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...
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
On a sunny day (Thu, 24 Jun 2010 09:30:22 -0700 (PDT)) it happened panfilero wrote in :
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.
On a sunny day (Thu, 24 Jun 2010 17:41:38 GMT) it happened Jan Panteltje wrote in :
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.
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
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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 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
formatting link
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...
Or, if you shy away in fear from transistors, a "howlin'" Howland current source will provide you with man-days of entertainment trying to get it to stay stable under all conditions.
--
Tim Wescott
Control system and signal processing consulting
If the system is AC coupled and the calibration isn't critical, you could start with just a resistor to a DC voltage. +20 and 2.5K, or +30 and 5K maybe.
If you want to follow the example circuit, you could use a 1N5313 for a couple of dollars from Mouser.
You could also create this with something like a TL431 reference, an op-amp, a PNP transistor or small p-channel MOSFET and a few passives and get higher output impedance at low frequencies.
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.
--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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.
A constant current diode is just a JFET with the gate tied directly to the source. If you have a good collection of JFET's, you can likely find one where the I(sub)DSS is 4 mA. There is usually quite a spread in this current so picking a particular JFET should be done based on this spread. For example, the I(sub)DSS of a 2N4416 is typically between 5 and 15 mA. Some common JFET's and their corresponding I(sub)DSS ranges are shown in the table below.
Having used many PCB Piezotronics parts before, I can suggest that you buy their charge amplifier if you need the ultimate in performance. If you can wait a while for mail order, Mouser stocks the 1N5311 diode which is spec'ed to be within the range of 3.24 to 3.96 mA.
Dr. Barry L. Ornitz snipped-for-privacy@live.com
JFET Manufacturer I(sub)DSS Minimum I(sub)DSS Maximum MPF-102 On Semi
2N4416 Vishay 5 mA 15 mA
2N3819 Vishay 2 mA 20 mA MPF-102(obs.) Motorola 2 mA 20 mA
2N5457 Fairchild 1 mA 5 mA
2N5458 Fairchild 2 mA 9 mA
2N5459 Fairchild 4 mA 16 mA PN4303 Fairchild 4 mA 10 mA BFR30 NXP (Philips) 4 mA 10 mA BFR31 NXP 1 mA 5 mA
2SK722E Sanyo 2.5 mA 6 mA MPF-102 Fairchild 2 mA 20 mA
2N5557 Central 2 mA 5 mA
Note: P-channel or N-channel determines the "direction" of the constant current diode.
A constant current diode is just a JFET with the gate tied directly to the source. If you have a good collection of JFET's, you can likely find one where the I(sub)DSS is 4 mA. There is usually quite a spread in this current so picking a particular JFET should be done based on this spread. For example, the I(sub)DSS of a 2N4416 is typically between 5 and 15 mA. Some common JFET's and their corresponding I(sub)DSS ranges are shown in the table below.
Having used many PCB Piezotronics parts before, I can suggest that you buy their charge amplifier if you need the ultimate in performance. If you can wait a while for mail order, Mouser stocks the 1N5311 diode which is spec'ed to be within the range of 3.24 to 3.96 mA.
Dr. Barry L. Ornitz snipped-for-privacy@live.com
JFET Manufacturer I(sub)DSS Minimum I(sub)DSS Maximum MPF-102 On Semi
2N4416 Vishay 5 mA 15 mA
2N3819 Vishay 2 mA 20 mA MPF-102(obs.) Motorola 2 mA 20 mA
2N5457 Fairchild 1 mA 5 mA
2N5458 Fairchild 2 mA 9 mA
2N5459 Fairchild 4 mA 16 mA PN4303 Fairchild 4 mA 10 mA BFR30 NXP (Philips) 4 mA 10 mA BFR31 NXP 1 mA 5 mA
2SK722E Sanyo 2.5 mA 6 mA MPF-102 Fairchild 2 mA 20 mA
2N5557 Central 2 mA 5 mA
Note: P-channel or N-channel determines the "direction" of the constant current diode.
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