4mA current regulator [Basics]

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From: panfilero on 24 Jun 2010 12:30

can anyone offer any suggestions on how to make a simple 4mA current
regulator... I started looking at LM317 voltage regulators, but found
that they're gonna output a minimum of 10mA... I'm now looking at
JFETs with a resistor between the source and gate... most the JFETs
I'm finding can only provide currents less than 3mA...

thanks

From: dagmargoodboat on 24 Jun 2010 12:38

On Jun 24, 12:30 pm, panfilero <panfil...(a)gmail.com> wrote:
> can anyone offer any suggestions on how to make a simple 4mA current
> regulator... I started looking at LM317 voltage regulators, but found
> that they're gonna output a minimum of 10mA... I'm now looking at
> JFETs with a resistor between the source and gate... most the JFETs
> I'm finding can only provide currents less than 3mA...
>
> thanks

You didn't give any specs.

Here's a quickie, for lax needs:

(view in Courier font)

Vcc
-+-
|
.-.
| | R1 V
| | | | i.out
'-' | V
| |
| |/
+-----| Q1
| |>.
--- |
\ /~~> |
--- .-.
LED | | | R2
| | |
| '-'
| |
=== ===

--
Cheers,
James Arthur

From: Jim Thompson on 24 Jun 2010 12:41

On Thu, 24 Jun 2010 09:30:22 -0700 (PDT), panfilero
<panfilero(a)gmail.com> wrote:

>can anyone offer any suggestions on how to make a simple 4mA current
>regulator... I started looking at LM317 voltage regulators, but found
>that they're gonna output a minimum of 10mA... I'm now looking at
>JFETs with a resistor between the source and gate... most the JFETs
>I'm finding can only provide currents less than 3mA...
>
>thanks

Source (down from a positive rail)?

or

Sink (into ground/negative rail)?

or

Floating?

Accuracy?

Temperature coefficient?

Compliance (minimum and maximum voltages applied across the current
"regulator")?

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

Obama: A reincarnation of Nixon, narcissistically posing in
politically-correct black-face, but with fewer scruples.

From: John Larkin on 24 Jun 2010 12:44

From: Tim Wescott on 24 Jun 2010 13:34

On 06/24/2010 09:30 AM, panfilero wrote:
> can anyone offer any suggestions on how to make a simple 4mA current
> regulator... I started looking at LM317 voltage regulators, but found
> that they're gonna output a minimum of 10mA... I'm now looking at
> JFETs with a resistor between the source and gate... most the JFETs
> I'm finding can only provide currents less than 3mA...
>
> thanks

As mention, the problem is underspecified. Do you want a one-quadrant
outwardly passive device that -- when provided with enough voltage to
drop -- passes 4mA, nothing else? Or would you be satisfied with a node
that always draws 4mA from your positive rail, until some low-voltage
threshold is violated? Or do you need a node that'll source 4mA _to_
ground until some _positive_ threshold is exceeded?

I _think_ that there are voltage reference chips that will do the "one
component passive" thing. Some are essentially really low power (and
higher precision) versions of the LM317, some are more oddball.
Anything that's LM317-like will have some uncontrolled current coming
out of the ground pin, you'll have to take that current -- and it's
expected variation -- into account when you design your circuit.

I'd start looking for app notes. Note that some of these things are
called "programmable zeners" or some such.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

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