120VAC to 28VDC Question

[johnfgoodman]

Hello all,

I am new to arduino, and am still in the research stage of my project. I plan to use an arduino mega. My goal is to replace my home pool control board that controls 2 pool pumps and the saltwater chlorine generator. I think I have been able to research solutions for most of what I need (including controlling pumps and monitoring water levels) except for the matter of supplying power to the salt cells. Currently the salt cells are connected to the control panel (Autopilot DIG-220, malfunctioning at the moment).

There are different power levels for the cells, depending on how much chlorine needs to be generated, but the max level supplied is 26VDC at 8 amps. My supply could be either 120VAC or 240VAC, as both are present at the control board.

My question is what are the options for obtaining that voltage and current (which seems like a pretty strong current)? In my research I have not seen any power supplies that supply that voltage and amperage, at least not affordable. I have researched a little about bridge rectifiers and using transformer + diodes + capacitor + regulator. Is this the best method? Are there other methods I have not come across?

Would controlling such a current with an arduino be feasible? Would I be able to adjust the voltage and current depending on chlorine levels?

Thank you all for your help.

[allanhurst]

That's a >200 watt psu, so no route is going to be very cheap.

Does it have to be smooth dc? - I wouldn't have thought so for an electrolytic cell.

Does it have to be variable ? or would turning it on/off with a variable mark-space ratio be acceptable?

If your control board has a biggish conventional transformer for this job you may be able to salvage it.

If (as  probably) it's a switchmode device, no - and I suggest you don't try and build one unless you have considerable experience in that area.

Allan

[Paul_KD7HB]

What is wrong with using the power supply in your current controller? Or did it all go up in smoke?

Paul

[justjohn]

What is wrong with using the power supply in your current controller? Or did it all go up in smoke?

Paul

I'd agree, since the supply is what will cost the "big" bucks.

[johnfgoodman]

Thanks for the replies and the help!

Does it have to be smooth dc? - I wouldn't have thought so for an electrolytic cell.

I am not sure, and I had not considered that question. I will have to research that a little.

Does it have to be variable ? or would turning it on/off with a variable mark-space ratio be acceptable?

Good point. Actually, I think that is how it is currently set up to work, so probably being variable is not necessary.

If your control board has a biggish conventional transformer for this job you may be able to salvage it.

If (as  probably) it's a switchmode device, no - and I suggest you don't try and build one unless you have considerable experience in that area.

I am not sure if it is switchmode or not, I will have to look into it.

Thanks for the input, this helps point me in the right direction!

[johnfgoodman]

What is wrong with using the power supply in your current controller? Or did it all go up in smoke?

Paul

Well, it did not go up in smoke, but did receive water damage after the casing cracked. I tried repairing some of the obvious damage, but I am not sure of the total extent of the damage. So I could theoretically attempt to salvage the power supply, but will have to test to make sure it was not damaged.

Also, I was thinking of keeping the control unit for parts, in case I ended up going back and getting a new one of the same system and needed to do repairs later on.

I am not opposed to spending money on a power supply and the project as otherwise it will cost around $700 to replace the current unit. In researching a little more, it seems that desktop power supplies would be able to provide the power needed. Would that potentially be a viable option? One consideration is that it is outdoors, so I would have to come up with a way of sealing all the components...

Thanks!

[MarkT]

Hmm, water damaged not good for mains supplies...

I would suggest an open-frame 24V adjustable supply might be the best option - has to be mounted
in a case of course, but you should be able to get to around 28V on the adjustment, and they are available
in a large range of power outputs.

Quick search found this one at Farnell: http://uk.farnell.com/xp-power/ccb200ps24/power-supply-medical-ac-dc-24v/dp/2783921

Only 5% adjustment range though, but 25V and 8A ought to be usuable?

Simple PWM duty cycle control might be enough, at either a low rate (1Hz?) or with an LC filter you
could run at a high rate (ultrasonic).  The LC filter smooths the load seen by the supply if switching
rapidly.

[allanhurst]

Or for less money eg...

http://www.ebay.co.uk/itm/PANEL-TRANSFORMER-400VA-240-24VAC-16-Amp/182934183625?hash=item2a97ba1ec9:g:PnQAAOSwaSZaHnwV

You then need a 25A bridge rectifier (<5$), a suitable relay for the primary, and a smoothing capacitor of say 5000uF rated >40v

Allan

[johnfgoodman]

Thank you MarkT and Allan for the suggestions. I will look into those options.

[MarkT]

Thinking about it the old-fashioned transformer+rectifier approach might be a better fit, since it doesn't need
to be lightweight, and the simplicity means a longer life is likely - so long as the transformer is fully dipped/sealed one that won't be affected by damp.

[JohnRob]

If you chose a transformer / rectifier / capacitor solution,  be aware the current in the bridge rectifiers and capacitor is likely to be 2 to 3 times the load current.  There are many sites showing the calculations.
Essentially the larger the capacitor, the shorter the time the diodes are conducting and the higher the current.

Question.  It is possible you could use the raw rectified voltage to power the cell?   you could still PWM the output if you needed variable.   And I guess you could use phase control on the input of the transformer.

[johnfgoodman]

Question.  It is possible you could use the raw rectified voltage to power the cell?   you could still PWM the output if you needed variable.   And I guess you could use phase control on the input of the transformer.

Not sure about the answer to this question. I have not found a whole lot of documentation on this cell and am not very familiar with how they function.

After digging around a little more, refining my search with the help provided here, I found some products such as this:

https://www.amazon.com/ABI-Supply-Outdoor-Rainproof-Weatherproof/dp/B01IU8QBCO/ref=sr_1_5?ie=UTF8&qid=1512678252&sr=8-5&keywords=led+power+supply+24v+waterproof

Advantages: waterproof (so can install outdoors), inexpensive, will meet power requirements.
Disadvangtage: not as educational and fun as putting something together myself?

Thoughts on how something like this might work for my situation?

Thanks!

[MarkT]

The picture shows a fan through the grille!  That's some strange new definition of weatherproof I think, especially as it doesn't appear to have an IP rating even.

You need something certified IP56 or better for outdoors I believe.  This unit, if mounted the right way up,
might well survive the odd light shower, but that's not the same as being weatherproof.

[allanhurst]

Do you mean IP65 ? That's what I always specced for stuff like this...


Allan

[MarkT]

Water is the main risk, not particulates I think - immersion proof is pretty good thing to aim for in a
pool controller.  IP?6 or better?