I'm creating a diy sous-vide rig using esp8266, a tubular heating element (1.5 kW) and a solid state relay (zero- cross, Fotek SSR-25DA)
I use a relatively powerful heating element, so that the water can quickly reach the desired temperature. However, once it's reached, I want more granular control, so I use 10 Hz PWM to open and close the SSR.
The problem is, my ceiling lights start to flicker when the duty cycle is not 100%.
I think this is due to the starting current of the heater, but I don't know how to approach fixing it.
Maybe I should increase the PWM frequency? But I'm not sure it will work at all, considering that the SSR is zero-cross. Or should I add some fat capacitor in parallel with the heating element?
I want to be able to limit the heating power because it takes some time (about 300ms) to get the temperature from the sensor (ds18b20:
http://www.kynix.com/Detail/206274/DS18B20.html), and leaving the heating element on for that time on full power easily overheats the water.
Here are some specifics:
1.Water volume is about 3L
2.The readout time for the sensor is 375 ms
3.Typical temperature range is 60-65 degree Celsius
4.I use the following cycle:
read the temperature (t)
if t >= target
set duty to 0
if t < target - 3
set duty to 10/10
if t < target - 2
set duty to 7/10
if t < target - 1
set duty to 5/10
if t < target - 0.3
set duty to 2/10
else
do nothing