Current-dropping current source! Please help!

[vicious5id]

Good work, vicious5id.

LoL, I guess I need to express my gratitude to LC for that!

Oh, I actually didn't realise the current Ir (reference current) was through the ZR423, I had assumed it was the current through the 10k pot. So now it's a case of decreaseing the 70k resistance to obtain a current through the ZR423 of ~150 uA (according the the datasheet, that's the optimum current).

LC, did you just assume there was a 1.25 V drop across the 10k pot to work out the current through it and the ZR423? If so can I take the resistance of the ZR423 to be 50k (according to the thumbnail in post #20)?

In the graphs I posted prior, the one titled, "Vin+, Vin- VS Time", the blue line is Vin+; I want that line to be pretty much straight as soon as I set the current. I want the blue line to do what it does in the first ~200 secs, to do it in the first second or half-second.

Many thanks, v5.

EDiT: That 70k needs to be changed to a 43k or 43k6 to be precise.

 

[eblc1388]

So now it's a case of decreaseing the 70k resistance to obtain a current through the ZR423 of ~150 uA (according the the datasheet, that's the optimum current).

The value of the unknown resistor is not difficult to calculate if you know how.

Because of the 1.25V output nature of the ZR423, the voltage across the 10K POT will be fixed at 1.25V and this would result in a constant current flowing in the 10K pot and that's 125uA. If you want another 150uA to flow in the ZR423, then the total current flowing in the top dropping resistor will be the sum of the two, i.e. 275uA. The voltage drop across this resistor is 12V-1.25V=10.75V so Ohm's law give resistor value as 39K.

EDiT: That 70k needs to be changed to a 43k or 43k6 to be precise.

Your calculated value are close. See above.

 

[vicious5id]

Guess what? I'm back! I decided to go away for a while and try and sort this damn thing out myself, and fortunately I did (with little help). I got what I wanted with **broken link removed**, without a reference! That circuit I just linked to works from 0 - 120 mA, with a drift of 0.1 mA over 1 hr i.e. I really like it and it does the business. The problem I have is with **broken link removed**. The circuit with the reference only lets me get 0 - 60 mA output, but if I remove it I get 0 - 120 mA output.

How the hell is this working without a reference?! Can anybody explain?

 

[eblc1388]

You are posting a question without sufficient data again.

What is the voltages on the two opamp inputs when the circuit with the reference diode is outputting max. 60mA?

Added: Check to make sure that the voltage across the reference diode is 1.25V as intended. Is is possible that you have reversed the polarity of the reference diode by mistake?