Author Topic: UNO Water Heater Controller (OperaHouse)  (Read 13815 times)

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JohnWilliams

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UNO Water Heater Controller (OperaHouse)
« on: September 14, 2014, 11:04:11 AM »
OperaHouse, (BTW, splitting into a new thread)

To that point of needing a charge controller to maximize efficiency for passive water heating. Now that the days are getting towards the fall, want to start bread-boarding a controller to play with. May I see your program for the UNO and a high level sketch of the control schematic? I have a tendency to want to tinker in the shack when it starts getting colder. I have a scope, parts box and other assorted gear. Soldering iron is warming up... ;) I have some IRF510 Power FETs, will they work?

I can give you an e-mail addr if you want to keep the program private.

In order to start testing it, can I use the existing 4500W 230V element that is already in the passive tank or do I really need to go buy a 2000W 120V unit? Mainly want to see current flowing at this point so I can measure it. I can bridge the math to predict a more efficient element's behavior, I am sure.

As an aside, thinking of the WIFI enabled UNO. There is a group on the web that has written some code to parse the EnviR monitor, which spits out periodic stats in XML. I can program the UNO to output data in XML, I bet. I can then modify the web app to show a graph of the system energy production. Cool stuff.   8)

John
John

JohnWilliams

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #1 on: September 14, 2014, 03:37:17 PM »
To add to the post, I just started reading the UNO chest fridge thread. Beginning to understand. Programming is a lot like the assembly lang programming I used to do years ago. Basic integer programming without a lot of fancy language features. No problem.

Still trying to understand the overall code flow as well as the final circuit diagram using 48V input to generate 56V power point. Maybe that is a MPPT concept but not yet clear on how this is achieved. Guessing you have a nominal higher voltage than 56V on this panel and you vary the duty cycle to get the reduced voltage. Must be some sort of feedback loop to monitor voltage so that the iterations can be variable?  Don't understand the relationship of voltage to current that allows for consistent delivery of watts to the element. If you control the voltage does that cause the current delivered to the element to increase? Ohms law says it should. Or are you controlling current to keep the panel at the power point voltage?

Still learning....

John
John

JohnWilliams

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #2 on: September 16, 2014, 02:36:37 PM »
I have a new application of this that I want to try. I want to use a dedicated solar system panel to run a 120V water heater to supply floor based radiant heat for my shop...

Wonder how many watts of power I will need to obtain a consistent level of heat with such a large heat sink?
John

Bruce S

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #3 on: September 16, 2014, 04:55:50 PM »
NOT enough info given to give a decent answer. SO,,,
What is the wattage size of the 120V heater? what 's the incoming water temp? how many gallons of water ?
Then you'll need to do some math.
I think it is something fairly straight forward like weight of water x specific heat (1.00 for water?) x temp diffwill equal the BTUs needed <<<this of course is for a perfect system.
Then figure out how many watts you'll need for the amount of BTUs.

Knowing of course you will have to overcome the heat loss so maybe double the original number you come up with?



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JohnWilliams

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #4 on: September 17, 2014, 07:58:26 AM »
Heat loss calculator (using planned insulation values for walls and ceiling) shows 10K BTU per hour. Selected tank is a 19gal with a 120V 1500W element. On a normal install to grid, the calculator says 3 KWh is needed to provide the necessary BTUs to heat the slab and overcome building heat losses. Using a UNO controller approach, we need slow and steady heating to overcome the heat loss sufficient to get a consistent gain in heat over time.

Sounds like this will not work...
John

Bruce S

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #5 on: September 17, 2014, 09:48:14 AM »
It's not that it will NOT work , just not as you possibly envisioned.
OperHouse is x100 more knowledgeable on this and perhaps he will chime in with little tid bits.
There are other options since you are wanting to use passive heating.
Things like running blacked pipe inside of the windows were the sunshine can assist with the over all heating of the water to get it up to a higher temp so the need to raise the water/glycol temps isn't so far?
Best of luck
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OperaHouse

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #6 on: September 19, 2014, 01:23:19 PM »
Sorry for the delay on this.  I've been at my sisters house, high speed internet but no computer.  My laptop no longer allows me to sign into fieldlines.  I've been "timed out" for over a month and this is the only site it has a problem with.  Hasn't been much of a problem since I have to go to the library anyway to get internet access.  Have to get serious about it in a month when I move back home.

