My goal is to use the 2560’s PWM output to drive a Solid State Relay that controls ≈2000watts to a hot water heater. I also want a remote indicator of the PWM %.
I believe I am good to go.
Exactly which type of Solid State Relay will you be using, considering that you are driving it with a PWM signal ?
Do you have some questions or doubts?
Then good luck.
I don't think that you can control the output of that solid state relay using a PWM signal from an Arduino Mega2560.
However if you can show me a datasheet that says that you can, I will stand corrected.
I am hoping that this will be my first post that somewhere, someone does not find something wrong. 
It is a sanity check.
- I cannot imagine using high frequency PWM output to drive a Solid State Relay that controls ≈2000wattsto a hot water heater, 220VAC @ 9 Amps, is even close to being a smart thing to do.
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Most of SSRs are designed to do on/off switching for AC cycles, not phase cutting.
So something in range of >10ms.
I am assuming the 220 Volts is AC. There are some MOSFET solid state relays that will work at the Khz speeds but at last look they were pricey. In case you have built this I recommend since the inertia of the water temperature will be slow consider maybe a second or slower PWM cycle. Going faster would not improve the performance but will up the cost.
Another suggestion is move R55 to the PWM pin 2, that will eliminate the voltage divider for the MOSFET gate.
Not sure what the analog circuit does as the percentage information is available from the processor.
Let us know how you do.
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Can you show us a link to a water heater that DOES NOT have a built-in thermostat? Or have you bypassed it? If so, what safety feature do you have to monitor the water temperature and keep from burning someone or exploding the heater tank from over heating and creating a steam boiler.
edit: Now that I think more of this, the thermostat protects the heating element and has nothing to do with water heating. The water just cools the heating element until it can’t cool any more. The thermostat will go off and on many, many times before the water temperature reaches the thermostat setting.
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Photovoltaic excess diverter?
Use a low frequency RC filter to generate a set temperature signal and a comparator to the actual water temperature that controls the relay. Display whatever you like, I'd prefer the actual temperature.
Yes. Water heater rewired so upper and lower elements are in series. existing thermostats are functional.
Looks like I will need the more expensive “proportional SSR” with different circuitry.
MPC PC 24 25 A
Thanks to everyone for your input. I’ll get back to you.
You shouldn't be using PWM or "variable power" and PWM usually doesn't work with AC anyway.
Virtually all heating & cooling systems use on/off control. I'm sure you've noticed that with your home heater. Your refrigerator and oven work similarly.
Temperature can't change instantly so with a heater, heat is turned-on until the target temperature is reached, then it turns off.
In the real world there is usually some "swing" or hysteresis so the system doesn't turn on & off several times per minute or several times per second. (With a mechanical relay that would be "relay chatter".)
If you try to control it linearly (with PWM, etc.) you aren't applying full power so it won't heat-up as fast. And things get tricky... Because of the delay you can get an unstable system with overshoot-and undershoot.
Most solid state relays use TRIACs which latch-on until current falls to zero. With AC that's the next zero crossing. You can turn it on with PWM but it won't go off until the zero crossing.
AC light dimmers use something like PWM but it's synchronized to the AC cycle. A pulse turns it on at a fixed-point during the AC cycle and it stays on until the zero crossing at the end of the half-cycle. How Dimmers Work.
But again, heaters don't normally work that way.
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Hi, @barryfields
I agree, are you trying to control the heat output of the elements.
Or just turn them full ON when the water gets down to a certain temperature and turns OFF when a certain temperature is reached?
If the latter you do not need the PWM from the 2560.
Tom.... 
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And you get 2kW? So originally you had >8kW boiler??
That would work.
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How does the solar inverter react to sudden loss of load?
Are you sure it's ok with pwm instead of analog 0-10V?
R65 & C5 Is low pass filter that converts the 0-5v PWM to a 0-5vdc level. OA4 boosts that 0-5v dc level to 0-10v dc level. Desired because of the 40 foot distance to the SSR and a 0-100% signal for VM1 display. I have also changed R73 & R74 to 5k to drive Q1 & Q2 a little harder.
Ahh, ok.
I expect that the usable range is more close to 1-9V anyway.
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