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constant current sink understanding (10-50mA)

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Atom:
Hi to everyone, i need a little constant current sink circuit to put on the output of my bench psu so i can have always a little load for stability(that is not the main reason), but also for the turnoff, caps discharging ecc..

so i found this circuit , it a let's say crude but to me it's completly fine.

correct me if i'm wrong , if i understand correctly the 2 diodes in series generates a v drop of 1,4V .
by substracting the 0.7V of the vbe of the transistor we have a vdrop across R1 of 0,7V
we can calculate the current flowing with ohm's law so I=V/r.

i tried the circuit on a breadboard and it worked flawlessly but since the diodes v ref changes and the v drop of the transistor isn't constant the current taken varies let's say from 20ma to 30ma (also the transistor heats up)


then i remembered that the tl431 can be used as a control for a constant current sink but there isn't any kind of explanation on the datasheet of why it is used like that and since i don't like mindlessly copying schemes i wanted to understand it.
so could someone explain it to me or give me some hints ? any help is appreciated :-+

Benta:
An important piece of information is missing: what's the output voltage of your PSU?

Atom:

--- Quote from: Benta on February 04, 2019, 09:33:14 pm ---An important piece of information is missing: what's the output voltage of your PSU?


--- End quote ---

sorry the output voltage is from 0 to 32 V .

ahbushnell:

--- Quote from: Atom on February 04, 2019, 09:15:31 pm ---Hi to everyone, i need a little constant current sink circuit to put on the output of my bench psu so i can have always a little load for stability(that is not the main reason), but also for the turnoff, caps discharging ecc..

so i found this circuit , it a let's say crude but to me it's completly fine.

correct me if i'm wrong , if i understand correctly the 2 diodes in series generates a v drop of 1,4V .
by substracting the 0.7V of the vbe of the transistor we have a vdrop across R1 of 0,7V
we can calculate the current flowing with ohm's law so I=V/r.

i tried the circuit on a breadboard and it worked flawlessly but since the diodes v ref changes and the v drop of the transistor isn't constant the current taken varies let's say from 20ma to 30ma (also the transistor heats up)


then i remembered that the tl431 can be used as a control for a constant current sink but there isn't any kind of explanation on the datasheet of why it is used like that and since i don't like mindlessly copying schemes i wanted to understand it.
so could someone explain it to me or give me some hints ? any help is appreciated :-+

--- End quote ---
The TL431 will hold the voltage on that resistor at 2.5 volts.  It does it by changing the amount of current shunted from the base of the transistor. 

iMo:
The simplest current source/sink of around 10-20mA is a BF245C jfet.
Gate and Source wired together and to GND, Drain to Vcc rail (your PSU output).

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