Equipment Description
3-5
Revision 2 - 26 April, 2000
HP SERIES DIMMER
TECHNICAL MANUAL
The output devices are triggered with Siemens BRT12 opto-isolators. These are
optically triggered SCRs rated at 600 volts.
Opto-triggered dimmers have a lower ability to correctly drive low-power inductive
loads than DC-fired SCR dimmers. Although this is true of the HP12-TR, careful
snubber design and component selection guarantees that it will correctly drive all loads
from rated maximum down to 25 watt transformer pinspots.
When the lamp blows in a transformer-driven pinspot, the unloaded transformer primary
presents a very small, highly inductive load. The HP12-TR dimmer may misbehave with
such a load. However, if two or more pinspots are connected to a dimmer channel, a
lamp failure in one pinspot will have no adverse effects.
HP12-SC/HP6-C:
The SCRs used in the HP12-SC are 35 amp, 600 volt devices (type S6035K) with 2500
volt isolation to the mounting tab. They have a surge current rating of 425 amps for 10
milliseconds. The dimmer will function with replacement SCRs of lower ratings but its
ability to survive short circuits and overvoltages will be jeopardised.
The SCRs used in the HP6-C are 65 amp, 600 volt devices (type S6065) with 2500 volt
isolation to the mounting tab. They have a surge current rating of 800 amps for 10
milliseconds. The dimmer will function with replacement SCRs of lower ratings but its
ability to survive short circuits and overvoltages will be jeopardised.
The SCRs are connected in back-to-back pairs, such that during each mains half-cycle
one SCR is blocking (reverse biased) and the other will conduct if triggered.
HP12-SC: The SCR gates are triggered with continuous DC (as opposed to trains of
pulses), which forces the SCRs to conduct even if the load current is lower than the SCR
latching current. This allows excellent control over small and highly inductive loads.
The DC gate firing current for the twelve SCR pairs is provided by twelve power
oscillators. The 40kHz power oscillators operate from a 10V regulated supply, and are
controlled by phase-control signals from the CPU.
The oscillator outputs drive pulse transformers. The two secondary (mains-side)
windings of the pulse transformers are full-wave rectified and fed to the SCR gates.
Sufficient gate current is generated to correctly trigger the SCRs at very low
temperatures (minus 10
°
C).
Each of the twelve oscillators is protected by a 2R7 supply resistor, such that failure in
an oscillator fuses the resistor without disrupting other oscillators. Each oscillator draws
8mA when inactive, and around 80mA when that channel is driven to full.
HP6-C: The SCR gates are triggered with Siemens BRT12 opto-isolators. These are
optically triggered SCRs rated at 600 volts.
Opto-triggered SCR dimmers have a lower ability to correctly drive low-power inductive
loads than DC-fired SCR dimmers. Although this is true of the HP6-C, careful snubber
design and component selection guarantees that it will correctly drive all loads from rated
maximum down to 40 watt pinspot transformers.
When the lamp blows in a transformer-driven pinspot, the unloaded transformer primary
presents a very small, highly inductive load. The HP6-C dimmer may misbehave with
such a load. However, if two or more pinspots are connected to a dimmer channel, a
lamp failure in one pinspot will have no adverse effects.
Summary of Contents for HP6
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