Electronics > Projects, Designs, and Technical Stuff

Constant current source adjustable by PWM

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magic:
Nope, it's making the design more complex with more parts OR perform worse. The latter may be acceptable.

And LM358 can't implement this kind of current sink by itself, not without ugly kludges which may or may not work (tie OUT to V+ and IN- to GND, minimum current is 1mA, no warranty :P). So you are looking at more external components and more than 3 cents.

delphes123:

In order to clarify , it is not a led driving, it is more complex to describe the use case, but it is a resistive charge (with probably a very very low capacitive)

I need to produce a constant current at different level between 0 to 20 mA max during a certain amount of time (very slow events)

like 0 to 2 sec = 20 mA
2 to 4 sec = 10 mA
etc

Precision is not very impressive let say 10% is ok.

I would prefer for many reasons a very simple transistor solution with PWM to control the level of mA.

PCB.Wiz:

--- Quote from: delphes123 on June 09, 2023, 06:10:37 pm ---
In order to clarify , it is not a led driving, it is more complex to describe the use case, but it is a resistive charge (with probably a very very low capacitive)…
Precision is not very impressive let say 10% is ok.

I would prefer for many reasons a very simple transistor solution with PWM to control the level of mA.

--- End quote ---
Is that 10% of full scale, or 10% of output, meaning 2mA is within 200uA , or now 1% of full scale.

What voltage(s) is across your ‘resistive charge’ ? The power in the output device needs to be considered.

How many of these will you make ? What is operating temperature range?

Are you ok with a calibrate curve (look up table ) in flash ? That will be needed to get 2mA@10%.

You probably would not need to curve re-calibrate every single unit, but single point calibrate will give you better performance, so you should allow for that, in your production costing spreadsheet and jigs.
In practical terms, I’d expect same-reel calibrate to be ok here.
An opamp does not need a look-up-table, but you ( or your tutor ?) has excluded the opamp.

delphes123:

Is 10 % of ouput 10mA +- 1 mA

Voltage with be variable due to constant current, but it is powered by 12 Volts initially

it will operate between 10 and 30 °c

Yes of course I can have a lookup table and calibration can be made for each.

tszaboo:

--- Quote from: delphes123 on June 08, 2023, 01:19:19 pm ---


NS145020A is a good component but it has fixed 20 mA constant current, how I can manage with this component any other value of current between 0 and 20 mA ?

--- End quote ---
You drive it with a transistor, which has the PWM signal on it's base/drain, and then smooth the signal (bypass it) with a capacitor.

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