I'll pretend I didn't hear anything about giving up.  Everything in the house is running off a single UNO now.  For a while the water heater had its own because I didn't want the fridge shutting down while I was doing development.  The code is simple one liner IFs giving it a conversational feel.  No nesting or complex display drivers.  The hardware is primarily that which can be found anywhere for free, old PC power supplies and UPS.  A grinder with a thin cutoff wheel quickly makes useful subassemblies from circuit boards.  These are just hot melt glued onto a board.   I thought this would appeal to those who see an old rusty brake rotor as a potential mill.  At least it would make some feel less intimidated by electronics.  So far I can't even get a snicker, let alone build one.

The current program operates the solar panel at a fixed power point voltage of about 53V for a 36V panel string.  This voltage works well with common UPS power FETs which are around 60V.  The actual voltage is set to about a volt over the real power point.  This allows it to coexist with a MPPT charger. Power point will change with panel temperature but for a season it is close enough to use one value.  It could easily track if you wanted it to.  Since the resistance is fixed, panel voltage times the duty cycle of the heater will give a "power number" without the need for a current measurement.  If the voltage is higher than the power point, increase the count by one.  if lower, decrease the count by one.  The PWM output accepts numbers from 0-255.  Very low or high numbers will only cause FET heating due to turn on/off speeds.  Numbers less than 15 are output as zero.  over 240 are output as 255 or on all the time.

Power is stored in a capacitor bank.  I use at least ten 470uF capacitors in parallel removed from old PC power supplies.  These will handle the high current pulses.  The UNO outputs a 5V pulse.  This can drive many FETs but it is preferable to drive them with about 14V.  I use an opto isolator to drive the FET.  This has the advantage of taking the UNO out of the current path.  I place the Capacitor bank and the FETs right at the water heater for minimal wiring loss.  The opto isolator feeds about a 900 ohm gate to common resistor for the drive.  At these speeds (490Hz) rise and fall times are not an issue as well as placing the UNO 50 feet away from the opto isolator.

The A/D is read and that value id fed into a running average filter.  You may have ten bits but that is also a bunch of noise.  A running average not only filters that but multiplies up the number so it looks like the voltage.   That just makes it easier to deal with.  I run a loop counter and only make a PWM adjustment a couple of times a second.  That allows the voltage to settle in from the last adjustment before the next adjustment is made.  More later.

JohnWilliams

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #7 on: September 20, 2014, 08:40:42 AM »
Understood, so far. Will start scrounging. Have a lot of defective computer power supplies around.

On UNOs, is the code portable between versions or is it specific. If so, what version of UNO do you have.

What IDE do you recommend?

Awaiting installment 2 of the story...

FYI ... Since I build ham stuff, have the usual test equip - DVM, pwr supply, scope, etc.

John

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OperaHouse

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #8 on: September 23, 2014, 03:13:52 PM »
Just lost everything I typed and running out of time.  Here is the water heater schematic.  I used a 1000 ohm in the current version.  This seems to drive the FET just fine with absolutely no warming.  as I remember from about 2 years ago 300-470 was about the ideal resistance.  I use 2 FETs in the 36V converter w/ 900 ohm and they get a little warm.  Just bought another panel and am going 48V next year.  That takes me out of using 60V FET from old UPS.

Was using fast FR302 diodes 9 parallel.  They got a little warm.  Economically it was cheaper to use seven 3A schottky than to buy bigger diode.  This heatsink is VCR head shield.  So thin I cut it with a sissors.  Runs nice and cool.  .1 X2 MKP polypropylene capacitor, Avoid MKT polyester.  With 100 ohm resistor sucks up spikes and noise.

This is the 36V converter.  I was popping the 10A breaker so I would limit the upper end of allowed PWM.  Finally just paralleled two 10A breakers.  In some areas that might be a code violation.  They are only there if the diodes short.  You can see all the capacitors in parallel.  At the water heater there are just as many on the same bus.

