Ds1820 -> pid -> pwm

[aratti]

Working with ac and resistive load, phase angle control is the choice (dimmer principle), PWM works very well only with dc.

Since you will have 50 Hz main, every half cycle will last 10 millisecs, so once you have found the zero crossing moment, you will fire your triac with a delay from 0 millisecs to max 10 millisecs.

where:

0 millisecs delay = 100% duty
.
.
5 millisecs delay = 50% duty
.
.
10 millisecs delay = 0 % duty

Remember that your triac will turn off at every zero crossing, which make phase angle control a rather efficient and easy way to control resistive loads with ac.

Cheers

Al.

[HenrikOlsson]

Hi Al,

I agree, phase angle control is nice but IMO overkill for a system like this.

The heater itself and the system as whole is most likely quite "slow". Creating a low frequency PWM with a period of 1 or even 10 seconds will likely work just fine.

That way the slight error introduced by the solid state relay (or triac) not shutting off until next zero-crossing can be disregarded. (Max 1% with 1s period or 0.1% with a 10 second PWM period.) Depending on if the SSR is "random firing" or not it'll be a little bit different but still good enough IMHO.

Of course it depends on the power of the heater, the amount of water in the tank and the amount of "temperature ripple" you can tolerate but I don't think the tank will "respond" fast enough that a 1 or even 10 second PWM period won't work.

/Henrik.

[aratti]

The heater itself and the system as whole is most likely quite "slow". Creating a low frequency PWM with a period of 1 or even 10 seconds will likely work just fine.

Hi Henrik, Let me say that I could accept your statement only if we talk about an amatour project. But since the OP opened the thread with the following statement:

I need to do a university project, for "Computer Control. The project consists of performing a PID controller based on a Microcontroller.

In this case I will expect from him to challenge the highest performace he could think of.
He will be a tomorrow electronic designer and he will have to face an extremely aggressive global market, so I do expect from him something more than "something that will likely work just fine"

Herik, I Hope you will agree with me!

Cheers

Al.