Hot tub temp control

[HenrikOlsson]

But it's STILL a PWM signal, right? It's just that the frequency (ie the PWM period) is a lot longer than what you normally can generate with the CCP module of the PIC.

[MOUNTAIN747]

Thanks guys for all the input. I think Amoque has a point which is supported by Dave. I’m not worried about controlling power for any AC cycle but controlling the on-off function of the SSR for say an on cycle of 5 seconds or 5 minutes so I use a pulse stream like Henrik displays in his illustration for 5 sec. or 5 minutes. After which I monitor the temp change. I’m sure a relay would do the job just fine except I don’t want to have to replace a relay every other year. I’m sure it will take some experimenting to get the on-off times worked out depending on the volume of the water. A PID is most likely overkill. I see now there are many ways to control this function. Thanks again, you guys are awesome.
Wayne

[Amoque]

But it's STILL a PWM signal, right? It's just that the frequency (ie the PWM period) is a lot longer than what you normally can generate with the CCP module of the PIC.

I suppose it is... But, I would more typically describe PWM as a means of limiting apparent output to a percentage of fully on. As an example, with an LED: If the modulation is fast enough that the LED appears less than 100% bright it would be (in my mind) PWM; if the modulation was slow enough that the LED appeared full output, I would call that "blinking". I'm sure that there are a dozen issues with my definition/ description, but its what first comes to mind as how I'd differentiate.

[HenrikOlsson]

As we all know the term PWM means Pulse Width Modulation. There's nothing in it saying how long the period containing the "pulse" is, all it means is that the ratio between high/low is what carries the information. As with FM, AM etc, you can frequency modulate a 1mHz signal as you can with a 1THz signal etc and so on.

The LED in your example is either on or off (except during the transisitions) it's just that our eyes isn't fast enough to see it. The inertia in the "system" (the switch, the LED, our eyes and brain) "filters" the "high frequency" components out and we see it as 50% (or whatever) brightness.

Same thing with the low(ish) power heating element in a large volume of water. The "system" (the switch (SSD), heating element, water and even the sensor or our body if it's a hot tub) is so slow that you can have a PWM period of 60 seconds. If the element is on 30 seconds and off 30 seconds during that 60 seconds period you're effectively limiting the total power to 50%, just like you are with the LED.

Of course, if you reduce the the period too much (say a day or week or month, all depending on the system) you're going to get "blinking" with the heater/water as well. The effect of selecting a too long PWM period is obvious but it's still a PWM signal.

/Henrik.

[MOUNTAIN747]

Henrik,
Thank you once again for your eloquent technical explanations. I always enjoy reading your posts as I’m sure everyone else does as well. Now my only decision is how much time I want to spend on this code. Thanks.
Wayne