Equipment Description 3-10
Revision 2 - 26 April, 2000 HP SERIES DIMMER TECHNICAL MANUAL
3.6.7 5 volt power supply
All electronic circuits other than the fan controller are run from a 5 volt supply, regulated
from the 15 V supply with a LM2576-5 switching regulator (REG1) operating as a
continuous-mode buck converter at approx 52kHz..
The demand on the 5 V rail is around 750 mA. A conventional linear regulator would
need to dissipate around 7 watts. This loss is reduced to less than 0.5 watts by using a
switching regulator. . This circuit is reliable in operation but it may be damaged by long
periods of very high mains voltage, eg. over 300Vac.
If not working, check the regulator or flyback diode D1. Note that D1 should only be
replaced with a suitable high speed flyback diode designed for use in switchmode power
supplies. Also check inductor L1 for mechanical failure.
An MC78L12CG 12 volt regulator (REG2) provides a supply rail for programming the
FLASH devices. A two transistor switch (Q2, Q4) controlled by IC5 pin 13 connects this
12 volt rail to the FLASH device when programming is required. In general this circuit
should not be active - it is only active during software upgrades to FLASH.
3.6.8 Reset generator
The reset generator is based around a MAX691 (IC8). This IC monitors the +5 volt rail
and puts the CPU into reset if the voltage is below 4.6 volts. The RESET LED should
flash briefly when power is applied and removed, and when the RST terminals are
shorted together. IC8 also disables the SRAM via the CE pins and switches SRAM
power from the main supply to the backup battery during power up and down. Capacitor
C17 maintains the supply to the SRAMs during changeover - if a dimmer is dropping its
memory and the battery is OK this capacitor may be faulty.
The Lithium battery should last approximately 5 years from the date the battery was
made - note that a 4 year life from date of product sale would not be unexpected when
delivery and manufacturing times are allowed for.
CAUTION
Danger of explosion if battery is incorrectly replaced.
Replace only with the same or equivalent type
recommended by the manufacturer. Dispose of used
batteries according to the manufacturer’s instructions.
3.6.9 Rotary encoder
While the encoder mounts on the CPU card, most of it’s active circuitry is contained on
the front panel card. A pair of 220R/10K/10nF RC networks filter the encoder signals on
the CPU card before they are routed to the front panel card via the flat cable. The value
of these parts is quite critical to correct encoder operation.
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