Electromechanical thermostat

Here is teardown of Stego FZK 011 thermostat. Thar is an electronechanical thermostat that can control heating resistor and/or cooling fan.


The thermostat is based on bimetal strip that bends based on temperature. To get sharp transition and some hysteresis there is also magnet near silver calibration adjustment screw and electrical contacts. Components on board are small 4700 pF capacitor for NC contact filtering and one resistor (marked as RF in circuit wiring).


The electrical NC contact (heater) is rated for 10A current at AC230V and NO contact (fan) for 5A. The contacts can also be used for low voltage signals (24V DC minimum 20 mA current for reliable indication).



RF had value of around 237 kohms. When wired at mains power application means about 1 mA current at 230V AC goes through the RF resistor when the heater is turned on. Around 230 mW power loss on resistor, which means some heating to bimetal strip to reduce hysteresis according to document that came with the product. According to manual the N contact connected to resistor can be left not connected if higher hysteresis is wanted than what is got when N is wired to neutrak.


  1. Tomi Engdahl says:

    Interesting how much lower the DC ratings are than AC ratings for this thermostat:

    FZK 01170.0-000
    AC 240V NC: 10 (4)A, NO: 5 (2)A
    DC 30W (T=+5-60 C)

  2. Tomi Engdahl says:


    The mechanical thermostat is used for controlling heating and cooling equipment, filter fans or signal devices. The thermostat registers the surrounding air and can switch both inductive and resistive loads via snap-action contact. Functionality: The temperature setting on the scale equals to the upper switch point, which means that the NC contact opens. The temperature setting minus switch temperature difference (and tolerances) equals to the lower switch point, which means that the NC contact closes.

    Switch temperature difference 5K (-3/+2K tolerance)¹
    Sensor element thermostatic bimetal
    Contact type change-over snap-action contact
    Service life > 100,000 cycles
    Min. switching capacity 10mA
    Max. switching capacity, NC 250VAC / 120VAC, 10 (4) A
    DC 30W
    Max. switching capacity, NO 250VAC / 120VAC, 5 (2) A
    DC 30W
    Max. inrush current AC 16A for 10 sec.

    ¹ If the Normally Closed contact is used, the switch temperature difference could be reduced by connecting terminal “N” (RF heating resistor). It causes the thermal feedback which is subject to surrounding conditions and thus has to be determined for each individual application.

    ² When connecting with stranded wires, wire end ferrules must be used.

    Important note: The contact system of the regulator is subjected to environmental influences, thus the contact resistance may change. This can lead to a voltage drop and/or self-heating of the contacts.


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