# Small (few components) 15ma constant current regulator circuit 5v-30v dc

#### Karenzee

Joined Mar 6, 2015
44
Hi,

i already found a few circuits but i really need it to to be small.

Similar to this

Any suggestions?

Thanks!

#### ericgibbs

Joined Jan 29, 2010
11,082
hi K,
How many LED's in series and what are their operating voltages?
E

#### Karenzee

Joined Mar 6, 2015
44
It's not for leds but for electrolysis.

#### BobaMosfet

Joined Jul 1, 2009
1,222
This works with 5VDC in, and outputs 16mA or so. 2 BJTs, 2 resistors. It doesn't matter how much current the LEDs want to draw, all they get to fight over is 16mA.

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#### Alec_t

Joined Sep 17, 2013
11,730
How accurate does the 15mA have to be?

#### AnalogKid

Joined Aug 1, 2013
8,677
3 parts. This is a simple as it can be. Output current changes with temperature, expect only 10% accuracy.
R1 = 39 ohms
R2 = Vee x 1000
T = 2N4401, 3904, 2222.

ak

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#### Alec_t

Joined Sep 17, 2013
11,730
If the supply varies over a 5V to 30V range, here's a variant which keeps the current within a 13.8-15.8 mA range over a 10C to 30C temperature range:

Q2 will dissipate ~400mW when the supply is 40V, so will probably need a small heat-sink.

#### Karenzee

Joined Mar 6, 2015
44
i'd say between 14-15ma (as accurate as possible)

I can also test under 30v (5v-20v), in term of space the heatsink is an issue.

Thanks for the suggestions so far.

#### Bordodynov

Joined May 20, 2015
2,696
See

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#### crutschow

Joined Mar 14, 2008
25,633
What is the maximum voltage drop across the electrolysis load?

To minimize the ≈-0.3%/°C temperature drift due to Vbe variation, and also current change with voltage, you can use a TLV431 programmable reference in place of the current-sense transistor.
It has a much lower temperature coefficient and higher voltage gain than the transistor to significantly reduce the current variation from both temperature and supply voltage change.
It's main disadvantage is that it drops 1.25V across the current sense resistor R1 whereas the transistor drop is only about 0.7V, thus increasing the minimum operating voltage by about 0.55V.

LTspice simulation below:

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#### Bordodynov

Joined May 20, 2015
2,696
See

#### crutschow

Joined Mar 14, 2008
25,633
So you gain a little extra stability by using J1 as a constant-current source in place of a resistor and adding R2?

#### Bordodynov

Joined May 20, 2015
2,696
So you gain a little extra stability by using J1 as a constant-current source in place of a resistor and adding R2?
I get stability more than 10 times better than in your simple scheme. Although I expected better stability. Perhaps this is due to different models of reference voltage sources

#### crutschow

Joined Mar 14, 2008
25,633
I get stability more than 10 times better than in your simple scheme. Although I expected better stability. Perhaps this is due to different models of reference voltage sources
So the question is, how good is good enough for the TS.

#### Karenzee

Joined Mar 6, 2015
44
Bordodynov this seems accurate enough for what i need!

Sensacell i'll definitely look for one, thanks for bringing this.

Thanks again to everybody for their suggestions!

#### BobaMosfet

Joined Jul 1, 2009
1,222
3 parts. This is a simple as it can be. Output current changes with temperature, expect only 10% accuracy.
R1 = 39 ohms
R2 = Vee x 1000
T = 2N4401, 3904, 2222.

ak
show off

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,278
The LT part is sort of 3 parts too. Bypass cap(s) might add more. Real estate might still be about the same.

#### AnalogKid

Joined Aug 1, 2013
8,677
When I was in school, Tektronix was on campus recruiting, and came with a short quiz with some real-world problems. One was how to implement a 30 uA current source in a scope (early 1970's, solid state chassis with a CRT). The answer they were looking for was a single 10 M resistor to the regulated +300 V plate supply.

ak