# Need help with TL431 as Current Sink

Discussion in 'General Electronics Chat' started by Dharith, Feb 23, 2012.

1. ### Dharith Thread Starter New Member

Feb 23, 2012
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I need help with understanding this circuit. I understand that the circuit is acting like a voltage splitter for the supply. I feel as though it does not work but I must be wrong. I feel as though the current through the transistor is more than the allowable source and I get conflicts with the voltage supply. Can anyone help me understand the circuit?

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2. ### #12 Expert

Nov 30, 2010
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I see this as a 100 ma current sink through the 2n2222 and the string of 20 ohm resistors add up to 100 ohms, so the 25 volt supply can send more than 250 ma through the string.

100 ma through 100 ohms or less uses up about 10 volts.

Is this working for you?

Hang on...there is only 3.77 ma available to the base of the 2n2222. Probably a lack of current to drive the base. Is that your point of conflict?

No, the 2n2222 has a gain of 75. It should work.

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• ###### 2N2222.pdf
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Last edited: Feb 23, 2012
3. ### Dharith Thread Starter New Member

Feb 23, 2012
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It is actually a schematic that I received from someone who made a prototype but when I did the analysis it seems to me the base current is too high for the transistor. I am just trying to understand the design which it is trying to act like a string of battery cells that I can manipulate. I'm thinking that the supply to the base of the transistor must be lower.

Last edited: Feb 23, 2012
4. ### #12 Expert

Nov 30, 2010
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The 431 wastes the excess base current.

5. ### Dharith Thread Starter New Member

Feb 23, 2012
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I am so happy that you are helping me out on this circuit cause I was lost ... So the extra current is passed through the TL431. I'm a junior engineer so forgive this question but does the voltage at Ve equal 2.5? or what is the voltage at the base ?

6. ### #12 Expert

Nov 30, 2010
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Pin 1 of the 431 chip is at 2.5 volts. The chip dumps some of the current available at pin 3 to ground if the voltage at pin 1 is too high.

The base of the 2n2222 will be 2.5 volts plus whatever Vbe is required to operate the transistor...typically about .65 volts at 100 ma.

The fact that pin 1 is locked at 2.5 volts and the adjacent resistor is 25 ohms is how the circuit is set at 100 ma.

7. ### Dharith Thread Starter New Member

Feb 23, 2012
4
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Thank you for helping. I was struggling so much with this and I guess its because I've never used a TL431.

Thank you!

8. ### #12 Expert

Nov 30, 2010
16,298
6,808
Everybody learns as they go.
New parts become available almost every day and everybody has to keep up by reading and understanding the datasheets.