Current and voltage equivalent calculations.

[prabhukaran3]

Dear Guys,
Warm Greetings!

for long days I am having a doubt in Basics of Ohms law Understandings. I am explaining My understanding through one Transistor..

I m using One BC547 transistor. base resistor is 470E. collector resistor is 1K and i have connected one LED with 5v. Emitter is grounded.

Now, Current passes through LED---->Collector resistor--->transistor(coll-Emit)... Approximately current is 20mA. If current is less than 10mA some voltage will available between collector and emitter.. Are my right??? Kindly help me guys...


Thanks in advance...

 

[TommiRouvali]

In your case:

Calculate base current: (5v-0.7v) / 470 ohm = 9.15mA.
Transistor is current amplifier so Ic is hFE * Ib. Datasheet gives hFE 110..800. So Ic is 110 * 9.15 mA = at least 1A. But it is not true as collector resistor limits current. The transistor is saturated. Saturated means that it could conduct more current that is passed thru it. You have in series led (2V), 1k resistor and saturated transistor(~0.3V). so Ic is really (5V-2V-0.3V) / 1000 ohm = 2.7mA.

2.7mA may or may not show on led. You should have 10-20mA thru led.

Above I assumed base resistor is connected to 5v.

If you are trying to dim led, you have way too much base current. Should be uA range. (Ic / hFE)
You have too big collector resistor you are never going to have more thar 5v-2v/1000ohm= 3mA current with that.

Are you trying to use transistor as switch here (saturate it) or use linear religion (dimming)? Tell me so I can better guide you.

 

[prabhukaran3]

Dear tommiRouvali,
Greetings!
You are correct i am connecting base to 5v through 470E resistor.. your calculations and regarding post is very use full for me... and again i am having a doubt while reading data sheet...

In bc547

** Baes-Emitter voltage is About 5V.. It means what? However we are not going to connect with straightly to 5v.. If we put resistor between Base and Logic 5V,0.7V only apearing at base to groung.. Remaining voltage has been dropped out by resistor.. Then y they said like so in datasheet(As 5V)...
**Collector to Emitter Voltage is 45V Dc.. It means what? Voltage difference between collector and emitter is 45V? If it so Collector current supports for 100mA at 45V? I thing it is not currect..

** Continuous collector current 100mA means, it is maximum current we attain through BC547 or by adjusting base resistor we can get more current above 100mA...

All the times i m having some conflicts to choose a component and designing part.. How can i improve?

Thanks in advance...

 

[TommiRouvali]

You are reading Absolute Maximum Ratings section. If you go over these values your transistor will break.
'Base-Emitter' is actually emitter-base voltage, and means reverse voltage over base-emitter 'diode'. I do not think you are going to hit that very easily. You are right about forward current and resistor taking most of the voltage.
Collector to Emitter voltage means break down voltage. BC547 cannot handle more even when not conducting. Consider this as limitation of your power supply voltage.
Collector current means you cannot have more than 100mA flow thru collector or it will break. Design your circuit so that this won't hapen. (with collector resistor or some other way)
One thing to watch out is 'Collector Power Dissipation':
Lets say we have supply voltage 20V. And collector resistor 200 ohm, no leds etc. When transistor is not conducting, it has collector-emitter voltage of 20V we are safe, it is under 45V. When we saturate transistor collector emitter voltage is near zero, maybe 0.3V. Current is just below 100mA, so we are just safe. Power that remain on transistor is 0.3V*100mA=30mW. So we are safe from that too. But what hapens when we drive transistor in half conducting stage? with base current (50mA/Hfe). Now collector current is 50mA and collector-emitter voltage 10V that gives us power 10V*50mA=500mW. So we are in limit there.

 

[prabhukaran3]

Dear TommiRouvali,

You are great. Understood the concept very Well.... Thanks a lot

One think need to confirm... Collector current 100mA is at 45V(absolute maximum).. if we reduce the voltage level,can we get more current.. Means insead of 45v dc i m passing only 5V means what will happen.. that same 100mA only i can attain or something more...

 

[jayanth.devarayanadurga]

You can get 100 mA for 45V and also for 5V. It depends upon Ib and Rc.

 

[TommiRouvali]

No, those values are unrelated. You cannot have over 45V no matter what current. And you cannot have more than 100mA current, no matter what voltage. If you want more, find another transistor.

 

[prabhukaran3]

Hummmm! Now i m very clear in those ratings...

Can i get 5V 5A from 230V 2A current? By your input i came to one understanding level.. I think i am correct.. no matter in voltage, current is the actual think...

 

[jayanth.devarayanadurga]

You should consider max voltage and max collector current od transistor. It should never exceed max ratings. Ie = Ib + Ic. Ic depends upon Ib. If you need 100 mA Ic at Vcc = 5V then calculate Ib according to it. Is the transistor used as switch or amplifier?

transistor doesn't generate current. If your 5V supply to transistor is able to deliver 5A or more then if Rc is 1 Ohms (5V/5A = 1 Ohms) approx 5A will flow through Collector but Collector max Ic should be more than this current.

 

[prabhukaran3]

Dear friends...!
Greetings. Why in data sheet they mentioned Base-Emitter voltage is 5v However we are passing the pulse via resistor. What will happen if we give pulse at 10V using 1K base resistor. If we measure voltage base and ground while transistor is ON state i am getting 0.7V. Then what is Base-Emitter voltage?
In LED whatever the voltage we passed through 1K resistor LED is glowing and Voltage drop across the LED is 2V. Will same think happen on Transistor too?

Thanks in advance

Cheers guys.. Happy weekend

 

[LvW]

for long days I am having a doubt in Basics of Ohms law Understandings. I am explaining My understanding through one Transistor..

I m using One BC547 transistor. base resistor is 470E. collector resistor is 1K and i have connected one LED with 5v. Emitter is grounded.

Now, Current passes through LED---->Collector resistor--->transistor(coll-Emit)... Approximately current is 20mA. If current is less than 10mA some voltage will available between collector and emitter.. Are my right??? Kindly help me guys...

Thanks in advance...

Prabhukaran, your explanation is not good enough (for me).
Why don`t you show us your circuit?
Questions:
What means "base resistor 470E"?
LED connected at which node?
Why 20 mA? Measured? Simulated? Design value?