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As part of my graduate research on electrochemistry, I'm looking for a way to have 10 or 20 inexpensive, low amperage, constant current power supplies that simply hold at 10 mA with reasonable accuracy, +/- 1 mA.

My first thought was to purchase a bunch of wall chargers like these ones, cut the jack off it, split the wires, attach a 10 ohm resistor inline then attach some leads. Then I would buy a power strip and plug all of them in and have one "power supply" per electrochem experiment I'm running. But I'm not sure if that would work because I'm unsure if these adapters are constant current or constant voltage. I definitely need CC.

Even if I did make this small army of power supplies, how would I go about measuring them for accuracy? I'm not sure how good my cheap $10 multimeter is at measuring ~10 mA while the electrochem is running, and I'm not sure how much measuring the system affects the current while it's being measured. If anyone has a suggestion for a good multimeter/current measuring device at such low currents, I'd love to hear it.

The answers so far are very helpful. Thank you all very much. I have a lot of reading to do to learn everything presented so far.

Edit in response to expected voltages and chemistry details:

I ran a solvent system yesterday which required 5-7 V (currently using cheap, 10 A power supplies for our initial runs), but today I have another solvent which requires 22 V. I don't think the experiments will require anything more than 25 V.

The electrochem experiments involve using rare-earth metals to make/break chemical bonds. We'll be running them with 5 - 20 mL of solvent in a divided cell reaction flask using organic solvents like acetonitrile and acetic acid, and organic salts as electrolytes. The divided cell has a cathode on one side and an anode on the other, separated by a fine frit. The experiments will run for at least an hour, up to 24 hours in some cases.

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    \$\begingroup\$ What range of voltages/resistances do you expect? \$\endgroup\$ – W5VO Jul 6 '18 at 21:21
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    \$\begingroup\$ (1) Vast majority of consumer power supplies are constant voltage. The only type of consumer power supplies with constant current output that comes to mind are LED power supplies. (2) Could you describe your electrochemistry experiment in more details, please? (3) I'd like to second the question about the range of voltages. \$\endgroup\$ – Nick Alexeev Jul 6 '18 at 21:34
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    \$\begingroup\$ If you can have one common terminal (one side of the current sources or sinks) you can use one power supply and many simple current regulators. \$\endgroup\$ – Spehro Pefhany Jul 6 '18 at 21:55
  • \$\begingroup\$ Can you please edit your question to include an answer to @W5VO's question in the first comment? This could have an effect on the answer. \$\endgroup\$ – mkeith Jul 6 '18 at 23:04
  • \$\begingroup\$ Any DMM in current mode will give you pretty accurate readings, since the "burden voltage" will be negligible relative to supply voltage. \$\endgroup\$ – Ale..chenski Jul 7 '18 at 1:08
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The cheapest way is to buy some LM317 voltage regulators, and some 124 ohm resistors. The data sheet for the LM317 will show you how to connect them, and parts cost will be about 50 cents per channel. Plus, of course, a DC power supply of at least 3 volts greater than the electrochem potential, and at least 200 mA for 20 channels.

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  • \$\begingroup\$ +1, yes, LM317 is a classics for 10 mA, works like a charm. Maybe some trimpot is needed to compensate for 50uA of adjustment pin current, \$\endgroup\$ – Ale..chenski Jul 7 '18 at 0:53
  • \$\begingroup\$ NIST is redefining all metrology values based on universal constants to improve the present 7 significant figures. LM317L is about 10 cents in bulk (300pc) \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 7 '18 at 13:27
  • \$\begingroup\$ @AliChen - OP gave a 1% error margin, and that's 100 uA. \$\endgroup\$ – WhatRoughBeast Jul 7 '18 at 13:39
  • \$\begingroup\$ @TonyEErocketscientist - I used digikey 1 unit price, about 40 cents. \$\endgroup\$ – WhatRoughBeast Jul 7 '18 at 13:40
  • \$\begingroup\$ Sounds right... \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 7 '18 at 14:35
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A better way would be to use a wall-wart connected to one of these:

An LT3042 constant current regulator

They are low noise and easy to adjust with one resistor to set the current.

enter image description here

You can get an LT3042 on a dev board.

You can vary the current from 0 to 200 mA and it takes a 2 to 40 V input. You could measure the current with a picoammeter if you have the money. If you don't then just use a regular DMM. What it will come down to is accuracy and how accurate you need the current measurement. The accuracy can be found by going to the manufacturer's website and locating the specifications or buy downloading the datasheet.

enter image description here

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  • \$\begingroup\$ @W5VO 10 mA 30% ? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 7 '18 at 2:02
  • \$\begingroup\$ @Tony Ewww.. that was not the part I thought I was looking at... \$\endgroup\$ – W5VO Jul 7 '18 at 3:15
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schematic

simulate this circuit – Schematic created using CircuitLab

All you need is a current mirror.

All the same transistors. In 100pc < 10 cents/channel, if you know how.

Sources of error.

  • Noise ? use good EMI design practice
  • Actual mirror is less by 2*hFE/R+emitter so increasing by same cancels this error. e.g. If hFE ~ 100 this cancels a known 2% error by adding a 2 Ohm resistor.
  • Similarily if hFE differs by 2:1 value between transistors, Imirror output has 1% error.
  • R emitter tolerance in error, so use 1% or 0.1% matched arrays in SIP/DIP package etc.
  • Set 10mA using Ohm's Law for R Ref
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