constant current source

[brownt]

assuming a 5 volt supply across a 10K ohm potentiometer, how can i make it so there is a variable 5 volts with a constant current of about 15mA at the wiper?

[danadak]

What is the load on the wiper ?


Regards, Dana.

[Circlotron]

Do you mean that there is a 15mA CC load between wiper and ground?

Add a 15mA CC source from +5V to wiper.
Better still, use a unity gain opamp buffer between wiper and load.

[danadak]

Ap note on current sources and sinks.


http://www.ti.com/lit/an/sboa046/sboa046.pdf


Regards, Dana.

[newbrain]

I think until the OP clarifies their question, we are left in the realm of speculations.

What are you actually trying to achieve? In which context and to which purpose?

[Damianos]

This ^ ^ ^ ^
Answers to a vague question can lead to bad results.

[brownt]

Turns out everything works ok, as it seems the load is only 70 microamps and so even at 10K there is sufficient current through the wiper.

thanks

[brownt]

that is correct, there was i thought a potential 15mA load after the wiper.

add a unity gain buffer, ok thanks.

Add a constant current source? I do not understand. I mean, that was the question, how to make a constant current source.

[hamster_nz]

With a 10k pot, depending on the position of the wiper you can seen an equivalent of

- 5V with 0 ohms in series (i.e. the wiper at the 5V end),
- 3.75V with 1.875k in seres ((i.e. the wiper at 75% , 2.5k Ohm to +5V and 7.5K Ohm to 0V).
- 2.5V with 2.5k in series (i.e. the wiper in the middle, 5k Ohm to +5V and 5K Ohm to 0V).
- 1.25V with 1.875k in seres ((i.e. the wiper at 75% , 7.5k Ohm to +5V and 2.5K Ohm to 0V).
- 0V with 0 ohms in series (i.e. the wiper at the 0V end)

If you want to stay within 1% of the wiper setting (e.g. so 50% gives 2.5V +/- 25mV) you don't want to be sourcing/sinking more than 0.01 mA from the wiper.

If you are sourcing 70mA, you will be off by about 10% with the wiper in the middle position (around 2.26V vs what should be 2.5V).

Easiest solution is to use a potentiometer with lower resistance (maybe go 1k)

[Insatman]

Here is a ~50uA to 50mA adjustable precision current source I built some months ago.  The only expensive parts are the voltage source, 10 turn pot and the 100 ohm 0.01% Low-tempco resistor.  Truth is a lower tolerance resistor is fine so long as the tempco is very low, as you can calibrate the error out.   The calibrated setting of 50mA was tested to be +/- 2uA after warmup in numerous tests measured with a HP34401A.   

[Circlotron]

that is correct, there was i thought a potential 15mA load after the wiper.

add a unity gain buffer, ok thanks.

Add a constant current source? I do not understand. I mean, that was the question, how to make a constant current source.

You won’t get useful results supplying a 15mA load with a constant current source.