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[on:]

I am working on the electrolysis of molten salts and I need to design something that would regulate the voltage supplied to the cell so that the current would be constant. I have looked into different things but I am not sure what direction to take. I can get current readings with the Arduino, the problem is how to adjust the voltage consequently.

I looked into digital potentiometers but I am not sure how I would implement them in a circuit that would do this type of control.

I plan on using about 6V and 20Amps in the electrolytic cells.

Links to pages that explain how to use such ICs with the Arduino would be helpful (and also the design of a circuit that would allow the use of digital potentiometers controlling circuits with high currents). Other ideas on how to achieve this are welcome as well.

Thank you

[Reply #1 on:]

analogWrite generates a variable PWM output that could be used to drive a transistor.  A MOSFET such as
the PHW7N60E is rated for 7A continuous drain current and thus could be used to control the current.  Does
pulsed DC count or must it be continous?

(The transistor is just one I have lying around, there may be better ones, but that one can be controlled directly
from an Arduino pin, I would put a series resistor between the Arduino and the transistor gate to limit the current
when pulsing - say 100 Ohms.).

The transistor will need a heat sink.

[Reply #2 on:]

The transistor will need a heat sink.

It might help to keep the salt molten

More serious, what temperatures are you talking about and how do you monitor them? or don't you?

[Reply #3 on:]

The molten salt is kept at temperatures around 400C during the electrolysis and the current needs to be constant. Temperature is regulated by a temp regulator in the oven...
The MOSFET mentioned is rated to 7A but I need 20A, is there a way to design a circuit that would still allow the use of transistors?

[Reply #4 on:]

Sorry, I misread the V and I!

I've never used MOSFETS at that high a current, but ebay has lots of MOSFETS rated at 110A!.  Seem to be about £1.48 each, which will not break the bank.  The heatsink and power supply will be a lot more.

Also, from some quick googling, you can parallel N type MOSFETS because they have a positive temperature coefficient, though you should probably put them all on the same heatsink.

Its not clear to me from your reply whether "constant" means 1 "a steady DC that varies relatively slowly thus keeping the temperature correct" or 2 "a pulsed DC at about 480Hz, with the effect of a mean DC that keeps the temperature constant. 

If  1 you will have to average the pulsed PWM with a RC filter, and the transistors heat calculations will be more complicated - and they will dissipate more heat. 

If 2, then the PWM output will do, and the transistors will mostly be off or on, and so the mean power dissipated will be lower.

Another option might be an rectified output from an auto-transformer that is controlled up and down with a motor, controlled by the arduino. 

[Reply #5 on:]

Is there some reason you can't just build/buy a constant current source with the required voltage/current rating?

[Reply #6 on:]

- The current I am taking about is only for the electrolysis redox reaction, not for controlling the temperature. There will be an oven to keep the salts molten and at the right temperature.

- I have a power supply rated for the right voltage and current, but it only allows me to set a voltage, not a constant current. I need to design a circuit and program that will read the current passing through the load (that will vary with time) and make adjustments to the voltage to keep the current as constant and as close to 20A as possible.

I will look into those high current MOSFETS Shelleycat, Thanks

[Reply #7 on:]

- I have a power supply rated for the right voltage and current, but it only allows me to set a voltage, not a constant current. I need to design a circuit and program that will read the current passing through the load (that will vary with time) and make adjustments to the voltage to keep the current as constant and as close to 20A as possible.

Yes, I get that you want a constant current source. What I don't get is why you're trying to design and build your own. They aren't especially expensive to buy and will probably be cheaper than the Arduino you'd be using for your DIY one. For example, LT3741 can handle 20A at 6V and you can buy them individually for a few bucks. Do you actually need to 'design a circuit and program' to solve this problem?

[Reply #8 on:]

you will probably need to used a inductive filter in series with your load . This inductor in series will make sure that even if the voltage fluctuates (the PWM voltage) the current remains almost constant.

[Reply #9 on:]

Thanks PeterH! that might be just what I was looking for