Constant current limiting circuit

[FreshmanNewbie]

Can someone help me understand the working of this circuit:




My questions :

1. Please explain me how this circuit will work in its entire operation range.
2. I am not able to understand the purpose of the Zener diode and the 10K parallel combination. What purpose do they actually serve in this circuit?
3. The Q2101 transistor does not have a base resistor to limit its current. Isn't this circuit wrong?

 

[KlausST]

Hi,

1) R2101 || R2109 pull the base of Q2101 HIGH. BJT becomes conductive. Load current is flowing through R2104. causing voltage drop according load current.
This pulls the base of Q2102 HIGH (current limiting mode) and thus the base of Q2101 is regulated low to maintan constant load current.
2) This is a power dissipation limiter for Q2101 in case of low load resistance...reducing load current.
3) R2101 || R2109 limit the current

Klaus

 

[danadakk]

2 ) Zener is not connected across 2101 and 2109, its just in series with 2102 and 2110.


Regards, Dana.

 

[KlausST]

Hi:

Power dissipation limiter:
0...5V across Q2101_CE --> I_load about 35mA
5..10.5V across Q2101_CE --> I_load becomes reduced from 35mA to about 3 mA.
10V .. 36V across Q2101_CE -->I_load rises to about 13mA
[Edit: Corrected names "Q2101"]

If my mind calcualtion and circuit analysis is right. Please run a simulation to verify this.

Klaus

 

[FreshmanNewbie]

Hi:

Power dissipation limiter:
0...5V across Q2102_CE --> I_load about 35mA
5..10.5V across Q2012_CE --> I_load becomes reduced from 35mA to about 3 mA.
10V .. 36V across Q2012_CE -->I_load rises to about 13mA

If my mind calcualtion and circuit analysis is right. Please run a simulation to verify this.

Klaus

Thank you. Could you please explain in a little more detail step by step on how this happens and how the base current and Vce of both the transistors go through during this circuit operating condition?

 

[KlausST]

Hi

As already said: Use a simulation tool.
It shows you all the voltages and currents more precise than I can do.

In short: The current through the Zener pulls up Q2102 base.

Klaus

Added: Sadly there were typos in my post#4 BJT names. I've corrected them now. Hope it did not confuse.

 

[danadakk]

Not sure what this is doing -

Regards, Dana.

 

[KlausST]

Hi,

thanks Dana for taking the time to do the simulation.

In my explanation I referred to
* the "load current" = R5 current (of your circuit)
* Q2101_power_dissipation = Q2 power dissipation (of your circuit).
* the voltage across Q2101 = V_CE of Q2 (of your circuit)

It would be kind if you could spend the time to upload these diagrams.

(I see my load current estimation was a bit off, I expected earlier cutoff of Q2 base)

Klaus

 

[danadakk]

Simetrix simulator, file attached Klaus.

Regards, Dana.

 

[KlausST]

Hi,

Thanks.
I (maybe other members, too) don´t have installed this simulator software.
I just asked for the diagrams ... would be kind of you....

Klaus

 

[danadakk]

Klaus, that is the schematic used by, made by the tool. I do not have
any other doc. Note the sim is, in my opinion, much better user
interface than LTC Spice. It was the main sim used by Analog Devices
until they bought LTC, then they decided, unfortunately, to standardize the
company around LTC Spice.

Regards, Dana.