Author Topic: Current and resistance  (Read 124 times)

Offline Turbidaceous

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Current and resistance
« on: February 17, 2020, 04:15:44 PM »
I understand a constant current is ideal. But if I buy a constant current power supply and use a potentiometer to reduce the ma current to what I want, won't that cause varied results because of possible voltage fluctuations?

As I understand it the volts and the ohms of the resistor dictates the current. So if I buy a constant current power supply, then cap it to 3ma with the turn of a dial on the potentiometer, won't fluctuations in voltage cause the ma to go up and down? I'm not an electrical genius, so please if you could enlighten me on this.

I found this article but I am not sure what will happen. I am going to try with a constant current 24-42 volt range constant current power supply and a 50k Potentiometer. I just don't know what the results may be, based on this info:

Quote
A constant voltage power supply will attempt to hold the target voltage no matter how much current it's asked to source.  So for simplicity's sake, assume you have a variable resistor (a potentiometer) across the output of a constant voltage power supply and you then vary the resistance which in turn varies the amount of current drawn from the power supply.  For an ideal constant voltage power supply the voltage across the variable resistor will hold constant, no matter how much current its delivering.

Of course actual devices will have design limits and at some point you'll hit a current that's more than the circuitry of the power supply can handle and at this point the voltage will start to drop and you'll hit a maximum current.  Well designed power supplies will have current limiting or short circuit protection built in to they won't blow up if someone shorts the output terminals.

An ideal constant current power supply will deliver a constant current to the load no matter how much voltage it needs to do this.  So once again for simplicity assume the same variable resistor across the output of a constant current power supply.  This time assume that you set the resistance to zero - a short circuit, or very close to a short circuit.  The power supply will now deliver the few millivolts or whatever is necessary to drive the specified current, say 1 amp for example.

Now you increase the resistance to 1 ohm.  The constant current power supply will increase the output voltage to 1 volt so it can maintain the constant 1 amp current, and so on.

Once again, real world supplies will have design limits and at some point when the load resistance gets too high the supply will no longer be able to drive the voltage high enough to deliver the asked for current and you'll hit a maximum output voltage.

Offline Gene

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Re: Current and resistance
« Reply #1 on: February 17, 2020, 09:44:33 PM »
If you buy a constant current supply like that 300ma LED supply, it will try to maintain 300ma. If you try to reduce that to say 3ma using a pot, you're not going to like what you get because it will still try to maintain 300ma and as such, peg to ITS supply voltage and it becomes a really crappy constant voltage power supply at this point because it can no longer regulate current where its also not really regulating voltage - just connecting its supply voltage to its output basically.

I'm not sure what you mean "fluctuations in voltage" using a constant current source.  The whole purpose of constant current is that regardless the load (within reason) and regardless how the load resistance changes (for a colloidal silver cell the cell resistance drops a bit over a run as you accumulate silver ions in the water - I've seen about a volt drop give or take from start to the end of the run variation at the fixed current due to this change in cell resistance) the current source will maintain an accurate FIXED current flow. This is the whole purpose of a current limiter because without an accurate, fixed current flowing through the cell, Faraday's law of electrolysis doesn't work and you have no clue WHAT PPM you made.

The lower resistance of the cell due to accumulated silver ions, for a fixed current, as I said above would have the cell voltage drop for the lower cell resistance.

E = I * R (voltage equals current times resistance)

If R becomes less, E follows suit.

The purpose of a 10V minimum cell voltage is a bit different. It has to do with electrochemistry and the minimum potential you want for good quality production of colloidal silver (actually Ionic silver - the colloidal silver happens after reduction). You need to maintain 10V across the cell for this reason.  I usually set my cell to about 12V to account for the drop in voltage over the course of a run so I don't have to check it at all because its STILL higher than 10V at the end of a run.  I like simple!

Where things go haywire is when the voltage of your cell plus the headroom requirement of the current source (figure a volt for a 2 transistor limit and 3V for an LM317 style limiter) exceeds the applied voltage from your power supply because now the limiter can't function and its just pegged at its maximum output voltage and you no longer have a constant current.

If you use a fixed voltage power supply and a limiting resistor, as the resistance of the cell changes due to accumulation of silver ions in the water over the run, the total resistance (cell plus limiting resistor) will drop and consequently the current will go UP.  The voltage of the cell will change also where the cell voltage is a direct result of the current times cell resistance.

Offline Turbidaceous

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Re: Current and resistance
« Reply #2 on: February 18, 2020, 01:54:05 PM »
I understand potentiometers and getting the ma I need. This transistor business totally and complete looses me. I have not found anywhere enough information for me to make sense of it and put it into practise. I ordered that constant current power supply hoping that it would take the headache of the "constant" current part of it, so I can then just limit the ma. I am happy with my current setup but I really want this constant current people keep talking about. I have no problem stripping wired, bending them around contacts and electrical taping them securely. I just am clueless with regards to this transistor business, 1 of them? 2 of them? How do you use them? voltage headroom? When Amazon searching I found nothing but confusing things there too. I have not found a single shred of info that lets me know exactly what I need to do.

Right now I have a wall-wart 30v 500ma power supply which I have no idea if anything about it is constant, both the voltage and the ma fluctuate by a tiny amount every second but only a small amount 0.1 of a volt and maybe 0.02 of a ma. Then I have put a potentiometer inline to tune it down to the ma I want. What would I need in order to make this constant current?