# Constant current sink

Discussion in 'Electronics Resources' started by bedfordwheelnuts, Jun 7, 2011.

1. ### bedfordwheelnuts Thread Starter New Member

Apr 19, 2011
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Hi all,
I have viewed all the threads about constant current sink circuits, but one thing I am struggling with is this:
I have designed various flavours of constant current sink circuits, either by breadboarding or by using OrCad and Pspice.
My basic understanding of this kind of circuit is that for variations of input voltage, say between 20v and 30V the current sink should remain constant, fo my application 20mA.
In each of my circuit simulations the current varies as the voltage changes from 20V to 30V.
Have I got my understanding wrong?

Apr 5, 2008
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Hello,

LED driving

Bertus

Apr 14, 2005
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4. ### steveb Senior Member

Jul 3, 2008
2,433
469
Keep in mind that there are various voltages of concern here. A current source should not depend on the voltage connected to the other end of the load, but it may depend on its own power source voltage. Or, you may have a voltage controlled current source which of course would have load current dependent on the voltage set point.

I suspect that you are changing your bias voltages when your supply voltage changes. The solution here is to use some type of voltage regulator or preferably a voltage reference to control the bias/control voltages despite changes in your power supply voltage.

But, yeah, Ron H hit the nail on the head. Without a schematic it's hard to be certain.

5. ### hgmjr Moderator

Jan 28, 2005
9,030
215
Ronh is right, the quickest way to get at an explanation for your circuit's behaviour is to give us a glimpse of the schematic by posting it here.

hgmjr

6. ### bedfordwheelnuts Thread Starter New Member

Apr 19, 2011
7
1
Hi chaps,
Here are two of my designs. One uses a precision zener shunt regulator, the other a JFET.
Both exhibit the same variation in the current, when the voltage supply is varied.

Hope it helps.

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7. ### Ron H AAC Fanatic!

Apr 14, 2005
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Sink LED Strip 2 is not a constant current source, as you discovered. Sink LED Strip is mostly off the page. Did you look at it before you posted it?

Apr 19, 2011
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9. ### steveb Senior Member

Jul 3, 2008
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So, unless I'm missing something, I think I have to say that this second circuit should have performed much better because you are using a voltage reference to bias the transistor.

Obviously the first circuit is going to change current with the supply voltage changes because there is no stable reference to drive the transistor.

Dec 26, 2010
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Whatever circuit is used, the current can only remain constant over a certain range of input voltage. If the voltage is too low to supply the sum of the Vfs plus whatever the regulator needs, the current must be less than the required value. If the voltage is too big, the regulator may break down. What voltages did you try, and how did the current vary?

Edit: I now see that you said 20V to 30V, At the lower end you could be running out of volts. The LM431 is a 2.5V device, http://www.national.com/ds/LM/LM431.pdf to which you could add maybe 0.3V for the transistor minimum Vce, say giving 2.8V for the regulator. If the six LEDs are white at maybe 3.4V each the total minimum voltage would be 3.4V*6+2.8V = 23.2V.

What kind of LEDs are you using?

Last edited: Jun 8, 2011
11. ### Ron H AAC Fanatic!

Apr 14, 2005
7,049
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Your current is about (2.5V/130Ω)=19.2mA. You will have about a 10.7V drop across R1. If your input voltage is 20V, the transistor will saturate when the cumulative LED voltage drop is (20-10.7-2.5) 6.8V. This is not enough voltage for 6 LEDs. With a 30V supply, you will have 16.8V headroom for the LEDs. With a 20V supply, you don't need R1. In fact, you probably don't need it when the supply is 30V. If each LED drops 2V, the transistor dissipation will only be about 310mW. If the LED fwd voltage is more that 2V each, the transistor dissipation will be even less.

12. ### jaclement Active Member

Apr 15, 2009
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Easy current sink: 3 parts-a .1 uf cap ( anything between .1 and 10 uf), a lm317 regulator,and a 60 ohm resistor (for 20 ma current , current = 1.25/R). put the LED string from the positive supply to the input of the lm317. put the cap from the input of the 317 to ground. put the the 60 ohm resistor from the output of the 317 to ground. put the adjust terminal of the 317 to ground. put the negative output of the power supply to ground. The voltage drop of the LED string wil be about 3 volts per LED. the power supply should be at least 3 volts more than this to cover the 317 drop out voltage, and no more than 30 volts more than this to keep the 317 within its voltage rating. max power disipation of the 317 would be 30 x .02= .6 watts ,no heatsink needed for a to220