Dimensioning Resistor in Transistor Circuit

[malmoit]

Hi there!

Would be grateful for help regarding the following problem.

Translated from my textbook the problem sounds:


Quote-------------------

"By means of a photosensor and a comparator 5V is given when darkness,

and 0V when daylight. The signal generated is maximum 10mA. A lamp, which

consumes 800mA at 24V shall light when it is dark. To accomplish this, a

transistor swith is used. Your task is to calculate and dimension the

resistance, and draw the corresponding circuit.

The transistor withstands a current of maximum 1A, .i.e. Ic,max = 1A, and has hFE = 150 times (i.e. the beta-value).

Also U(BE) ~ 0,6V and U(sat) ~ 0V.

The circuit does not have to include the photocell and the comparator,

instead only the darkness/light signal is assumed." -----------end quote


I have attached a picture-file where my idea of how the circuit should look like, is shown. The answer to this problem according to the book is:

440 < Rb < 810 Ohm


Would be very thankful for some help here. .....

Thanks!

 

[ericgibbs]

hi,

Assuming that the transistor saturates at 800mA/150hfe, the base current would be 5.3mA to cause saturation.

If Vsignal is 5V and Vbe is 0.6 then for Ib= 5.3mA thats [5-0.6]/0.0053 = 830R

If the Vsignal can only supply 10mA then Rb = 4.4/0.01 = 440R.

Does that help.?

EDIT: I would use a 470R base resistor.

 

[audioguru]

The book and Eric are wrong.

hFE is used when a transistor has plenty of collector to emitter voltage like an amplifier. All datasheets for transistors show the max saturation voltage when the base current is much, much more than the hFE would make it.

For this transistor to saturate then the base resistor should have 80mA and be 55 ohms.

A 2N3904 little transistor has a typical hFE of 230 but its max saturation voltage is shown when its base current is 1/10th the collector current.

EDIT:
The source can supply only 10mA so the transistor will not saturate, the load will have reduced voltage and current and the transistor might burn up.
The load is an incandescent light bulb? Then it draws up to 8A when it is cold. It won't light with this transistor with not enough base current.

 

[ericgibbs]

I suspect the author of the text book is talking about a theoretical transistor, not a 2N3904.

I am aware of the decrease in hFE versus hfe.

The book also gave a Vsat 0f 0V, which I am sure you know is also theoretical for a transistor.

I would say to the OP that the calculations I gave are within the parameters supplied in the book.

 

[malmoit]

Thanks Eric!

Just as Eric is writing the author is assuming a theoretical transistor. Also his answers are the right ones. Thanks Eric!

Have a good one you all!

/Pierre

 

[MrAl]

Hi there,

Sometimes when a student posts a question there are certain restrictions
in place that are not there with a real life question. Sometimes the teacher
even wants the answer in a certain form, such as "1/3" instead of 0.3333 or so.
This means sometimes to answer a students question more info might be
needed, and in this case more info would have helped too.
I assumed we were working with an ideal transistor too because of the
0v Vsat spec, which spells "IDEAL DEVICE" to me.
I would have probably calculated 825 ohms, which also doesnt fit the
supposed answer range. We also dont have any more data on the
transistor so we cant look up the gain at high collector currents.

 

[audioguru]

A real transistor does not saturate when it has a high current gain and therefore has a low base current. It needs to have a lot more base current to make certain that excessive base current forward-biasses the base-collector junction.

 

[ericgibbs]

A real transistor does not saturate when it has a high current gain and therefore has a low base current. It needs to have a lot more base current to make certain that excessive base current forward-biasses the base-collector junction.

If the question from the OP was based on a 'real' transistor, the values I calculated would have been different, having taken into account the change in hFE at high currents.

So, as the transistor in the question is theoretical,your 2N3904 example is not relevant.

The base resistor value calculated for the theoretical transistor is still correct, so whats the point you are trying to make.?

 

[audioguru]

Hi Eric,
High collector current is not the issue. The transistor in the book has a high current gain at the required current.

Any transistor needs much more base current than its current gain would calculate for it to saturate.
I used a little 2N3904 transistor as an example because its base current for it to saturate is so much higher than calculated from its very high current gain.

A BD139 little power transistor has a typical current gain of 90 with a collector current of 500mA and a collector to emitter voltage of 2V (its base current is only 5.6mA and it is not saturated).
But its max saturation voltage of 0.5V with a collector current of 500mA is when its base current is 50mA.

 

[ericgibbs]

Hi Eric,
High collector current is not the issue. The transistor in the book has a high current gain at the required current.

Any transistor needs much more base current than its current gain would calculate for it to saturate.
I used a little 2N3904 transistor as an example because its base current for it to saturate is so much higher than calculated from its very high current gain.

A BD139 little power transistor has a typical current gain of 90 with a collector current of 500mA and a collector to emitter voltage of 2V (its base current is only 5.6mA and it is not saturated).
But its max saturation voltage of 0.5V with a collector current of 500mA is when its base current is 50mA.

hi,
I know that you are keen to point out to newbies the limitations of certain devices, but I'm sure at their early stages of learning some of the information you post is more likely overwhelm them.

I would hope once they have learned how to actually do the base calculations they will either do more self study or come back with more questions.

I also find it hard sometimes just to answer the question.

BTW: more of a technical thing, did you experiment with the LM393 amp version, in seeing how low in value the 0.47uF[0.5uf] could be in order to extend the limits of the -3dB point freq.? before any instability.

 

[audioguru]

I use opamps (TL07x) that have a -3dB cutoff frequency of 31kHz at a gain of 100, not just 20Hz like the comparator circuit. The high bandwidth opamps oscillate when a small capacitance to ground is on their output.

It is interesting that the low bandwidth comparator amplifier has a much higher output resistance than an opamp.

 

[malmoit]

That's right!

Yes, you're absolutely right, Eric. From a newbie point-of-view it's definitely easier to understand the elementaries.