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Books => The BJT Transistor Theory => Topic started by: kam on November 14, 2013, 10:46:17 AM
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Ok, I will start designing the very first circuit for the Pair Book. Here are the suggestions:
- DC Transistor Switch
- Transistor Voltage Regulator
The first circuit should be simple (IMO). Do you have any other suggestions???
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I think that DC transistor switch would be a nice start!!!!
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I think both are interesting for starters.
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I would like to see a circuit for motor controls, something for a fan (like the ones used in computers with 3 wires). And how to reverse the direction of rotation.
Thank you
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I would like to see a circuit for motor controls, something for a fan (like the ones used in computers with 3 wires). And how to reverse the direction of rotation.
Well, PC Fans cannot change direction of rotation. I will make make a linear PC Fan controller using only transistors to control the speed of a PC Fan. Maybe a H-bridge for DC motors as well to demonstrate how to change the direction of rotation of a simple DC motor (brushed).
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Well first questions have arrived!!!!!! Please everyone be kind and gentle...... ::) .
1) Why do we have to use a base resistor ?
2)Why our calculations are concentrated for the current that needs the transistor to "turn on" instead of the saturation voltage ?
3)Also in the very first example, the solenoid needs 16.6ma to actuate and we run our calculations on 35ma why ?
Sorry for the amateur questions........ :-[
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1) Because the BJT are controlled by current. We know that our voltage is constant, so we can control the current by using the right resistor value.
2) When we turn the transistor on, the resistance drops to a very low value. This is because it is saturated. If the base resistor value is too high (the current decreases) the BJT won't be saturated anymore. So the resitance won't be so low.
(correct me if i am wrong)
3) It is good practice to use some safety margin in your calculations. But i think in this case it is just a typo followed by copy and paste. (kam?)
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Well, here is what i do not explain clearly in the book. As mein Freund Cheerio said, transistors are current devices. There is no direct link between voltage and transistor operation. I have to clearly explain this in the book. Transistors only amplify CURRENT. The voltage amplification is a sort of "byproduct" as a result of ohm's law: by increasing the current through a resistor (for example the collector resistor) the voltage drop across this resistor is increased.
That being said,we need to find a way to "convert" voltage into current. The easiest way is by using a resistor. Since the supply voltage for the base is constant, we can alter the base current with a series resistor (I=V/R). We need to drop it as low as required. How much? this depends on the collector current requirements and the hfe. Say we want 20mA collector current and the hfe is 100.. the base current must be 20/100=0.2mA or 200uA. So we use the ohms law to calculate what resistor we have to use on the base to allow only 200uA of current thorugh the base:
R=V/I = 5/0.2 = 25KOhm
As for the 16.6 to 35mA, as Cheerio already said, we always overpower the outputs. Since the transistor that i use can provide up to 800mA, there is absolutely no problem to double the current. This is how the 35mA comes out of the 16.6, not a typo ;)
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That being said,we need to find a way to "convert" voltage into current.
How can a resistor convert current into voltage? I do not get it what you mean.
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Ok, you have this resistor that is connected across a current supply. This current supply is able to deliver specific current. Say the resistor is 100 Ohm. Say again that the current supply delivers 100mA of current through the resistor. The voltage drop across the resistor is:
V = I x R = 0.1 x 100 = 10 Volts
Now the current source delivers 50mA. The voltage drop across the resitor is:
V = 0.05 x 100 = 5 Volts
and so on. This is what i mean. If you connect a volt meter across this resistor and you start alter the current through the resistor you will see that any change in the current is "translated" into voltage across the resistor.
We will discuss about this on later circuits with amplifiers, when we discuss how to amplify the input voltage of a signal using transistors (which are actually current devices)
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Very nice explanation......!!!!!! (To both of you!)Waiting for the breadboard circuit to make experiments and measures!!!!
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Very nice explanation......!!!!!! (To both of you!)Waiting for the breadboard circuit to make experiments and measures!!!!
almost ready
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Ok, i've prototyped the first circuit and took some measurements. Next step is to do the calculations. Here is the link:
http://www.pcbheaven.com/userpages/bjt_dc_load_switching_transistor/?topic=worklog&p=1 (http://www.pcbheaven.com/userpages/bjt_dc_load_switching_transistor/?topic=worklog&p=1)
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Ok !!!!!!! FABULOUS!!!!!!!!! Really amazed!!!!! The continuation of the transistor theory is going to be pioneer!!!!!!
(Something that i think that you didn't explain).At the beginning of the calculations you made an educative guess for the hFE which was 250.
This number means that we want to have a current gain of 250 times more?Did we achive it or it was much more (as i think?)(624 times?)
Perhaps i am wrong but i want to clear it to my mind.....
ps Also i would like to ask why we calculate the collector current to IC=35mA although the relay coil needs 16ma to operate???. (Because of the led that was added?)
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this hfe value is typical for this transistor. After many tests and measurements this is the most common value that i come up with. Other transistors may have different hfe though.
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i must say that i didn't understand clearly your answer..... :-[ :-[ :-[ Perhaps my questions were bad?
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hfe is the amplification parameter. take for example a magnifying glass. It has for example 4x magnifications. The "hfe" of the glass is 4. A magnifying glass has a fixed "hfe" whereas a transistor varies. The specific transistor can "magnify" the current within a range of 100 to 600. From my experience i can tell beforehand that the specific transistor will have a magnification range of 200.
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The specific transistor can "magnify" the current within a range of 100 to 600. From my experience i can tell beforehand that the specific transistor will have a magnification range of 200.
This specific parameter is needed in order to start or continue our calculations....because IB = IC / hfe so if i pick up a transistor that i have in my drawer how would i be able to define the hfe because of my lack of experience?
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well... you can consult the datasheet, but for a precise value... you have to make tests. Make the circuit, measure base current, measure collector current, do the division (IC/IB) and there you have you hfe.
Remember, hfe (as all hybrid parameters) are not stable. You may have 2 similar transistors in your drawer, and both may have very different hfe.
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ok i will test my first circuit on a breadboard and i would post my progress and results......
Something else that i would like to ask.....We calculate the collector current for 35ma....the relay needs 16ma...and our calculations were based on 35ma but finally it proved that the relay consumed 16ma....How did that happen?I mean why the coil didn't consumed much more (because the collector current can give double 35ma) as a result the coil to be burnt?What would might have happened if we had used a led instead of a relay coil?
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Say you have a 24V power supply able to supply up to 10amperes of current and you connect this 24V relay, will it burn? No, because:
THE CURRENT DEPENDS ON THE LOAD
So, the transistor is able to provide up to 35ma, but the final current depends on the load. Clear?
Now, if you put an LED instead of the relay, you will have fun! But only for a short while...
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Sometimes the head stucks!!!!!!!SORRY.........
Also i forgot to mention a very important parameter.I wanted to say what would might have happend if the voltage was 3v instead of 24v.
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Then the transistor would saturate and all the current (provided by the 3V) would go through the relay, but of course it would not arm as it needs 24V
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so far so good......Another intresting calculation might be switching on-off leds with a transistor...Shall we analyze a circuit together?
Transistor as switch again ,but with different kind of load....
LEDS!!!!!!! Controlled by PWM....... (Corrected..... mistake by rush)
opinions?
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what do you mean that the current is not stable???