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From: dagmargoodboat on 25 Jun 2010 03:20
On Jun 24, 6:09 pm, "k...(a)att.bizzzzzzzzzzzz"
<k...(a)att.bizzzzzzzzzzzz> wrote:
> On Thu, 24 Jun 2010 09:38:24 -0700 (PDT), dagmargoodb...(a)yahoo.com wrote:
> >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
> >    |      | |
> >    |      '-'
> >    |       |
> >   ===     ===
>
> Same idea but (a lot) better accuracy...
>
>                              |
>                 VCC          |   |
>                  |           |   |  I(sink)
>                 .-.          |   V
>                 | |          |
>                 | |          |
>                 '-'        |/
>            +-----+---------|
>            |     |         |>
>            |   |/            |
>            +---|             |
>                |>            |
>                  |          .-.
>                  |          | |
>         LM385    z          | |
>          -or-    A          '-'
>         REF1112  |           |
>                  |           |
>                 ===         ===
>                 GND         GND
>
> (created by AACircuit v1.28.6 beta 04/19/05www.tech-chat.de)

Nice. I (think) this might be one less part and even one step better
(accuracy, drift), but it's 3:20 a.m. so it could be broken and
stupid ....

|
VCC | |
| | | I(sink)
.-. | V
| | |
R1 | | |
'-' |/
+---------| Q1
| |>.
.-----' |
^ |
TLV431 / \<---------+
'---' |
| .-.
| | | R2
| | |
| '-'
| |
=== ===
GND GND

If Q1's a FET (or Darlington) the i(b) error is minimized, but even
with a high-gain BJT i(b) will still cause far less absolute error
than the TLV431's tolerance.

Now to be turned into a current source--like the OP wanted--left as an
exercise for someone less sleepyzzzzzzzzZZZZZ <snore>... :-)


--
Cheers,
James Arthur
From: Spehro Pefhany on 25 Jun 2010 10:56
On Thu, 24 Jun 2010 16:02:50 -0700, John Larkin
<jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:

>On Thu, 24 Jun 2010 15:25:59 -0700, dplatt(a)radagast.org (Dave Platt)
>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...
>>
>>The old 2N3819 seems to have a very wide range of Idss values (not
>>unusual for JFETs). The version from Temic, for example, lists Idss
>>as being "min 2, typical 10, max 20", with cutoff voltages of -3
>>typical and -8 maximum.
>>
>>If you don't mind buying a few and hand-selecting you can probably
>>locate one which will fit your circuit. They're all of $.09 each from
>>Jameco (quantity 10).
>>
>>If you want something a bit more modern for some strange reason:
>>Fairchild has a whole bunch of JFETs with minimum Idss well above 4
>>mA: J110, J309, J310, PN5434, lots more, in several different package
>>types. Mouser carries numerous types, mostly under $0.20 in single
>>quantity (a few are more but not by all that much).
>>
>>Heck, I think I have a bag of 100 2N3819s I bought a few years ago
>>when they were on-sale at Jameco for under a nickel each (overstock
>>sale, I assume). If you're in the U.S., send me an email with your
>>mailing address, and if I can find the bag I'll stick a few in an
>>envelope and mail 'em to you to dig through.
>
>If Idss is over 4 mA, you can stick a selected resistor or a pot in
>the source and tune the drain current. TC will be mediocre, but that
>may not matter to the OP.
>
>John


If it's less you can always stick two or more in parallel.

From: JosephKK on 26 Jun 2010 13:38
On Thu, 24 Jun 2010 10:56:14 -0700 (PDT), panfilero <panfilero(a)gmail.com>
wrote:

>Thanks for the responses... what I'm trying to do is power an
>accelerometer.... I'm trying to re-create the "Constant Current
>Signal Conditioner" side of the circuit shown in Fig. 4 of this link
>
>http://www.pcb.com/techsupport/tech_signal.php
>
>but in that circuit they use a "Current Regulating Diode" I don't have
>one of these, but I do have an electronics store down the street, so
>basically that's what I'm trying to re-create, and for the
>accelerometer I have, the current would have to be regulated to 4mA...
>
>thanks

Since the absolute DC accuracy of constant current diodes (basically a
jfet at IDss) is rather poor, but the effective impedance is really high
i recommend a Wilson current mirror (positive side) circuit instead; much
easier to get parts.
From: Jim Thompson on 26 Jun 2010 14:23
On Sat, 26 Jun 2010 10:38:09 -0700,
"JosephKK"<quiettechblue(a)yahoo.com> wrote:

>On Thu, 24 Jun 2010 10:56:14 -0700 (PDT), panfilero <panfilero(a)gmail.com>
>wrote:
>
>>Thanks for the responses... what I'm trying to do is power an
>>accelerometer.... I'm trying to re-create the "Constant Current
>>Signal Conditioner" side of the circuit shown in Fig. 4 of this link
>>
>>http://www.pcb.com/techsupport/tech_signal.php
>>
>>but in that circuit they use a "Current Regulating Diode" I don't have
>>one of these, but I do have an electronics store down the street, so
>>basically that's what I'm trying to re-create, and for the
>>accelerometer I have, the current would have to be regulated to 4mA...
>>
>>thanks
>
>Since the absolute DC accuracy of constant current diodes (basically a
>jfet at IDss) is rather poor, but the effective impedance is really high
>i recommend a Wilson current mirror (positive side) circuit instead; much
>easier to get parts.

See...

http://analog-innovations.com/SED/EnhancedCurrentMirrors.pdf

I don't see anything about the OP's circuit that would require
absolute accuracy.

...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: dwight on 28 Jun 2010 17:57
On Jun 24, 9:30 am, 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

Hi
Depending on the dynamic range of the voltage, you could put
a resistor from the output lead to ground to supply a minimum
current.
If the voltage range was high enough, you could even use a second
LM317 as the minimum current. Sink enough extra current in
the second regulator to keep the first one in range.
My reading of the spec is that
5ma is the worst case minimum and not 10ma.
Dwight
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