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Vary bias to a plain transistor through a potentiometer. Constant current goes to the load regardless of load.
View attachment 149296
The pot was dialed to one extreme and back.
100k 2w 10 turns which gives great precision. Can this circuit be made to work with a 100k pot?
Since it's rated 2W then it can handle 80V going through it.
Add a series bias resistor. Max current through the pot is 1-2 mA.
View attachment 149301
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Pot was dialed from one extreme to the other and back. Response appears proportional.
Hi,
About schematic of post#7:
It seems you didn't do a search for proven circuits.
And it seems you don't care about typical OPAMP circuits and OPAMP operating limits.
Why don't you choose a working circuit and modify it for your needs?
There are many tutorials and design examples in the internet ... and in this forum.
For a high side current source even I posted some kind of tutorial here: https://www.edaboard.com/showthread.php?376094-Design-of-high-side-current-source
Many of the considerations apply for a low side current source, too.
Your circuit issues:
* weak power supply of 20V (voltage depends on supply_load_current
* no bulk capacitor and no fast capacitor at the power supply
* 20V input voltage a noninverting input is beyond input voltage range of usual Opamps
* no stabilisation circuit to prevent oscillations
...
Klaus
I ran the step simulation with R12 set to 1k not 100k, the circuit does work with 100k pot.
The noise is well calculable..and not that high.and its prone to amplifying noise
Hi,
The noise is well calculable..and not that high.
Noise reduces precision, because it introduces some uncertainty.
It seems you don't like this uncertainty....thus you use the fet solution.
But the fet solution introduces other errors, like unlinearity and thermal drift. I'd say this uncertainty is much higher than the Opamp circuit uncertainty.
With an Opamp the input referred noise is in the low microvolts RMS.
If you use a shunt of 1k Ohms, then this means low nanoamperes RMS.
Klaus
Hi,
Useful shunt value depend on current range.
With 1k and 30mA there will be 30V across the shunt.
The 1k of my post#15 is just an example.
You can use 100 ohms, 10 ohms or even 1 Ohms shunt with the Opamp solution and it will stll be more precise than any solution without Opamp.
V_be will drift with about -2mV/K..... this is huge.
What noise current can you accept?
What thermal current drift can you accept?
Klaus