[HELP] Transistor current value...calculation

[scdvom]

Hi, can anyone show me the way to calculate the output current on collector, 178mA?
[It's better to include what will happen when 5V is turning off]

Thanks for helping!!!

 

[keith1200rs]

It is simply controlled by the current gain of the transistor. You lose 0.7V roughly across the base-emitter leaving 4.3V across R1 so base current is roughly 4.3/2k = 2.15mA. The current in the collector then depends on the transistor current gain. If the gain was 100 then the collector current would be 100x2.15mA = 215mA. That would change if the transistor could saturate, but 215mA in 1.8 ohms is only 0.39V.

I guess you have simulated it and so the collector current is slightly different to my rough calculation.

Keith.

 

[leozuo]

(5V-0.7V(VBE))/R1=Ib
Ic=Ib*beta.
find beta from datasheet.

 

[scdvom]

thanks for you two helping.
but inside the datasheet, there got few value of beta, which one should i choosed?
here is the datahseet link,


https://www.fairchildsemi.com/ds/2N/2N3904.pdf


thanks!!

 

[keith1200rs]

thanks for you two helping.
but inside the datasheet, there got few value of beta, which one should i choosed?
here is the datahseet link,

It doesn't really matter, in a way. The current gain of transistors varies enormously so designs have to take that in to account. So, while it might be of academic interest, you cannot design a circuit to generate a specific current in that way. You have to accept that the current could be maybe 100mA to 600mA depending on the beta, temperature and the specific transistor.

What are you trying to achieve?

Keith.

 

[scdvom]

ok.thanks for your helping.
what I going to do is just trying to amplify the output current and voltage from few Amp and 5V to 160mAmp and 9V.
ermm..1 more equestion.

what will happen if I remove the 1.8ohm (R2). whats the purpose to put it inside?
thanks

 

[keith1200rs]

ok.thanks for your helping.
what I going to do is just trying to amplify the output current and voltage from few Amp and 5V to 160mAmp and 9V.

I assume you mean "... from a few mA to 160mA..."?

What is the resistance of the load you are wanting to drive? 1.8 ohms?

How precise do you need the 160mA?

Keith.

 

[scdvom]

Hi keith,

ya, from few mA to 160m-200mA with 1.8ohms

can tell me whats the problems i encounter?
thx

 

[keith1200rs]

You haven't said what precision you need for your current or what the load resistance is, but a crude circuit is attached. Really you need more current gain than a single transistor if you want the circuit to generate 160mA without being too dependant on the transistor beta and without drawing too much current from your 5V signal so I have drawn two. A single MOSFET would be another option. A far more accurate circuit can be made by using an opamp.

Keith.

 

[leozuo]

op amp with feedback will be a good choice if precision is needed.