Hot tub temp control

[MOUNTAIN747]

I am considering building a hot tub with a standard 220v hot water heater element. I would use a 40 amp solid state relay to control power. I have never used a SSR before but think it would control with PWM. What do I need to know about the SSR to use PWM. I’m not worried about temperature measurement or the PID. What other consideration should I think about?

Wayne

[HenrikOlsson]

Hi,
Have you seen Scampys recent thread on pretty much the same topic?
In my experience using "normal" PWM (normal as in hundred to thousands of Hz since that what the PIC usually generates using HPWM etc) doesn't work with SSR for controlling AC and I try to explain why in the other thread. However a hot tub is a relatively slow system (if you don't have a heater in the megawatt range) and you can easily do a software PWM with a period of something like 10 seconds (or even longer). Do a timer interrupt at 100Hz, keep track of 1000 interrupts and you have your PWM signal with almost 10bits of resolution.

What other consideration should I think about?

Safety safety and of course safety!

[mackrackit]

http://www.compliancesolution.com/co...96354293_3.jpg

[MOUNTAIN747]

Thanks,
Dave - Now that looks like a great way to enjoy your last Sunday afternoon
Yea, I saw the post but I wasn’t sure exactly how it would relate to the hot tub control. Now I see that Henrik's illustration makes it clear, great illustration by the way. Hot tub would be much simpler due to the long time frame to move temperature. After reviewing the post I think a standard 1k pulse stream driven by PID on-off time would do the trick. I shouldn’t have to worry about zero-crossing, phase angles, half cycles, or any of that stuff taken into consideration when speed is a concern. Does that sound logical?

Wayne

[HenrikOlsson]

Not sure what you mean by standard 1k pulse stream but again, as long as your PWM period is MUCH MUCH longer than a half cycle you shold be fine. With SSD I'd go for 10 seconds or something, with a mechincal relay 1 minute but it obviously depends on how much the heater is able to "move" the temperature in that timeframe (which isn't much I'd guess) - as you say.

[MOUNTAIN747]

Not sure what you mean, MUCH MUCH longer than a half cycle. You mean the period being the time for the full on and off portion of a single cycle? Why is it important for the pulse to be longer than a half cycle? Does it have to do with the time needed to turn on the SSR? Looking at your illustration it would seem that any positive going pulse would turn on the SSR for the completion of that cycle? Or, am I making this more complicated than it is? So why would you switch on the SSD for only 10 seconds? Why not much longer? Water temp is not going to move very fast at 220V&18.5Amps Lets say 250 gallon tank . I’m sure I’ve missed the point. Please explain.

Wayne