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Range swichable constant current source
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OM222O:
Hello
I need to have a range switchable constant current source for a project with outputs of 10mA 100mA and 1A. I create a 1V reference using 2 fixed resistors and a digital pot which acts as a crude DAC. my first idea was to amplify the feedback signal from shunt resistor in order to trick the main op amp into seeing different values across the shunt and therefore making adjustments to keep the "1V" across the shunt. (see the circuit below)
http://tinyurl.com/y7zr7zqq

This worked really when even simulated in pspice using real world models of op amps but there was a ton of oscillation on the output of the main op amp when I built it. I tried a 1k resistor and 1uF ceramic cap as a low pass RC filter which dampened it down but there was still oscillations across the shunt resistor which obviously wasn't ideal (I also tried different op amps, same results!) ... also unlike the simulation, when all switches were disconnected there was about 200 too 300mA which I'm guessing was cause by electrostatic build up or random noise ... so to say this was a bad idea would be a gross understatement. I'm really limited on the PCB space so I'd rather avoid having 3 separate op amps with 3 separate output pass elemnts (BJTs for FETs) as well as 3 shunt resistors ... I know this is in theory the best method but I'd like to see if you guys have better suggestions on how to achieve this ... One other idea that I'm having right now is to use a unity gain amplifier on the 1V reference, then have the decade divider resistor network (3k 27k 270k) on it's output and use the switch to feed that into the non inverting input of the second amp. also include a RC snubber network which also acts as a pull down when no switch is connected, so the load will be off. That would probably require a split supply or some weird trick as even the "rail to rail" op amp I'm using rn has about 25mV swing so it can't each the desired 10mv .I'm not sure how it works and haven't simulated it yet, I'm just really frustrated after wasting so much time for nothing. I would really love to hear your advice first, thanks.

P.S: another reason I'm trying to avoid 3 of each component is because of the fact that one side of the PCB has to be used as a front panel so vias can't be used except for a really small part where the 1inch oled display will fit onto, but if nothing else works, I'll give that a shot.
OM222O:
Here is what I meant as my second attempt:
http://tinyurl.com/y848xrb7

the 560k is a pretty weak pull down but still seems to be causing issues ... maybe a better solution is to connect the output of the op amp to the BJT using another switch? There are so many options when it comes to analog circuits, it's seriously overwhelming  :-//
soldar:
What kind of voltage and what kind of precision do you need because this is the kind of thing I would do with a single transistor and a zenner diode.
Zero999:
You need a Howland current pump. Here's a link to one I designed awhile ago. It can both source and sink current and is bridged but as you only need to source, it can be much simpler.


https://www.eevblog.com/forum/projects/need-help-with-bi-directional-constant-current-source-(100ma)/msg1972685/#msg1972685

Here's some links giving more information.
http://www.ti.com/lit/an/snoa474a/snoa474a.pdf
https://www.edn.com/design/analog/4430167/Op-Amp-Current-Sources--The-Howland-Current-Pump

Simulation:
http://www.falstad.com/circuit/e-howland.html

OM222O:
Max voltage is 9v (battery) and I need it to be fairly accurate as it will be used with an ADC to measure resistance values (not shown in schematic) but the digital pot can compensate to +-10%


--- Quote from: Zero999 on January 15, 2019, 05:18:44 pm ---You need a Howland current pump. Here's a link to one I designed awhile ago. It can both source and sink current and is bridged but as you only need to source, it can be much simpler.


https://www.eevblog.com/forum/projects/need-help-with-bi-directional-constant-current-source-(100ma)/msg1972685/#msg1972685

Here's some links giving more information.
http://www.ti.com/lit/an/snoa474a/snoa474a.pdf
https://www.edn.com/design/analog/4430167/Op-Amp-Current-Sources--The-Howland-Current-Pump

Simulation:
http://www.falstad.com/circuit/e-howland.html



--- End quote ---

That was my first idea as well but I'm not sure they are suitable for currents of about 1A ... I read a lot of articles which suggest they are good for low current situations.
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