Analog PID controller - settings wrt $V_{err}$ - setpoint always higher than room temp

I designed from scratch an analog PID controller to mantain a temperature selected in the range of 60 to 80ºC with a room temp say = 20ºC. The "plant" is a simple 4R7 5 W resistor heated by a PWM-controlled current (100% to 0% range).

For any given setpoint, say 67ºC, I know that room temp is lower, so, if heat is not supplied, the cold source (room) will be pulling the temperature continuously down.

My question: where should I "center" my PWM duty cycle excursion?

1. If $V_{err}$ = 0, DC = 50%? I heat the resistor with more or less current, pivoting on $V_{err}$ =0

2. For $V_{err}$ =0, DC = 0% (minimum or null current)? I heat the resistor with a varying current only if Verr is positive

Option 2) seems somewhat reasonable to me, but, at the same time, I see that I am just controlling the heating side of this. Do I leave the room alone doing the cooling with no current supplied by me?

• What do you mean by PWM duty cycle excursion? Also, a simple diagram of the two situations you describe would make this question much more clear. – Chris Mueller Oct 21 '15 at 21:12
• With respect to cooling it's quite typical that a PID controller will only control adding energy to the system rather than adding and removing. For example most motor control PID controllers will only control voltage to the motor rather than also operating a brake when speed is overshot. – Myles Oct 21 '15 at 22:08
• Hi Myles, I am not asking if adding or removing but adding more / adding less for option 1 or, in option 2, adding when needed and let the system to cool by itself. – atferrari Oct 21 '15 at 22:18
• For excursion I meant "variation" of the duty cyccle. – atferrari Oct 21 '15 at 22:20
• I don't follow what the alternative is to "let the system to cool by itself"/"Do I leave the room alone doing the cooling with no current supplied by me?". If you overshoot your desired temperature what other than the room would do the cooling? In a typical PID controller you would initiate action not just if Verr is positive but also if it is negative but rising too rapidly (the D portion of PID covers this). – Myles Oct 22 '15 at 14:59