4mA current regulator
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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: dagmargoodboat on 25 Jun 2010 03:24 On Jun 25, 1:13 am, Grant <o...(a)grrr.id.au> wrote: > On Thu, 24 Jun 2010 18:09:30 -0500, "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) > > I've read a green LED (old style ~2V) balances out the transistor's > tempco? > > Grant. > --http://bugs.id.au/ Yep, that's why it's there. Strangely, many LEDs of all colors sport -2mV/ºC tempcos, neatly offsetting a BJT's Vbe drift. -- Cheers, James Arthur
From: Phil Hobbs on 25 Jun 2010 10:32 On 6/25/2010 2:13 AM, Grant wrote: > On Thu, 24 Jun 2010 18:09:30 -0500, "krw(a)att.bizzzzzzzzzzzz"<krw(a)att.bizzzzzzzzzzzz> wrote: > >> On Thu, 24 Jun 2010 09:38:24 -0700 (PDT), dagmargoodboat(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 04/19/05 www.tech-chat.de) > > I've read a green LED (old style ~2V) balances out the transistor's > tempco? > > Grant. Using a 1.2V bandgap gives you a sink current that's nominally proportional to absolute temperature (PTAT). That's often useful, e.g. in stabilizing the transconductance of undegenerated differential pairs--but mostly inside ICs. Using a LED is usually superior, since (a) you get first order temperature compensation (i.e. not PTAT), and (b) it's much, much quieter than a bandgap. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs at electrooptical dot net http://electrooptical.net
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. |