I'm looking for a way to control the output current between 0 and 2mA.
My first design uses a LM334 current source that is adjusted using a digital pot. With the LM334, I need 34 ohms for 2mA and 68 ohms for 1mA. Using a 1K digital pot with 256 taps, I'd have about 8 steps in between 1mA and 2mA. Are there any other current sources out there that have a larger range of resistances between 1mA and 2mA (e.x. 100 ohms for 2mA, 500ohms for 1mA)?
I've never used an LM334... The spec sheet it goes down to 1uA and there might be a "trick" for changing it's range.
I think you should be able to build a [u]transconductance amplifier[/u] (a voltage-to-current converter) with an op-amp. I've never built one of those either, but I assume, like most op-amp circuits, if you choose the resistors correctly you could build something that puts-out 2mA with 2V in... Or 2mA out with 5V in. That should give you plenty of resolution.
Or, National makes a transconductance chip, the LM13700, and it might be worth checking-out.
I have seen circuits that pull current down instead of up. Like a voltage divider to set the max and a transistor to drain some or all of that. In my own clumsy way I have run limited current to the collector of an NPN transistor and controlled that 0-100% but I can't say exactly that transistor throughput is only C-E or if the bit on the grid goes through too, and then there's the drops due to the transistors to take into consideration.. the EE's would use math and I would screw around with trimmers.
If you put the digital pot in parallel with a ~73ohm resistor you'll get about 240 steps between 34 and 68 ohms.
I went the extra effort and made a graph to demonstrate.. but then I fall into the non-math GoForSmoke type and that was the only way I could calculate it as well.
What is the load, and what is the approximate range of its resistance?
Maybe an lm317 as a current source? Your 1k digipot would work as 2ma is 625 ohms and 1ma is double that so add a 460 ohm resitor before the pot and you get a nice adjustment from .0027a to. 00085 a and a larger digipot would get you lowrr just less precision, and maybe a mosfet to turn it off entirely
My first design uses a LM334 current source that is adjusted using a digital pot.
It would be good if you posted that.
Also remember that you will only be able to get 2mA if the impedance of your load is such that you can get 2mA with the voltage you have available. Will this impedance change while the current you fix at the arduino remains the same?
