Current source with low voltage drop.

[joble]

Hi,

I'm looking for a 15 mA current source. The application is place
critical so a discrete circuit is no option. The voltage drop between
in and out should be as small as possible. The input voltage is 20 to
24 Vdc.

I've tried an LM317 (in SOT23 package), but the voltage drop is to
large.

Does somaone know an alternative for the LM317, but with a lower
voltage drop?

Thanks!

 

[Jim Thompson]

Hi,

I'm looking for a 15 mA current source. The application is place
critical so a discrete circuit is no option. The voltage drop between
in and out should be as small as possible. The input voltage is 20 to
24 Vdc.

I've tried an LM317 (in SOT23 package), but the voltage drop is to
large.

Does somaone know an alternative for the LM317, but with a lower
voltage drop?

Thanks!

Dream on!

...Jim Thompson

 

[John Fields]

Hi,

I'm looking for a 15 mA current source. The application is place
critical so a discrete circuit is no option. The voltage drop between
in and out should be as small as possible. The input voltage is 20 to
24 Vdc.

I've tried an LM317 (in SOT23 package), but the voltage drop is to
large.

Does somaone know an alternative for the LM317, but with a lower
voltage drop?

 

[joble]

OK,

The load is 5 leds (about 3,5V forward voltage per led --> 17,5V).

Maybe a FET current source could do the job. This is just one J-fet
with a resistor between gate and source.

Those things also exist in a 2 pin package. But i can't find one at 15
mA. Does anyone know a manufacturer who makes those???


Greetz

 

[Frank Bemelman]

Hi,

I'm looking for a 15 mA current source. The application is place
critical so a discrete circuit is no option. The voltage drop between
in and out should be as small as possible. The input voltage is 20 to
24 Vdc.

I've tried an LM317 (in SOT23 package), but the voltage drop is to
large.

Does somaone know an alternative for the LM317, but with a lower
voltage drop?

Thanks!

+20-24V---string of leds----------------.
|
|
|
-------------------'
| |
| |
| |
| |
| | low voltage opamp
| |\| sot-23
| .-------|+'
' | | >---.---.
.-. | .-|-. | |
| | | | |/| | |
| | | | | | '
'-' | '---|----' .-.
---------- | | |
| | | |47 ohm
V | '-'
- | |
. diode | .
| | |
GND -----------------------------------------------
created by Andy´s ASCII-Circuit v1.21 Beta www.tech-chat.de

 

[Fred]

OK,

The load is 5 leds (about 3,5V forward voltage per led --> 17,5V).

Maybe a FET current source could do the job. This is just one J-fet
with a resistor between gate and source.

Those things also exist in a 2 pin package. But i can't find one at 15
mA. Does anyone know a manufacturer who makes those???


Greetz

[/QUOTE]

Put a few in parallel.

Fred

 

[Spehro Pefhany]

OK,

The load is 5 leds (about 3,5V forward voltage per led --> 17,5V).

Maybe a FET current source could do the job. This is just one J-fet
with a resistor between gate and source.

Those things also exist in a 2 pin package. But i can't find one at 15
mA. Does anyone know a manufacturer who makes those???

Current regulator "diodes" need a lot of voltage (maybe 5V) across
them to work.

This has only 4 parts (two resistors and two SOT-23-x) and will work
down to around 1.3V. As a two terminal device it would drop maybe 1V
more, still better than the LM317, but not by much.

...
+24
|
| V
| -
.-. |
| | V
| | -
'-' |
| |
| |/ MMBT3904
+------------|
| |>
| / |
----- |
/ / \-------------+
/ \ |
----- |
| |
TLV431 | .-.
| | |
| | | 82R
| '-'
| |
| |
=== ===
GND GND

You could also use an LM334 (plus a booster circuit) but I think there
will be more parts and cost that way @ 15mA output.


Best regards,
Spehro Pefhany

 

[Ban]

Hi,

I'm looking for a 15 mA current source. The application is place
critical so a discrete circuit is no option. The voltage drop between
in and out should be as small as possible. The input voltage is 20 to
24 Vdc.

I've tried an LM317 (in SOT23 package), but the voltage drop is to
large.

Does somaone know an alternative for the LM317, but with a lower
voltage drop?

Thanks!

cheap and only 2mA self consumption (with 1mA through the zener). For the
reference voltage you can use a 7805L or so or a 6.8V zener with low tempco.
The current mirrors 1mA to 17mA approximately. Transistors are BCV61(dual)
or 2N3904, 2222A etc.
o-----+
+5V .-.
reference 3k9
Voltage | | 25Vmax/17mA
'-' LED-string
| |
+---+ |0.5Vmin
| | |
\| | |/
NPN |-+-| NPN
<|1V |>
| |.25V
.-. .-.
270R 15R
| | | |
'-' '-'
| |
+-------+
|
===
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

ciao Ban

 

[John Fields]

OK,

The load is 5 leds (about 3,5V forward voltage per led --> 17,5V).

