manual current control

[plumber]

Is there a novel/simple way to manually adjust amperage without affecting voltage? At this point, I am trying to determine an effect, with out selling the farm or frying my fragile plumber brain. I really don't want to buy a regulated, variable power supply for my simple experiment. Building my own current limiting circuit, or achieving constant voltage and constant current would be a bit much for me at this point. Cant I just turn some dials for crying out loud? I could probably struggle through a schematic and bread board...if I had the specific resistors/components,their values and order in the circuit. But as for calculating their end result values and placement...who do I talk to? What do I do? I know the needed end result, I just cant wrap my brain around designing the circuit. Sooo, If there is no temporary and simple way to do this, Here are the parameters: 30 vdc supply that can vary maybe 15% (perfect would be constant 30 vdc) . Current must stay between 2 and 6 mA. max. (perfect would be a constant 4 mA). The kicker: The experiment will start with almost no mA. A half hour into the project, the mA will begin to increase. Over the course of several hours the amperage will continue to climb. Without control, the amperage could go to 400mA to 800mA. Sooo, Any cave man workarounds or simple solution? Or where can I go, or who can I talk to for design help? Any input would be greatly appreciated....fried plumber

 

[Resqueline]

So, the resistance of your project goes from almost infinite down to 37 ohms.
To get the minimum 2mA you need 34.5/.002=17250 ohms total, and to get the max 6mA you need 25.5/.006=4250 ohms total resistance.
The simplest solution would be a fixed resistor in series with a variable resistor - that you would have to adjust throughout the experiment, looking at a mA instrument.
The fixed resistor would need to rated at 34.5*0.006=0.25Watts. If you cant get one at 4200 ohms then settle for a standard 3900 ohm.
The variable resistor would have to be 17200-4200=13000 ohms. Use a standard 10000 ohm, or 15000 ohm if you can get one. The wattage doesn't even need to be as much as the fixed resistor so a carbon track potentiometer like the ones you find in audio equipment would suffice, but it needs to be a linear one.
For a mA meter you can use an ordinary digital (or analog) multimeter, or even a dedicated analog 10mA dial instrument.
Everything connects in a series loop, order unimportant.

 

[plumber]

voltage drop

Thank you very much!!! I had to do a lot of reading and web surfing to realize your reply was awesome and to the point. I even dabbled with some long forgotten algebra. But I think I got a handle on it. Sooo, as far as voltage drop...do those resistance values indicate the lowest voltage I will encounter to be around 24 -25 vdc ? If so great! By the way, is it possible to maintain a set voltage when limiting current? This is for a borderline electrolysis using distilled water. It proceeds very slowly at first, but as the water takes on metallic ions the process starts moving faster and faster. Thermal and mechanical stirring controls the current very well...up to a point. Once the current goes over 10mA, the results are less than optimal. So my objective is to keep it going as long as possible. Thanks again, and I welcome any further input. I've been plumbing for 26 years...so if you ever need help, you are welcome to pick my brain!

 

[plumber]

linear?

I cant find enough stuff on linear pots to grasp the difference. Is it simply the slide type? And why would I need a linear pot as opposed to a non - linear pot? I'm just trying to learn.
Thanks, plumber

 

[Resqueline]

You're welcome.
Yes, I calculated the resistance values from the worst-case scenarios using 30V +/- 15% (25.5-34.5V) supply.
This setup is neither ideal current nor voltage limiting, It's much like adjusting a tap from a reservoir feeding a garden hose used on a sloping back yard. Flow depends on where you are, so it requires relatively constant attention, but at least changes won't happen fast & drastic with this resistance setup.
One can have both current and voltage limiting in the same supply, but they can never both be in effect at the same instant.
Volume pot's are logarithmic (& marked log) whereas balance and tone controls are usually linear (marked lin). It doesn't matter if they're rotary or sliding. Volume needs to be logarithmic (only very little resistance change happens in the beginning) due to the way the ear registers sound levels.
Thanks for the offer. I guess I approach plumbing the same way you do electronics. I fixed a leaking faucet at home (where the water had dug a hole in the seat) with gel superglue.. It worked out nicely, but the plumbing have to be replaced (& altered) at some time.. Last friday I passed by & noticed something strange on a pipe at work, so I just had to Google it - and learned that there are plumbing stuff called steam traps. I guess they're important..

 

[Lowerider]

I just had to add a thanks, this was a very helpful thread.