Espresso machine heater control

Discussion in 'The Projects Forum' started by moniker, Jun 18, 2010.

  1. moniker

    Thread Starter New Member

    Jun 17, 2010

    I am looking for info re: making up a cheap and simple temperature control for my home espresso machine. I do not want to use a PID because of the complexity and cost of such a design.

    The heating element consumes about 7A at 120VAC and is controlled by a mechanical thermostat. The thermostat has a fairly wide dead band and there is physical hysterisys due to the mass of the boiler and the water being heated (water continues to get hotter after the current is halted and temperature will continue to decline after the current is initiated).

    I want to center the temperature between 92-98C and then after testing, be able to change the set point up or down a bit. Of course, there will be undershoot and overshoot in the physical system and Therefore I suspect that my circuit should not add additional hysterisys if not necessary.

    Because of the current that will be seen, a low or non-existant hysterisys in the circuit is probably a good idea. I suspect that an SSR or a Triac will be necessary to actually switch the AC to the heating element. If there is any chattering around the set point the semiconductor switch should handle the problem with silence and aplomb.

    I found the attached circuit and really like it because I understand its basic operation. But I am not sure how to handle certain problems. I have attempted to contact the author with no luck. I hope someone can help with the following questons.

    How low a voltage can this circuit use? Six and 9 volt supplies are easily available to me. Twelve volts is inconvenient. Suggestions?

    The circuit is set up to use an electromechanical relay. Which should I use, an SSR or a Triac? Suggestions?

    How should I set up the circuit to handle an outboard SSR or a Triac? Suggestions?

    If you have another circuit that will do a better job than this one for the intended application, please give me the URL.

    Thank you all for any help you can give. Suggestions, reccomendations and alternatives will be greatly appreciated.


    Last edited: Jun 18, 2010
  2. bertus


    Apr 5, 2008

    For the power stage you could use a MOC30XX optocoupler with a suitable triac.
    In the datasheet there are example schematics.