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LDmicro Forum - Should I increase the PWM frequency when controlling the powerful heater ?

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Should I increase the PWM frequency when controlling the powerful heater ? (by HELENWANG)
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
Thu Mar 1 2018, 21:47:09, download attachment fY2v6.jpg
(no subject) (by Tom J.)
It would seem adding a frequency to voltage stage would give you control without the recurring inrush current.
Thu Mar 1 2018, 23:05:53
(no subject) (by Ziggy)
You are using a solid state relay with internal zero cross detection and triggering?

You are therefore not using phasecontrol of solid state relay but rather block control.


Block controlin the sense that the PWM signal controls duration of firing block and the firing block is 100% on when conducting.

You will see large switching transients.

In order to eliminate the visible effects of block control, block firing would need o occur at a rate no less than 18Hz ( to do with persistence of vision).

At 18 Hz you might loose the visible dimming of the lights but You may also loose granularity of control compared to 10Hz timing.

The other option is to reduce the heater rating and limit the distrurbance to power grid.
Thu Mar 1 2018, 23:24:01
(no subject) (by MGP)
You can not use a ds18b20 because LDmicro does not support 1 wire communication.

A SSR25-DA is zero crossing, phase control or PWM is not possible, beter to use block control as Ziggy quoted (counting the periods of the sinus)
Sun Mar 4 2018, 09:26:37
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