Maybe a FET current source could do the job. This is just one J-fet
with a resistor between gate and source.

Those things also exist in a 2 pin package. But i can't find one at 15
mA. Does anyone know a manufacturer who makes those???

---
First things first.

If your source voltage varies from 20 to 24VDC and your load voltage
needs to be 17.5V, then the voltage drop across whatever you use as a
constant current regulator _must_ vary from 2.5V (with a source voltage
of 20V) to 6.5V with a source voltage of 24V. Period.

If you can guarantee that your source voltage will never fall below
20.75 and your load voltage will never exceed 17.5V, then you could use
an LM317L like this:

lm117L
+-------+
VIN>----|IN OUT|--+--18.75V
| GND | |
+---+---+ |
| [RS]
| |
+------+--17.5v
|
[RL]
|
GND>---------------+

The output current is defined as

Vref
Io = ------ + Iadj
Rs

Since the Iadj term is small (100µA max) compared to the load current
(15mA) we can neglect it and rearrange to solve for Rs:

Vref 1.25V
Rs = ------ = -------- = 83.3 ohms
Io 0.015A

The closest 5% value is 82 ohms, which would raise your output current
to 15.2mA +/- 5%, so on the high side you'd be pushing 16mA through the
LEDs.


If you need a little higher compliance you could do this:


+V +V +V
| | |
[620] | [LED]
| | |
+-----+ +------|+\ C
| | | | >---[1K]---+-------B 2N4401
[Z6.8][5.76K]| +---|-/LM358 | E
| | | | | | |
| +---+ +---------------------------+
GND | | | |
[1.02K] | [0.1µF] [68]
| | | |
GND GND GND GND

 

[Robert Baer]

cheap and only 2mA self consumption (with 1mA through the zener). For the
reference voltage you can use a 7805L or so or a 6.8V zener with low tempco.
The current mirrors 1mA to 17mA approximately. Transistors are BCV61(dual)
or 2N3904, 2222A etc.
o-----+
+5V .-.
reference 3k9
Voltage | | 25Vmax/17mA
'-' LED-string
| |
+---+ |0.5Vmin
| | |
\| | |/
NPN |-+-| NPN
<|1V |>
| |.25V
.-. .-.
270R 15R
| | | |
'-' '-'
| |
+-------+
|
===
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

ciao Ban

....and there are dual NPN transistors in a SOT-23 package; even Zetek
has a rather hi beta dual in the SOT-23, if i remember correctly.

 

[Keith Buck]

Hi
This problem is usually solved using switched mode technology.
Regards keith buck.

 

[Robert Baer]

...and there are dual NPN transistors in a SOT-23 package; even Zetek
has a rather hi beta dual in the SOT-23, if i remember correctly.

Here is the Zetex (correct spelling) info..
8-MSOP package:
ZXT12N20DX 50V, beta(min/max) 300/900
ZXT12N50DX 100V, beta(min/max) 300/900
SM-8 package:
ZDT690 45V, beta(min) 400
ZDT651 60V, beta(min) 100
There are a number of others in this package running from Vceo of 15V
to 120V and betas from 75 to 2000.

There is a tricky way to make a 2-terminal current source that will
work down to about 650mV across it:
R2
+ O--*---- ^------*--/\/\/--*-----------O -
| \ / | |
| ---- ---- |
R1 -/\/\--|-------/ \-------|

Set R2 so that the I*R2 is about 600mV, where I is the current you
want.
Using high beta transistors means R2 can be fairly large, making the
I*R2 closer to what you want, and R2 making a less of a burden on the
"current source" result. This can be useable from microamps to a few
hundred milliamps (depending on dissipation).

Another way is to use a MOSFET, tie the gate to "ground" (-) terminal,
and use a current setting resistor from drain to same "ground" (-)
terminal. This can be useable from nanoamps to at least an amp
(depending on dissipation). Ther are a lot of MOSFETs in SOT23, SOT-223
etc packages.

Both schemes have a temperature dependence that you may or may not
want; compensation can be done by using an appropiate thermistor for R2
or the drain to "ground" resistor.

BTW, "LDO" is used for *voltage* regulators, and is inappropiate and
inapplicable for *current* regulators.

 

[Robert Baer]

Here is the Zetex (correct spelling) info..
8-MSOP package:
ZXT12N20DX 50V, beta(min/max) 300/900
ZXT12N50DX 100V, beta(min/max) 300/900
SM-8 package:
ZDT690 45V, beta(min) 400
ZDT651 60V, beta(min) 100
There are a number of others in this package running from Vceo of 15V
to 120V and betas from 75 to 2000.