I have both R2 and R3, suggest buying the 2011 version as they are cheap at $8.

OperaHouse

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #9 on: September 24, 2014, 04:45:27 PM »
I bought a NYLE heat pump water heater for home and bought one of these 12V temp controllers on ebay for under $5 shipped.  Liked it so much that I got another one for the camp.  I was looking for a temperature display and easy way to modify parameters before including it in the UNO.  I still have one analog input left for temperature.  This has a single NO contact on the relay.  It can be set up as heating or cooling, differential temp and time delay.  There ate ten parameters that can be changed.  Note that the temp sensor (10K thermistor) leads pass several turns through a toroid to eliminate noise.  There are two tanks, 10 gal primary and 20 gal secondary.  The voltage supply to the tanks is about 60V.  The tanks are placed in series and the relay shorts out one tank or the other.   That limits the open contact voltage to about 30V which most relays can take.  This is still rough duty for a relay and I use one that is rated for 16A.  I thought this was a novel way to do it and eliminate big arc problems.  The temperature controller time delay is set to a couple minutes and differential set to 1C.  I just bought another panel and next year will be going to 48V panel array.  Each tank will have its own power FET drive.

The previous schematic of the tank FET drive did not show the  storage caps.  Without the caps to store the energy the system will not work.  There are the same number of caps at the 12V charge controller which is in parallel.

In doing any development there are some things to be aware of.  The A/D inputs want to see a low source impedance, think about 10K.  You should have at least a .1uF cap at each A/D pin.  The channels easily ghost.  If the input becomes disconnected or in err read the next channel you will easily see 80% of the prior channels data.  It looks very realistic!  When you plug in a USB cable, the UNO will likely power itself from the laptop as the voltage will likely be higher.  That screws up the A/D reference and values will be off when it runs stand alone.  I modified a USB cable by cutting the 5V power lead.  I reattached with back to back diodes so it is still roughly connected.
« Last Edit: September 24, 2014, 04:57:57 PM by OperaHouse »

OperaHouse

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #10 on: September 27, 2014, 12:58:09 PM »
Every project has the following:
Known knowns
Known unknowns
Unknown unknowns

I can see the controller from my bed if I stretch a little.  One morning I hear the fridge start.  and notice the blinking indicator denotes full battery charge.  Hook up the laptop and the fridge screen says the battery is at 27V. Remember my comment about ghosting another channel.  Assume the connector got some corrosion.  A carton of salt will turn into a rock in about three days from the dampness.  Had to add an upper voltage alarm to the program.

Another morning the fridge just didn't come on. Computer said it was at minus 30.  Found a broken wire.  Had to add an open sensor alarm to the program.

I run on a single battery.  Two other batteries are basically just cores.  They probably consume more than they are worth.  I got into the habit of disconnecting them from the system and running the house on them at night.  One morning wife says the coffee grinder ran two seconds and stopped.  Forgot to connect the batteries for two days.

I added this code which does the connecting for me.  The relay draws 190ma normally.  By applying full power and then PMW it down to 31%, final consumption is only 60ma.  Just two mote lines of code.




//                    CHARGING OF SECOND BATTERY & RELAY POWER SAVE

if (batcheck == 1 && inhibit == 0 && FRIDGEON == 0 && battery < 14000)  PWM11 = 0;   
 // DISCONNECT second battery Set PWM to zero
 
if (batcheck == 1 && inhibit == 0 && FRIDGEON == 0 && battery > 14400 && PWM11== 0)  PWM11 = 255;  // CLOSE RELAY OK to charge second battery
 
if (PWM11 > 80) PWM11 = PWM11 - 1;   
 // Decrement relay current down to 1/4 power if on

analogWrite (11,PWM11);  // Output PWM to pin #11


JohnWilliams

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #11 on: October 03, 2014, 10:04:18 AM »
How are you measuring the voltages for the various items (power point on heater/fridge, bat voltage, etc). I see you control the output current using the pwm port. Just not getting how you measure a voltage then adjust the pwm value to arive at the new setting.

