collector to the ground and it acts as a closed switch and a small voltage ,Vce(sat) is present across the collector-emitter and this output (10E) is given to the mcu ,indicating the over voltage condition. This is what I mean by "pulls the output low ". I don't understand why this is wrong. Help please.
 

(*steve*)

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We are just going around in circles.

You are asking questions that have already been asked and answered.
 

Akshatha Venkatesh

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We are just going around in circles.

You are asking questions that have already been asked and answered.
But when I posted my analysis , you told me that the word "dummy load" is not appropriate , and I assumed my analysis was right except for that word. But now I'm confused. Can you please spare a few minutes and help me out ? This is kind of important.
Thank You
 

(*steve*)

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Have you simulated the circuit to see what happens?
 

(*steve*)

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Draw the circuit in falstad again, export it and I will see what you did wrong.
 

(*steve*)

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Hi Steve . Can you pls tell me what I've done wrong ?

Several things...
  1. The red dots indicate where you didn't join nodes correctly (you did that several places)
  2. The input is best as a sawtooth. This is a bit tricky.
I've fixed both of these here.

You can see the voltages on the circuit and on the scope traces. If you stop the simulation you can move your mouse cursor over the scopes to see how the voltages compare. It may be a little easier if you reduce the frequency of the sawtooth (try 3Hz). To do this, right click on the sawtooth, pick "edit", change the frequency, and click on OK. Then reset or restart the simulation.

The behaviour you see is not exactly as predicted by the datasheet, but this is because the datasheet doesn't describe what happens at intermediate voltages and it assumes a much higher collector current.

However the behaviour is, exactly what you would expect, given sufficient experience.
 

Akshatha Venkatesh

Jan 14, 2017
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Several things...
  1. The red dots indicate where you didn't join nodes correctly (you did that several places)
  2. The input is best as a sawtooth. This is a bit tricky.
I've fixed both of these here.

You can see the voltages on the circuit and on the scope traces. If you stop the simulation you can move your mouse cursor over the scopes to see how the voltages compare. It may be a little easier if you reduce the frequency of the sawtooth (try 3Hz). To do this, right click on the sawtooth, pick "edit", change the frequency, and click on OK. Then reset or restart the simulation.

The behaviour you see is not exactly as predicted by the datasheet, but this is because the datasheet doesn't describe what happens at intermediate voltages and it assumes a much higher collector current.

However the behaviour is, exactly what you would expect, given sufficient experience.
Thank you so much. One little doubt, diode "Fwd Voltage @ 1A" =805.904783m . The diode I need has fwd voltage of 715mV at 1mA. So would I be right in editing the diode parameter in the simulated circuit to 715V at 1A ?
 
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