Constant current source

[julian403]

What do you think about the next current source? It has to drive a current, in RL, of 0.1 [A] the most exactly it can be. For that, I put a transitor which has a Vbe= Ic * 6.8Ω ; so if the current of 0.1 [A] goes up, the transistor drives and the currents goes down.

The hfe of the transistor are 256.



I do not know if I must put an Rc in the Q2


PD: I saw the other topic of the constant current source and I think this topic are different

 

[crutschow]

That current in that circuit will vary about 0.3% per degree C change in temperature due to the Vbe voltage variation of Q2 with temperature.
Is that acceptable?

P.S. What's the purpose of R1? That will reduce the accuracy (feedback gain) of the circuit.

 

[erikl]

View attachment 115846 I do not know if I must put an Rc in the Q2

Sure, otherwise you would short the power supply by Q2.

P.S. What's the purpose of R1? That will reduce the accuracy (feedback gain) of the circuit.

Right, but I think it's intended to limit the power dissipation for this TO-92 (250 °C/W_j-a) transistor.

 

[crutschow]

Originally Posted by crutschow
P.S. What's the purpose of R1? That will reduce the accuracy (feedback gain) of the circuit.

Right, but I think it's intended to limit the power dissipation for this TO-92 (250 °C/W_j-a) transistor.

If that's its purpose then it would be better in the collector circuit so it won't affect the feedback gain.

 

[julian403]

If that's its purpose then it would be better in the collector circuit so it won't affect the feedback gain.

the purpose of this circuit it's to measure the resistor value of RL which goes from 0.1Ω to 10 Ω, and with a current of 0.1 [A], measuring it whit a voltimeter :bsdetector: , with a value of 10 [mV] there is a resistor of 0.1 [Ω]; a value of 22 [mV] a resistor of 0.22 [Ω], etc . I put the resistor of 47 Ω in emitter because it's reduce the temperature effect with temperature. But I want to reduce it more, so I use the transistor Q2.

Do you think it's so necesary the Q2? I put it because the teacher said that I have to be a good source and it has to be a better thing that just one transistor in emitter commun mode. So, I thought to put Q2 :razz:

This would be the final circuit.



But I do not know what value must be R2. Because I do not know what value would have Ic2 (Q2).

 

[erikl]

This would be the final circuit.

View attachment 115848

But I do not know what value must be R2. Because I do not know what value would have Ic2 (Q2).

R2 is totally unnecessary. With the shown value it would just reduce your constant current to ≈75mA.

And crutschow's above post is absolutely correct: Put R1 into the collector branch.

 

[julian403]

R2 is totally unnecessary. With the shown value it would just reduce your constant current to ≈75mA.

But, without resistor R2. How many current will flow by Q2? when the Ic1 will pass the 0.1 [A]

 

[erikl]

But, without resistor R2. How many current will flow by Q2? when the Ic1 will pass the 0.1 [A]

Let's say you put R1 into the drain branch of Q1, as suggested (and better). R2=0 . At current limit, base of Q2 resp. collector of Q1 will be at ≈ 2*0.65 = 1.3V. At nom. power supply of 9V , there are 7.7V over R3, which drives a current of 0.7mA through R3. Let's say Q1 has B=500 , so it needs 0.2 mA base current for Ic1=100mA, Q2 takes the rest, hence Ic2=0.5mA - neglectable against Ic1.