Analogic output for chilling (not relay based)

General
1

My idea is to use a mixing valve to modulate the cooling power into fermentor, avoiding themal shocks when using a powerful chilling method.
To make a control like this, an analogic output should be necessary, repolacing the relay, and the prediction algorithm should be turned off (is it right?).
Is a 1-wire output (0-10V or 4-20 mA) available? Is the software ready for this?
Thank you!

2

I am actually currently modifying the firmware for PWM (which is what you mean basically).

By default heaters will have a 4 second PWM period, but for a fridge compressor, you will have take into account a minimum off time. When the compressor is turned on, it builds up pressure. If you turn it off, and then back on again, without giving it enough time to equalize the pressure from the previous run, this will damage the compressor: the compressor motor cannot overcome the pressure and cannot start. It will get hot and break.

So for the fridge, a PWM period of 10 minutes I think is necessary. To protect the compressor, I plan to have a minimum ON time of 2 minutes and a minimum OFF time of 5 minutes.
The PWM algorithm will self-correct in the next cycle if the minimum ON or OFF time resulted in a different duty cycle than what was required.

I am still trying to figure out how to use peak detection to auto-tune the PWM, preferably online.
Ideas for that are very welcome.

3

Hi Elco,
what I actually would realize is not related to the fridge-logic, but to the brewery logic.
I’d like to apply a modulating valve to the tank glycole jacket, so the cooler has not to be switched on/off from brewPi.
If the valve is opened (PWM > 0), another digital output could help, switching on the glycole pump.
Is it possible?

4

The PWM signals will be quite slow, with a period of 4 seconds. A bit faster is possible, but they way it is coded now does not support PWM signals in the 100 Hz range.

With the new valve controllers that are coming, they do have a feedback signal to know when the valve is open or closed.
It would be possible to time how long opening takes and to stop halfway.