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power regulator

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Zocco

New Member
Hi,

:confused:I am a newbie so I hope my inquiry makes sense. I need to control the output of an immersion heater so that I can set the temperature of a liquid at a constant temperature. A thermostat is not suitable as the variations of the temperature would be too great.

Ideally feedback from a thermistor could be provided so that a stable temperature can be automatically maintained in varying conditions

The heater is 1500 watts and operates at 240 volts.

I would be delighted if anyone could supply a schematic and parts list.
 

Hero999

Banned
Regulating the power won't do any good. The temperature of the liquid will still vary widely as the ambient temperature changes.

Then there's the problem that the liquid near the element will always be warmer than the surrounding liquid; this is more of a problem with viscose liquids such as tar.
 

marcbarker

New Member
Eurotherm/Barber-Colman 2132 & 2116 Controllers

Those work very well. You need a temperature sensor, probably a PT100, and a relay as the immersion heater is too big to run directly from the controller.
That's a good controller example. There are other manufacturers making the same. They nearly all use '3-term control' which is auto-tune.

They use a PWM variable-power (via your relay clicking periodically), to accurately control the temperature (measured at the pt100) to much much closer to set point, than you'll get with a bang-bang thermostat.
 
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Zocco

New Member
Thanks Guys. I will look at these options. In the interest of costs I was hoping to be able to construct my own equipment.

It is not important that the current remains continuous. I was thinking along the lines of switching the power on and off at intervals to reduce the output and as long as the off period was not more than a few seconds it should work fine

The problems that Hero999 suggests should not affect the process too badly as the equipment would be controlling a distillation process which consists mainly of water. The boiling point will change as the distillate is removed. It is this that I wish to control as it should boil continuously but not be allowed to boil too violently.
 

Diver300

Well-Known Member
Most Helpful Member
The boiling point will change as the distillate is removed. It is this that I wish to control as it should boil continuously but not be allowed to boil too violently.
In that case, you don't want to control the temperature. The temperature is set by the boiling point of the liquid, and that changes.

You want to control the rate of evaporation. If you measure the flow of vapour away from the liquid, that can be controlled by varying the heat input. You could measure the vapour flow by feeding it though a slight restriction and measuring the pressure drop, or by seeing how much the cooling water in the condenser heats up (if you have a condenser!)

Auto-shut off electric kettles have the temperature sensor set at about 50 deg C, but put in the vapour above the liquid. When the kettle boils, there is suddenly a lot of vapour and that heats the sensor, so the kettle shuts off. They don't try to measure the water temperature, because it wouldn't work. Slight manufacturing or altitude variations would mean that some kettles would never boil and others would never turn off.

You could measure the rate of rise of temperature. That would need a microcontroller. When the rate of rise was over a certain value, the immersion heater should be full on. Otherwise, it should be at a low power. When the liquid is heating up, the rate of change would keep the immersion heater turned on. As it boils, the temperature stops going up and the power is reduced.
 

marcbarker

New Member
If you're happy to just set the power input, how about a stove power control? (you know, the square thing inside the stove/cooker, behind heat knob) They produce a PWM control ideal for manually controlling the heat input.
 
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Zocco

New Member
Thanks again,

I much appreciate your input.
Diver300, your reply makes a lot of sense and gives food for thought. My set up is experimental at the moment and not yet been tried out. I can try running it at full power and if the heater is oversized the idea from marcbarker appeals as an easy fix which is a good starting point and will allow me to adjust for optimal performance and power saving.
 
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