How Current Limitation is happening in the circuit???

[MrAl]

Hi all,

I found this circuit in google, Where they have used BD139 for current limiting function.

For example: now the LM317 produces 12V output, It has to pass through BD139(we assume that it is working in saturation), then drop between collector will be 0.2V. That means Vce=0.2V, Then a potentiometer is present between Collector to emitter terminal which takes this 0.2V.

1)How this 0.2V will forward bias base t0 emitter junction which is 0.7V? how current limiting is happening?

2) now the voltage has been reduced t0 11.8V and that will be given TIP3055, that takes minimum of 1V between base and emitter.
now output will be 10.8V, and input should be (12V+3V for regulator) 15 V for obtaining 10.8V. is it correct?

thank you all.

Hi,

That's an interesting circuit, but unfortunately that seems that is all that is good about it.
Without answering you question just yet, it looks like the connections with the 2N3055 destroys the voltage regulation capabilities of the LM317 due to a lack of feedback that comes from the very output, which is the emitter.  I do not think I would want to use this circuit at all for that reason alone.  Why use a circuit that is not as good at regulating as other circuits that are similar.
There also seems to be no good reason for using that BD139 transistor.  It appears to just drop voltage.  Then there is also a 1N4007 diode in series with that, which drops even more voltage.
Then there is another 1N4007 diode that drops voltage getting to the fan.  That reduces air flow to the heat sink(s), maybe to keep the fan quieter?

This is after taking a quick look without doing any analysis just yet.  You could verify if this is true or not with a simulation.

[ommsiva]

Hence we need 5V headroom voltage in this design.

For example: now the LM317 produces 12V output(17V input), It has to pass through BD139(we assume that it is working in saturation), then drop between collector will be 0.2V. That means Vce=0.2V, Then a potentiometer is present between Collector to emitter terminal which takes this 0.2V.

1)How this 0.2V will forward bias base t0 emitter junction which is 0.7V? how current limiting is happening?
2) In what configuration the transistor is connected?

[Zero999]

What have you being doing for the last 7 months? The deadline for your project must be drawing near by now.

It's a bad circuit. as mentioned above.

The correct way to do this is to put a PNP transistor, to bypass the LM317, when the current exceeds a certain threshold. It will only add 0.8V to the drop-out voltage.

[MrAl]

Hence we need 5V headroom voltage in this design.

For example: now the LM317 produces 12V output(17V input), It has to pass through BD139(we assume that it is working in saturation), then drop between collector will be 0.2V. That means Vce=0.2V, Then a potentiometer is present between Collector to emitter terminal which takes this 0.2V.

1)How this 0.2V will forward bias base t0 emitter junction which is 0.7V? how current limiting is happening?
2) In what configuration the transistor is connected?

Hello again,

Why do you care about current limiting in this circuit if it is an inferior design?
The output transistor works as a voltage follower, but just like other transistor voltage followers, it's imperfect which means we lose some aspects of the ability to regulate the output.
As I was saying, it's an interesting design because it makes a simple current booster for the LM317.  A voltage follower cannot increase the voltage at the emitter, but it can increase the current though the emitter.  That's the current boost.
I believe the design can be improved but you would have to find a way to regulate the output voltage.  Just guessing for now, maybe connect the emitter to the voltage adjustment divider with the pot.  Just a guess for now I did not evaluate this yet.

As to current limit, it's going to be the same as with any of these.  When the LM317 puts out it's max current, that's it, the 2N3055 gets as much base current as it's ever going to get, and that limits the total output current which is the sum of the LM317 output and the current though the collector of the 2N3055.  If the Beta of the transistor is 10 and the LM317 innate current limit is 1 amp, then the total output current will be 11 amps.  If the gain of the transistor is 20 and the LM317 current limit is 1 amp, then the max output current is 21 amps.

So it's an interesting idea but needs improvement.  My guess is that connecting the emitter to the adjustment divider might help with the voltage regulation, although the current limit set point is only a rough estimate because of the unspecified transistor Beta, which can vary.  In some cases that is good, in others not good at all.  Doing that the other stuff like that extra 1N4007 diode can be removed as well as the BD139 transistor I would think because they would no longer be needed in the circuit.

[ommsiva]

Hi to all,

I went through the TL494 for designing the same regulated power supply. From the data sheet , it is found the design for output voltage of 5V@ 10 ampere.

1) how to design for variable power supply  ,  Whether inductor calculation will differ?

2) how to design inductor . Give me some references.


Thank you all.

[Zero999]

The TL494 is a switched mode power supply.
https://www.ti.com/lit/ds/symlink/tl494.pd

Adding a potentiometer to the reference voltage and connecting it to the input of the error amplifier will result in a variable output voltage.

The inductor doesn't need to be changed, but it just won't be optimal, at the extremes of output voltage.

Have you built anything yet?

When is the project due?

[ommsiva]

no sir, not started.

its my interest to built this.

[ommsiva]

Hi to all,

This was the circuit i build using LM723. It was doing both voltage as well current control.

Some month ago i connected a DC motor across it to check. Due to back Emf , the circuit has failed.

Please give me some advice to protect the the RPS.