Garmin GPS 155 Repair Manual

Also see for GPS 155: Pilot's guide addendum

Garmin GPS 155 Repair Manual Manual pdf 7 page image

The block diagram in Figure 3 below shows the interaction between the various Main Board circuits.Figure 3Main Board Block Diagram1.2.2.2 Power Supply CircuitsThe power supply section of the Main Board supplies +60 volts for the display drivers, ±12 volts, +15volts, +5 volts (VCC), on/off circuits, and the battery charger. In addition, the main board contains com-parator circuits for detecting low NiCad battery level, low memory battery voltage, and external power.The external power input is through the rear connector (J101, Figure 5) and from the AC adapter througha DC power jack located near the rear connector. Both of these sources as well as the battery input arediode isolated to prevent parallel sourcing. The unit is turned on and off by a digital latch circuit operatedby a knob located on the front panel adjacent to the NavData® card opening. The switching circuit is afly-back design. The switching regulator operates at a frequency of approximately 40 KHz and uses a highefficiency switching regulator for the switching device. The high efficiency switching regulator drives atransformer that is configured in a fly-back mode. The +15 volt output is post-regulated to provide 11.75to 12.25 volts. The +5 volt output is regulated at 4.8 to 5.2 volts. All other voltages are ±20%.4