GV 640 Service Manual 75
For internal use only
7.9.2 Structure of the mechanical reed contact
Reed contact switches activate or cut off electric circuits. They comprise contact studs melted into a
glass envelope hermetically sealed with vacuum or inert gas; they form both the contact spring and the
armature. The name originates from the reed of wooden wind music instruments which are similar to the
moving contacts. The contacts are made of ferromagnetic material plated with a thin layer of precious
metal (e.g. soft iron). The contacts are activated by a magnetic field from the outside which is generated
electrically by a permanent magnet installed close by or in a corresponding solenoid. The magnetic field
attracts the two contacts thus closing the electric circuit. As soon as the magnetic field weakens or falls
below a specific level, the contact opens again due to the spring effect. Reed contact switches are very
sensitive to mechanical effects such as distortion.
1 Circuit board with a reed contact switch
2 Glass envelope
3 Contact studs
7.9.3 Replacing the flow sensor
This is only possible if the side panel on the left is removed.
Risk of breakage! Do not bend or kink circuit boards with a flow sensor in a glass envelope!
Very sensitive component!
1. Carefully bend the plastic flap on the heat exchanger out-
wards.
2. Disconnect the plug connection.
3. Remove the whole circuit board and the flow sensor from
the catches.
4. Unpack the new flow sensor, take the circuit board out of
the transport securing device and dispose of the holder.
5. Carefully engage the new circuit board in the heat
exchanger.
6. Restore the electric connection and bend the plastic flap
back again.
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