constant current source - Page 1
Forum section: electronics / beginners

author: brownt
author_rank: Regular Contributor
timestamp: February 20, 2018, 09:46:40 am
content: 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?

author: danadak
author_rank: Super Contributor
timestamp: February 20, 2018, 12:29:02 pm
content: What is the load on the wiper ?Regards, Dana.

author: Circlotron
author_rank: Super Contributor
timestamp: February 20, 2018, 12:33:34 pm
content: 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.

author: danadak
author_rank: Super Contributor
timestamp: February 20, 2018, 12:49:44 pm
content: Ap note on current sources and sinks.http://www.ti.com/lit/an/sboa046/sboa046.pdfRegards, Dana.

author: newbrain
author_rank: Super Contributor
timestamp: February 20, 2018, 12:52:26 pm
content: 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?

author: Damianos
author_rank: Frequent Contributor
timestamp: February 20, 2018, 07:45:53 pm
content: This ^ ^ ^ ^Answers to a vague question can lead to bad results.

author: brownt
author_rank: Regular Contributor
timestamp: February 20, 2018, 11:41:51 pm
content: 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

author: brownt
author_rank: Regular Contributor
timestamp: February 20, 2018, 11:43:27 pm
content: 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.

author: hamster_nz
author_rank: Super Contributor
timestamp: February 21, 2018, 01:10:55 am
content: 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)

author: Insatman
author_rank: Frequent Contributor
timestamp: February 21, 2018, 01:25:08 am
content: 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.

author: Circlotron
author_rank: Super Contributor
timestamp: February 21, 2018, 07:09:52 pm
content: Quote from: brownt on February 20, 2018, 11:43:27 pmYou won’t get useful results supplying a 15mA load with a constant current source.