constant source of 0.1V and 5mA - Page 1
Forum section: electronics / beginners

author: TiagoPT
author_rank: Newbie
timestamp: October 20, 2022, 02:35:27 pm
content: Hello all, some time ago i decided i wanted to crystalize copper using electrolysis. In order for the crystals to grow slowy but euhedral the voltage/current supply must be low (not considering the other parameters), the projects i've seen tend to use a regulator and some diodes to get 10-20 mA and a 0.1-0.2V however, i would like to go even lower to see what will happen. Even though i enjoy electronics a lot, i don't have any background in this area thus, im asking for a little help. For my voltage source i plan to use a old phone charger, i can't recall the exact specifications but, i think it's around 5v 300mA. Is it possible to go this lower and, possibily, even lower, if so how?Thank you allPS: this is my first topic here

author: MikeK
author_rank: Super Contributor
timestamp: October 20, 2022, 04:19:24 pm
content: A constant voltage source will vary its output current to maintain the output voltage at a fixed level. A constant current source will vary its output voltage to maintain the output current at a fixed level. One or the other. You need to first decide which one you want.

author: TimFox
author_rank: Super Contributor
timestamp: October 20, 2022, 04:25:34 pm
content: Practical constant voltage sources have an auxiliary limit on the current, either by control or practical maximum.Practical constant current sources have a "compliance voltage", which is the maximum voltage they can provide into a load, again set either by control or practical maximum.Which parameter (voltage or current) is more important to your application?

author: jeroen79
author_rank: Frequent Contributor
timestamp: October 21, 2022, 12:01:59 pm
content: Quote from: MikeK on October 20, 2022, 04:19:24 pmSort of but not entirely.You can make a circuit that will set limits to U and I and switch to CC or CV depending on which limit it reaches.(like a lab powersupply)A problem is that OP wants to work very close to the 0V output but is working with a 5V phone charger.That means that he should either select parts that can work at the 0V rail (like a true 'rail to rail' opamp) or find a way to detach the output's 0V from the input's 0V.(create a virtual ground or add a negative rail)

author: Zero999
author_rank: Super Contributor
timestamp: October 21, 2022, 12:33:22 pm
content: Quote from: jeroen79 on October 21, 2022, 12:01:59 pmThere are plenty of op-amps which will work down to 0V. The LM358 is cheap and will work from 5V, with its inputs down to 0V and output close enough for most applications.

author: magic
author_rank: Super Contributor
timestamp: October 21, 2022, 04:41:57 pm
content: Exactly.I suspect that OP basically needs a single resistor

author: Terry Bites
author_rank: Super Contributor
timestamp: October 21, 2022, 05:19:36 pm
content: Pois...Here is a simple voltage controlled current source that can go down to zero volts and zero amps. Its easy to change the scaling factor by altering Rset.This circuit is fine for a few uA to a few mA. Most opamps will work.

author: macboy
author_rank: Super Contributor
timestamp: October 22, 2022, 03:39:20 am
content: Quote from: Terry Bites on October 21, 2022, 05:19:36 pmThat is definitely a current sink, not a current source. I know that's being nitpicky but let's call it what it is.

author: Zero999
author_rank: Super Contributor
timestamp: October 22, 2022, 08:24:31 am
content: Quote from: Terry Bites on October 21, 2022, 05:19:36 pmThat doesn't limit the output voltage though.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 22, 2022, 11:37:24 am
content: Adding a CV or clamp function kinda defeats the ccs requirment. Simultaneous clamping of current and voltage is not possible.But if voltage limiting is needed a simple error amp and an npn will rob the ccs current closing a CV control loop.The cell votage is a function of the electrolyte/ electrode chemistry. It will vary a bit with temperature and also as the endpoint of the reaction aproaches. The energy gaps are in eV. So isn't the cell potential self limiting?Just for fun, I've added a super simple current source circuit for for a few mA down to uA. Select Rset for the current range. You need a decent rail to rail low voltage opamp.I've been looking in one of my old text books- it would seem that the voltage is the prime factor. Current limiting controls the rate. I which case you need a current limited voltage source. So let me think on that