4mA current regulator
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From: Jan Panteltje on 24 Jun 2010 13:41 On a sunny day (Thu, 24 Jun 2010 09:30:22 -0700 (PDT)) it happened panfilero <panfilero(a)gmail.com> wrote in <1f931fa4-0974-4f2e-9a4e-f166a28ba8be(a)w31g2000yqb.googlegroups.com>: >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 current mirror U U | | R1 | |----- i1 | | | c | c b--------------- b e e | | /// /// i1 = approx (U - 0.7) / R1 So if U is stable, and temp does not change too much, i1 is stable. Else use the LM317 with an opamp: + U -------- in out------------ + 1.22----------- + LM 317 out ------ adj --- - | | | opamp R1 /// | | -------------------| | i1 | /// The output of the opamp will rise until the voltage drop over R1 is equal to the output voltage of the LM317 (about 1.22 V here). So the current i1 here is 1.22 / R1. The opamp will have to be able to do the 4 mA, and needs a sufficient high supply. You could use the LM 317 with a resistor divider to make a stabilised positive supply, and use a voltage divider or trimpot to set the + input of the opamp.
From: Jan Panteltje on 24 Jun 2010 13:49 On a sunny day (Thu, 24 Jun 2010 17:41:38 GMT) it happened Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote in <i005cp$au6$1(a)news.albasani.net>: Oops, reversed i1 and R1: + U -------- in out------------ + 1.22----------- + LM 317 out ------ adj --- - | | | opamp i1 /// | | -------------------| | R1 | /// The output of the opamp will rise until the voltage drop over R1 is equal to the output voltage of the LM317 (about 1.22 V here). So the current i1 here is 1.22 / R1. The opamp will have to be able to do the 4 mA, and needs a sufficient high supply. You could use the LM 317 with a resistor divider to make a stabilised positive supply, and use a voltage divider or trimpot to set the + input of the opamp.
From: panfilero on 24 Jun 2010 13:56 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
From: panfilero on 24 Jun 2010 13:57 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. 7 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
From: panfilero on 24 Jun 2010 14:26
It's figure 7, not 4 sorry, tried deleting the mistake but it's still popping up |