Also, how do you determine when there is no sun and thus do you shut down everything or just let it sort of suspend itself at the current pwm value? What happens during restart when there is just a little bit of sun.

Net is I get the part of output of PWM pulses at a set duty cycle holding the current at a fixed power point, but do not understand the feedback process to adjust for changes over time. Startup, shutdown, less power, more power, etc...

Been on ebay to get a UNO. No R2s seen. Do you have a preferred vendor on ebay or elsewhere? Assume a clone is just fine...

John
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OperaHouse

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #12 on: October 04, 2014, 01:09:31 PM »
I power up everything with a relay attached to the raw solar panel voltage.  A typical 12V relay pulls in at about 8V and cuts out about 3V.  Seems I put about a 300 ohm resistor in series with the relay to delay cut in and protect the relay from over voltage.  Part of my system uses a 110V DC relay that cuts in at about 47V.  So the system only powers up sunlight hours.  The switched power for the micro comes from the battery for a more stable voltage.  I had a fairly simple resistive diode voltage drop so the UNO regulators saw about 11V during normal operation.  That worked fine for a long time till I started hanging on additional current draws.  That sorta snuck up on me.  Every once in a while first thing early in the morning I would swear the fridge turned on for a second and shut off.   Looked for a software logic error.  Eventually I found that supply voltage dropped enough to change the 5V reference a few mV.  That was enough to cause a sensor temperature drop and cause the fridge to shut off from low limit.  By the time the inhibit enabled turn on again there was enough sun so everything ran fine.  I now use one of the $2 2596 regulator modules to supply 10.5V to the board.

The voltage dividers shown in the fridge thread feed the A/D converters.  Choose a voltage divider that gives about 3V normally to protect against over voltage situations.  Try to keep the lower resistance to about 10-15K and use a cap to ground of.22uF for a low source imp.  I just use a counter limited to 0-255 to drive the PWM. 

if (pv > setpoint) PWM = PWM +1;

if (pv < setpoint) PWM = PWM - 1;

and numerous other conditions......

I have two PWM battery controllers and the water heater PWM.  Everything acts on each other so there are minor control loop problems.  The A/D values the decisions are made from arrive from a running average of at least eight values so the voltage delayed, not real time.  To solve this, decrements are immediate, increments happen about every six loops of the program.  Also, each PWM increment is staggered in time.  Just remember that all the energy is stored in a capacitor bank.  Eventually it goes to the right place and nothing is wasted. 

Most of the day the chargers sleep.  I still have two 10A Morningstar controllers in the system that control battery niceties.  Most real systems never get out of bulk charge.  There was a minor issue when the fridge turned on.  The PWM count had to ramp up the count from zero and that took a little time.  I added a line of code that if fridge was on and PMW count was zero, the PWM count was preset to a typical value. 

That is the nice thing about software, you can create the world you want.  Throw in the junk batteries and charge them when there is excess power and disconnect them near the end of the day, turn the pump on when nothing else is running, etc.  Just got an induction cooktop.  Next year I'll use that for lunch.  Press a button and the inverter switches from fridge to cooktop for a set period of time should I forget to turn it off.

JohnWilliams

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #13 on: October 06, 2014, 08:07:48 AM »
What is the total capacitance of the bank. Assume it is attached across the 56V imput and ground? I see another white cap in the picture next tot he breaker. Waht is that for?

Where are the diodes in the pic and what purpose to they serve?
John

joestue

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #14 on: October 06, 2014, 01:42:12 PM »
What is the total capacitance of the bank. Assume it is attached across the 56V imput and ground? I see another white cap in the picture next tot he breaker. Waht is that for?

Where are the diodes in the pic and what purpose to they serve?

http://johansense.com/bulk/pwm_resistor_2.png

understand that current flowing in an inductor stores energy according to amps squared times inductance/2
when the fet turns off, the current continues to flow until that energy is dumped somewhere.. if there is no capacitor, then that energy is dumped into the mosfet, as an over voltage avalanche, meaning the mosfet clamps the voltage at whatever its data-sheet voltage rating is, plus about 5%.
--this high voltage spike is usually significantly higher than the battery voltage, so the energy stored in the inductor is very quickly dumped into the mosfet, within say, 10 microseconds of the mosfet turning off.

