42089201TH Rev.4 29 /Oki Data CONFIDENTIAL(2) Operation of printhead (See Figure 2-3.)(a) When the printhead is idle, the armature is attracted by a permanent magnet andthe spring fixing the armature is compressed. The print wires fixed to each armatureare thus concealed under the wire guide.(b) When a signal for a character to be printed is detected, a current flows through thecoil. When the coil is activated, the magnetic flux (caused by the permanent magnetbetween the armature and the core) is canceled to eliminate the attraction force. Thearmature is driven in the direction of the platen by the force of the armature spring.The print wire fixed to the armature protrudes from the tip of the wire guide, strikesthe paper through the ribbon and prints a dot on the paper.(c) After the character has been printed, the armature is magnetically attracted againand the print wires are again concealed under the wire guide.A thermistor in the printhead prevents burning caused by over-heating of the coilduring extended continuous bi-directional printing. When the temperature of the coilexceeds a pre-determined limit (about 119°C) the control circuit detects a thermistorsignal. Printing will then be intermittent or stop completely until the coil temperaturefalls below the limit value.
