80 Circuit Descriptions TM9100 Service Manual© Tait Electronics Limited August 20053.6 Power Supply CircuitryIntroduction For a block diagram of the power supply circuitry, refer to Figure 3.6.The power-supply circuitry consists of the following main sections:■ supply protection■ supervisory circuit■ internal power supplies■ control of internal power supplies■ control of external power supplySupply Protection Electrical protection to the radio is provided by the clamping diode D600and by 20A fuses (for the 50W/40W radios) and 10A fuses (for the 25Wradios) in the positive and negative leads of the power cable. This providesprotection from reverse voltages, positive transients greater than 30V, and allnegative transients. An ADC monitors the supply and is responsible for theprotection of internal devices, which have an operating voltage of less than30V. The ADC also ensures protection if the radio operates outside itsspecified voltage range of 10.8V to 16V.Supervisory Circuit The supervisory circuit comprises a reset and watchdog timer. The circuitprovides the reset signal PSU SYS RST to the digital section, which in turnprovides the watchdog signal DIG WD KICK required by the supervisory circuit.Internal PowerSupplies There are nine internal power supplies:■ one SMPS■ five linear regulators (+9V 0, +6 V 0, +3V 3, +3V 0 AN, +2V 5 CDC)■ three switched supplies (+9V 0 TX, +3 V 0 RX, +13V 8 SW)The SMPS is used to regulate to 3.3V from the external supply +13V 8 BATT.The four lower voltages required are then further stepped down with linearregulators. These all take advantage of the efficiency gain of the SMPS.The 9V regulator and the 13.8V switched supply are connected to +13V 8BATT. The two remaining switched supplies (9V and 3V) use P-channelMOSFETs.Control of InternalPower Supplies The radio can be switched on using the ON/ OFF key on the control head orby means of external signals. For the latter case hardware links are requiredand there are several power-sense options; these are discussed below. Someinternal power supplies can be controlled by means of digital linesdepending on the mode in which the radio is operating.