10 uf per amp is probably sufficient to absorb this current depending on some other details.
but its easier just to throw a few large electrolytic caps at the system and not worry about it.

figure the amps flowing through the capacitors will be about half the amps flowing through the resistor at 50% duty cycle, so yes of course they can get warm (capacitors have a resistance in series with them), especially if they are old surplus crap.

the diode across the resistor takes care of the resistor's inductance, but it does not fix the problems caused by supply side inductance, for that you need the capacitor.

« Last Edit: October 06, 2014, 01:47:15 PM by joestue »
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OperaHouse

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #15 on: October 07, 2014, 11:03:36 AM »
The total capacitance on the input is about 10,000uF which is about minimal, there is a second bank in parallel at the water heater.  These caps are 330 to 470uF each. The water heater is almost always on so you really shouldn't be asking more than a half amp max from each cap.  As mentioned before capacitors have an internal resistance and will heat up and their life will shorten.    Remember this operates at 490Hz and all PWM turn on at the same time, I wish it was random, so battery charging and water heater draw current pulse can be quite large.  The white cap is on the output of the 50V to 12V battery charge converter.  I have an electronic current in the ground leg of the output.  The cap is there to make the readings more stable.  The diodes are part basic buck converter.   I haven't shown a schematic of this.  I mentioned before that these were 3A each.  They are actually5A  SR5100 schottky diodes, That would be a 3A package if it was a standard diode.

DamonHD

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #16 on: October 07, 2014, 02:42:00 PM »
OH: do you want a nice simple small fast PRNG for the Arduino to randomise your outputs?

http://sourceforge.net/p/opentrv/code-0/HEAD/tree/trunk/Arduino/V0p2_Main/PRNG.cpp

Rgds

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joestue

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #17 on: October 07, 2014, 02:51:54 PM »
randomizing the pwm frequency is great for reducing emi but it will not reduce the capacitance needed to prevent the mosfets from exploding.

the first photo in post #8 is pretty scary.
try a topology like this: http://johansense.com/bulk/50_buck2.jpg
where the top and bottom copper are used as nearly zero inductance wires.

--yes the fets can handle 200C temps needed to solder them to the board.
in fact.. they can survive operation at that temperature too. i've heard it takes 400C to intrinsically damage the silicon.
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OperaHouse

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #18 on: October 07, 2014, 09:33:25 PM »
joestue, you live in the fastlane.  My rise and fall times are too slow to have exploding FETs.  You'll be scaring Mr. Williams to death with this talk.  Scary, that was the improved version.  Here's scary.  This is one of the beams holding up my camp.  I'd tear the place down in a heartbeat if they would ever let me rebuild on the lot.   As long as the beer is cold and the milk doesn't stink I don't have time for updates.

Damon,  I've gone from a customer call to 100 micro controllers out the door in just ten days.  I can build nice but I'm trying to show that electronics doesn't have to be scary. With an angle grinder, a hot melt glue gun, and common junk very intelligent systems can be built. My whole camp runs off very shamefully simple code t6hat anyone here could develop.  I'm showing the basics, no fancy code beyond an IF.  I don't want to be writing code for people.  I'm getting enthused about this water heating.  It could be a product for those wanting to go green but don't have an easy way to do it.  If people will spend $1,200 on a HPWH to save half on electric, they ought to be willing to spend the same to get hot water for free. 

John, joestue was just answering the question he wished you had asked.

Mary B

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #19 on: October 08, 2014, 01:37:05 AM »
sister in a pressure treated beam next to the bad one...

Bruce S

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #20 on: October 08, 2014, 09:19:08 AM »
OH;
I like the idea!! What used to be guerrilla electronics , til the whole guerrilla word got caught up in all the nasties going on in the world..
Keep it up! I'm still trolling the alley and we now call 4th Monday days (that's the day the large pickup items get set out, for wire n stuff!!
The old VHS units are great for motors, and diodes.
Your quick code stuff is inspiring!!
Cheers;
     
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DamonHD

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #21 on: October 08, 2014, 04:30:34 PM »
Yes, no point in being precious about it.  If it works and is safe, ship it...

