Equipment Description 3-3
Revision 2 - 26 April, 2000 HP SERIES DIMMER TECHNICAL MANUAL
3.1.4 Low Voltage Transformers
HP12-TR: The transformers are nominally rated at 375V (primary), 23V (secondary).
The primary windings of these transformers will tolerate 415 volts without overheating.
They are protected against secondary-side overloads by card-mounted 30mA PTC
thermistors. These thermistors are sized NOT to trip during mains overvoltages, but
WILL trip if a low voltage fault occurs. Thermistor cutout will turn off the frontpanel
PHASE LED.
HP12-SC, HP6-C: The transformers are nominally rated at 240V (primary), 15V
(secondary). The primary windings are protected against secondary-side overloads by
card-mounted 30mA PTC thermistors. These thermistors will trip during continuous
mains overvoltages of more than 340V. Thermistor cutout will turn off the frontpanel
PHASE LED.
If thermistor cutout occurs, they may be reset by turning off power to the dimmer for
five seconds. However, since thermistor cutout will never occur during normal
operation, the cause of cutout should be investigated.
3.1.5 Control Tone Rejection
Control tones are superimposed on mains voltages by supply authorities to control off-
peak heaters etc. Tone frequencies of 750 Hz and 1050 Hz are common, although
frequencies from 167 Hz to 1750 Hz are in use. Amplitudes are typically less than 5
volts, although higher levels have been reported at some locations.
These tones may cause flicker in dimmed lamps, due to jitter in the detected zero-
crossing position.
The effect of control tones is minimised by filtering at the secondary windings of the low
voltage transformers. Resistor-capacitor networks (R1-CF3, R2-CF4, R3-CF5), in
conjunction with inherent transformer inductances, form a 12 dB/oct filter at 400 Hz.
Control tones at levels up to 15 volts at 750 Hz, or 25 volts at 1050 Hz, have little effect
on performance.
3.1.6 Low Voltage DC Supplies
Each of the three transformers has two associated rectifiers. One set of rectifiers feeds
the common DC supply filter capacitors, while the other provides full-wave rectified
waveforms to the zero-crossing detectors and three phase indicator LEDs via the Front
Panel PCB.
The dimmers’ unregulated supply is filtered by C9 (2200uF) and directly feeds the fan
controller, the overvoltage detector and the temperature sensors. All other circuits are
supplied via the OTEMP overtemperature cutout.
The dimmer will shut down under software control at 85°C. If the dimmer manages to
reach higher temperatures than this, the OTEMP cutout will remove power from all
circuits except the fan controller and associated circuits. The fan speed will increase due
to lighter supply loading.
The unregulated dc voltage at the common supply filter capacitor (C9) is normally
around 14 ~ 15 volts but may drop as low as 11 volts with low mains voltages, or rise in
excess of 20V during severe mains overvoltages.
Mains overvoltages are sensed as high unregulated DC voltages by op-amp U1b. Since
the DC supply voltage is dependent on dimmer loading as well as mains voltage, the trip
point of this circuit may vary from 260V to 280V AC mains voltage.
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