# Variable Constant current in micro-amp range?

Would like help with finding a IC that can be controlled directly or indirectly using an arduino.  It needs to go from 10 microamps to about 200 microamps. I have a 5 volt power supply. Is there an IC for this, or how should I do it. I couldn't find anything on google that i understood. Thanks.

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1 year ago

I am thinking some kind of op-amp circuit could serve for this, provided the load, the thing through which you are driving your constant current, has a sufficiently low impedance.

I mean, what I am hoping is true, is that your load has I-V character, such that when the current through it is a maximum, of 200 uA, the voltage needed for this is less than 5 volts.

The reason why I am hoping that is true is because I was planning on supplying that 200 uA from the output of an op-amp, which has a maximum voltage roughly the same as the supply voltage, 5 volts, and the output voltage from this op-amp has to be large enough to push that 200 uA current through your load, and a current sensing resistor, wired in series together.

I have attached a circuit diagram for this, and I think this is actually the easy way to do it, provided your load is some low impedance thing, that just has a few volts, or even close to 0 volts, across it when your max current, of 200 uA, is flowing through it.

Guessing an op-amp like maybe TLC272,

http://www.alldatasheet.com/view.jsp?Searchword=TL...

an op-amp intended to work with a single sided supply, and with output that can swing rail to rail, will work.

The equations scribbled on this drawing are based on the usual assumptions of ideal op-amp behavior, i.e. currents flowing into the op-amp inputs are zero, and negative feedback adjusts Vout in such a way as to push the input voltages, V+ and V-, equal to each other, i.e. V+ = V-

In the case where your load is some big impedance thing, requiring big voltage across it, bigger than 5 volts, some other mojo, like maybe a bigger voltage supply, will be required.

BTW, you can find a whole bunch of circuits like this, just by asking Google(r) Images to show you, uh, "op-amp voltage controlled current source", or even just "voltage controlled current source", or similar.

https://www.google.com/search?hl=en&site=imghp&tbm...

1 year ago

1+
Although in the Arduino world we are used to get a module for everything it only means someone already went through the trouble of making the circuit so he can earn money from it ;)
Microamps are not as easy as you might think and although a good op-amp should be able to do the hard work you have to consider a lot of factors to actually make it work as planned.

The 8bit Arduino only allows for 256 different PWM values, so getting the resolution in 1 microamp steps would work but above that it will be very hard unless you switch to a 16 bit Arduino or some dedicated microprocessor poweder shield that can be "misused".
IMHO the best approach would be a good op-amp as suggest above and from there converting the input side to work with the Arduino.
Measuring the output without interference might be tricky considering the range.

headslant (author)  Downunder35m1 year ago

Do you suggest a sound shield, if i can find one with enough bits?

1 year ago

Do you really need the current for something or is it just to "detect" the tongue?
Maybe a simple capacitve or moisture sensor would be easier to do.
Plus you would need far less voltage to make it work sefely.

headslant (author)  Downunder35m1 year ago

Yes I do need the current. But I am now on the fence as to if I will actually do the project

rickharris1 year ago

Ah Digital taste project.

Heed my warnings in your original question about current voltage and the human body.

You may want to do the tongue on a 9 volt battery and work out the current to get an idea what it is like. (at your own risk)

headslant (author)  rickharris1 year ago

I have considered the risk, and I am now on the fence as to if I will do this project.

steveastrouk1 year ago

What's the load resistance ?

headslant (author)  steveastrouk1 year ago

A human tongue... Based on some research its between 500 an 2000 Kohms

1 year ago

At 2 meg, you'd need 400V !!! Hence me asking the question. I think a wet tongue though is very considerably lower, since there are electrolytes in saliva, and you'd be looking at much lower loads.

Its important to know the expected load resistance, and the range you're suggesting is too high for simple solutions, if its believable.

1 year ago

How about going with op-amp based voltage to current converter, plus a second op-amp wired as comparator for to watch the output of the first op-amp, to drive a LED, for to indicate if its output goes all the way (or very close to) to the positive rail?

In other words, you throw in a little light that turns on, or maybe toggles from green to red, when the current source is overloaded, for to let the user know the current source is not achieving its programmed current.

I mean since we don't yet have a believable number for resistance of this, uh, tounge-electrode load.

1 year ago

I think you'd have to go for that solution wouldn't you ? I am not sure whether to read the OPs figures as 500 Ohm to 2000K or 500...2000K ?