Rgds

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JohnWilliams

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #22 on: October 09, 2014, 07:59:59 AM »
In my opinion (based upon years of engineering best practice) is to create a complete system schematic, with all details and specifications documented, and a copy of the code. I have a pretty good feel for what is being done here but nothing beats a comprehensive schematic by the orginal inventor... Then we can propose changes based on a common understanding of the circuit.

For instance, is there any over-temp capability? Can the system run-away and over-current and let smoke out? What is the behavior at maximum Voc? Since I have not seen the code, I don't know if there are any safeguards implemented. These are the things I need to have if I am to install this in my home. Otherwise this is just an interesting experiment....
John

DamonHD

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #23 on: October 10, 2014, 03:13:54 AM »
Also good.

Document at least snapshots of the schematic and code.

Rgds

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JohnWilliams

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #24 on: October 14, 2014, 08:35:46 AM »
Would this type of Power FET overcome the reverse current issue that requires the external paralleled diodes in OperaHouse's design?

http://www.mouser.com/ds/2/205/DS100311A%28IXFH-FT-FQ60N50P3%29-314222.pdf

Or perhaps this one is enough?

https://www.fairchildsemi.com/datasheets/FQ/FQI13N50C.pdf

Prices are quite reasonable ...

John

joestue

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #25 on: October 14, 2014, 03:12:31 PM »
John there is no way around it.

A diode gets connected across the resistor, a capacitor across the incoming battery lines, electrically close to the mosfet and diode.
Electrically short means the wires are low inductance,. --those three components should all be soldered to the same board, right next to each other.

Any generic diode will work provided the pwm frequency is low, and it doesn't need to be high voltage either, for a 48 volt battery it would be safe to use a 100 volt Schottky diode.  98% of the mofet losses are just amps squared times resistance, so it really doesn't matter what you buy.
I would buy a fet that has a user friendly package. to-247 rather than a surface mount device for example.
« Last Edit: October 14, 2014, 03:20:58 PM by joestue »
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OperaHouse

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #26 on: October 15, 2014, 02:52:44 PM »
"Would this type of Power FET overcome the reverse current issue that requires the external paralleled diodes in OperaHouse's design?"

There are no diodes in the water heater circuit.  It doesn't need them.  Those are in the 50V to 12V converter.

OperaHouse

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #27 on: October 28, 2014, 02:20:41 PM »
I'm back home now.  I miss the reassuring whine of the converters as I wake up and stretching my neck to see the LED flashes
indicating the state of the fridge.  The last several weeks have been the big crunch to get projects done leaving little time
for the internet. It is difficult to leave the camp to the elements for seven months.

It would be a good idea to read my posts again as there is a lot of information there.  I consider the various functions of
the controller all and the same as I switch from talking about one to the other.  Heating water is just a tease as it is
something we all use.  The same functionality of the water heater can be used as a linear current booster for a remote water
pump with no battery.  Everyone has something they want to turn on and off.  This controller gives that capability to the non
technical person.  The purpose of my posts is to illustrate intuitive lines of programming and simple circuits that work.
The logical choice for a temperature sensor would be the DS18B20.  Simple and accurate.  However, the vast majority of first
time programmers can not begin to understand the multiple explanations on the internet of how to use it.  I won't show any
software that can't be used "right out of the box", not even going beyond 16 bit integer math.  This is read all over the
world and I also won't use any specialty parts. Anyone can find a couple of diodes or a thermistor out of an old PC power supply.
This also teaches them to use the MAP function that will allow them to easily convert raw numbers into something useful. 
Someone may see that as a way to PWM a windmill.  If someone looks at the pictures of my construction or lines of code and thinks
they can do better than that goof, then I have done my job. 