There is a tricky way to make a 2-terminal current source that will
work down to about 650mV across it:
R2
+ O--*---- ^------*--/\/\/--*-----------O -
| \ / | |
| ---- ---- |
R1 -/\/\--|-------/ \v------|

Set R2 so that the I*R2 is about 600mV, where I is the current you
want.
Using high beta transistors means R2 can be fairly large, making the
I*R2 closer to what you want, and R2 making a less of a burden on the
"current source" result. This can be useable from microamps to a few
hundred milliamps (depending on dissipation).

Another way is to use a MOSFET, tie the gate to "ground" (-) terminal,
and use a current setting resistor from drain to same "ground" (-)
terminal. This can be useable from nanoamps to at least an amp
(depending on dissipation). Ther are a lot of MOSFETs in SOT23, SOT-223
etc packages.

Both schemes have a temperature dependence that you may or may not
want; compensation can be done by using an appropiate thermistor for R2
or the drain to "ground" resistor.

BTW, "LDO" is used for *voltage* regulators, and is inappropiate and
inapplicable for *current* regulators.

********* correction on "schematic":
R2
+ O--*---- ^------*--/\/\/--*-----------O -
| \ / | |
| ---- ---- |
R1 -/\/\--|-------/ \v------|

On the MOSFET, use of a logic version will reduce the needed drop
across the resistor; "ordinary FETs (by IRC) need about 3.5V to produce
10-100uA, and a logic FET needs about 1.5V for the same current.
The Vgs versus current plot is an excellent log relationship ofer a
wide current range, almost as good as the Vbe versus Ie plot.

 

[Ian Bell]

OK,

The load is 5 leds (about 3,5V forward voltage per led --> 17,5V).

What kind of LEDs are these. I never heard of ones with this big a
forward volts drop.


How critical is the 15mA? A 17.5V drop at 15mA implies an equivalent
resistance of 1166 ohms. For 24V operation an additional series
resistance of 430 ohms will achieve a 15mA current. When the volts
falls to 20V the current falls to 20/(1166 +430) = 13.36mA.

Ian

 

[Spehro Pefhany]

What kind of LEDs are these. I never heard of ones with this big a
forward volts drop.

That's a fairly typical Vf for blue and some other short-wavelength or
white LEDs (which generally use a blue die).

How critical is the 15mA? A 17.5V drop at 15mA implies an equivalent
resistance of 1166 ohms. For 24V operation an additional series
resistance of 430 ohms will achieve a 15mA current. When the volts
falls to 20V the current falls to 20/(1166 +430) = 13.36mA.

LEDs don't behave that way. To a first approximation the voltage drop
across the string is fixed so long as they are forward biased. To a
better approximation, it's like a fixed voltage source with a resistor
in series- a fairly small valued resistor.

For example, a Panasonic LNG992CFB blue LED has a Vf of 3.5V at 20mA
and 3.25V at 10mA, so the series resistor is r = 0.25/0.01 = 25 ohms.
The equivalent Vf is 3.25 - 0.01 * 25 = 3.0V.

So, if you put a resistor in series with 24V you need R= (24
-15)/0.015 - 125 = 475 ohms. At 20V, the current will be
I = (20 - 15)/(475 + 125) ~= 8.3mA

Best regards,
Spehro Pefhany

 

[R.Legg]

Spe

Current regulator "diodes" need a lot of voltage (maybe 5V) across
them to work.

1N5283 - 1N5314

xx Ir Vf
83 .22
84 .24
85 .27 1v00
86 .3
87 .33
88 .39
89 .43
90 .47 1v05
91 .56
92 .62
93 .68
94 .75
95 .82 1v20
96 .91
97 1.0
98 1.1
99 1.2
5300 1.3 1v45
01 1.4
02 1.5
03 1.6
04 1.8
05 2.0 1v85
06 2.2
07 2.4
08 2.7
09 3.0
10 3.3 2v35
11 3.6
12 3.9
13 4.3
14 4.7 2v9

To make your own, the figures of interest are IDss and Vp.

Look for JFet with high IDss and low Vp, for a self-biasing fet
limiter with higher current and lower voltage headroom. The
gate-source resistor determines current, below the IDss value, with
~Vp across it.

Types like 2N4393 (150mA 3V)are a good bet.

RL

 

[normanstrong]

Hi,

I'm looking for a 15 mA current source. The application is place
critical so a discrete circuit is no option. The voltage drop between
in and out should be as small as possible. The input voltage is 20 to
24 Vdc.

I've tried an LM317 (in SOT23 package), but the voltage drop is to
large.

Does somaone know an alternative for the LM317, but with a lower
voltage drop?

You've managed to ask the question without providing any information
except that it is a 15mA current source.

1. It needs the lowest burden voltage possible; so what is the
minimum burden voltage?
2. What is the range of the output voltage?
3. How accurate does the current have to be?
4. What is the minimum output impedance of the current source.
5. Does this have to be a 2-terminal source, or can it be 3-terminal?
If 3-terminal, what is the maximum voltage available for the 3rd
terminal?

Give us some help here.

Norm Strong