It is an experiment. I live comfortably for five months off the grid. Each piece is built in about an hour from junk I find
in the garage. This is Survivor Electronics.  It is all temporary as I constantly look for a better way to do it. Next year
I go to 48V panels.  And all of you are the experiment. You are all the lab rats. Can you take these concepts and put some
automation into your life.

The data sheet is a good place to start.  Yes that zener diode in the FET package is there to protect the FET.  It is the last
line of defense.  It is considered bad form to use it in high power applications where the fet is already getting hot. You don't
want to add any extra heat into the package. In this water heating example there are three FETs with over 160A total capacity.
The actual current load is about 8A.  There are three zeners in parallel. I never looked till I took the system down, the FET used
are IRFZ46 50 some amps at 55V.  These are used right at their voltage limit.  If there was to be any zener heating from inductive
spikes, I would have seen it.  These FETS run at absolute ambient temperature with no indication of heating. No heating, no
exploding.  A water heater just isn't that inductive and can't store that much energy. Adding a diode won't hurt if you like
adding parts to the system.

Scan down the data sheet a little further and look at gate capacitance.  Any power fet is likely to have over 1500pF gate
cap.  Put three of those in parallel and on/off speeds are going to be a little sluggish.  That saves you a little bit.
Look at the turn off of the scope trace. The charge on the gate has to be dumped through the 1K resistor. That is why I don't
allow short on/off periods in the code.  That would just lead to FET heating.  If you want to look like a professional and use
a gate driver chip that can supply several amps of drive, then you might to get some nasty spikes.  This isn't the way we do it
in the hills.  There is a little thought here.  The opto allows you to put the controller 30 or more feet away from the power
electronics and not worry about ground loops.  The construction methods aren't that critical because of the speeds.  The two
scope pictures are of the simple 50V buck converter.  The first is connected right to the heat sink of the FET. Nothing that
is going to kill anyone.  The second ia across the diode, output of the FET.  I only get to run the scope when doing laundry,
no picture of the water heater waveform, can't believe anything interesting there.

It wouldn't be Halloween without another scary picture.  Here is the actual water heater electronics screwed to the side of
the house.  Bubble wrap protected it all season.  More dangling electronics without a ground plane.

Mary B

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #28 on: October 28, 2014, 05:33:27 PM »
Looks like some of the stuff dangling behind my desk I use for ham radio  ;D been an interesting read. With the charge controllers I am using they have an optional relay controller that does switching for me based on set points for my on/off needs. I use a large SSR on the positive lead to each inverter and power them down based on voltage. That in turn drops whatever I am powering back on the grid via the transfer switches. I use a UPS on each to smooth out switching spikes because nothing is synchronized.

OperaHouse

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #29 on: November 29, 2014, 02:52:19 PM »
Not such a new ides.  The incomplete kit version is selling on ebay for $265    http://www.ebay.com/itm/Solar-Water-Heater-EZ-Kit-Save-No-Pipe-Changes-Hot-Water-System-PV-MPPT-/251735069810?pt=LH_DefaultDomain_0&hash=item3a9c945472    This is their technical link http://techluck.com/

kitestrings

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #30 on: December 05, 2014, 03:53:22 PM »
This is quite interesting, and something I'd probably play with if I were in a more 'conventional' setting.

What I find a little misleading on the link, and related discussion, is that they are recommending only 750 watts of panels.  The one testimonial I recall a user was happy to see ~114 degF.  What's often overlooked is the 'work' of water heater is in raise the temperature, and is nearly linear based on family size (more water; proportionally higher usage).  If you look at the math a bit:

On average typical family uses ~20 gal/person/day of hot water.  Assume the water is in at 50 degF and heated to 120 degF.  For a family of four then, we need (delta T = 70 x 20 x 4 x 8.33#/g = ) 46,000 Btu/day, or about 18 kWh/day.  To do this our 750 watt array would need to be at rated output 18 hours a day.  Here in VT we get a yearly average of about 3.5 hours.  Your mileage may very and all...

Not trying to be wet blanket; I think this is conceptually, electrically very cool.  I just don't like consumer information that is not framing things entirely for people.

~kitestrings

OperaHouse

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #31 on: December 05, 2014, 07:26:49 PM »
I thought it mirrored my experience within reason.  At home I have a heat pump water heater and for an eight day period I
used 38.8KWH to heat water.  That averaged out to 4.85 KWH a day.  Given that the unit has a COP of about 2, that would work
out to be 9.7 KWH a day for purely resistance heating.  Prior testing indicated that 3.2KWH of that was the normal heat loss
for this 30 gallon heater with an additional blanket or 135 watts per hour.  Anything over that goes into heating water.

Their claim is for supplemental heating durring the day to eliminate a majority of grid power heating. The water would start
at 120F.

At ny camp, water usage is even less. I had 30 gallons total, but the primary tank was only 10 gallons. Initially I monitored
total power into the heater each day.  I found that as little as 1 KWH would allow us to both take a hot shower. That water
would only be about 100F, but it was sufficient for our purpose.  Bear in mind that the second 20 gallon water tank provided
some temporing of the incoming water.  The final system for this year was a 36V array consisting of 720W of panels. 
Besides heating water these panels also powered the refrigerator and charged the batteries.  The panels are nearly flat, being
only at about 15 degrees, and they face southwest. Being in upstate NY, I have no better solar performance than you do.

Summer performance was quite impressive. By 11am the primary tank would be up to 40C.  It would switch over to the second tank
at 46C.  Often after we both took a shower the water would be hotter than when we started.  It is easy to understand how peoples
emperical results without instrumentation would seem better than they actually were.  You are also ignoring how much cumulitive
power there is in the morning and on cloudy days when operated at power point.  Even with a 7.2 ohm element, I was unable to use
all the power generated by a 36V array.  Next year it will be configured as a 1200W 48V array and I will be heating an additional
30 gallon tank for clothes washing.  In October the performance of the system did drop considerably with the colder weather and
shorter daylight hours.  The tanks were also sitting outside with only a tarp covering them.  There is no place in the house for
them.  Next year they will be in a well insulated shed.

I firmly believe that water heating is the most cost effective use for 300-600W worth of solar panels.  The commercial unit shown
in those videos has some technical flaws.  By using a IGBT the board has to dissipate 20W of power for no good reason. They say
pass through is 97% efficient when it can be considerably less than that.  Those three dinky capacitors probably won't be working
very well after about five years of operation.  They really glance over a major issue saying some temperature control should be
provided.  The water heaters internal temperature controls won't like it if they are switching when the duty cycle is near 100%.
Still, all appearances are that many people are quite happy with this system.




kitestrings

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Re: UNO Water Heater Controller (OperaHouse)
« Reply #32 on: December 06, 2014, 11:14:01 AM »
I had a typo in my earlier response - it should have been 13+ kWh per day (18 hours) in my example.  This is not out of line - 40% of a household kWh use can easily be heating water (if resistance electric).  The usage that you're describing is a better fit, but pretty modest use by conventional home norms.  Again it is a wide, wide range based on occupancy, patterns of use.  Throw a teenager in the mix with 30-minute showers, or hot water clothes washing and it can be notably different.

We've been diverting excess PV, or 'opportunity' diversion as it's sometimes called for a number of years.  It works really, really well and like this, the appeal is you effectively are getting Btus packed away without plumbing, penetrations, antifreeze or any of the normal details of solar dhw systems.

Quote
You are also ignoring how much cumulative power there is in the morning and on cloudy days when operated at power point.

I understand that the energy is useful, even in small amounts over an extended period.  On the other hand we haven't talked about "recovery" much either.  If the use is conservative and accumulated over the course of a summer day it is definitely measurable.  In a more consumptive home, the 3rd or 4rth shower would be pretty miserable.

I like the approach and the concept.  As I said earlier, I just have trouble with this:

Quote
Their claim is for supplemental heating during the day to eliminate a majority of grid power heating. The water would start at 120F

Leave out majority, or increase the array size, and I'm on-board with the message.  Otherwise, I liken it to the days when they tried to convince us that a 7 watt CFL could replace all our 75 & 100 watt incandescent bulbs.

Still very clever.  Regards, ~ks