Chromalox MiniMax 1, User Manual, Page: 15 / 28

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SCR Control Board
The Basic control board provides the following functions:
The low voltage dc to operate the circuitry:
A switching regulator circuit converts the instrument power voltage to +8Vdc.
The power distribution for the cooling fans:
The incoming instrument power is routed to the fan power terminals. Add fusing as required
for power fan requirements
The signal condition for the on/off input and analog inputs:
The 120 to 240 on/off input is isolated by an opto-coupler. The dc and contact closure inputs
are buffered by the circuitry.
The drive signal to the SCR trigger boards:
The temperature alarm:
The heat sink temperature is derived from a resistive temperature detection (RTD)
mounted on the heat sink. This is then compared to two set points. The first alarm is
a warning and activates the externally connected device. This allows time to correct
the problem
before the second alarm inhibits the firing circuit.
The Shorted SCR Alarm:
When a short is detected the externally connected device output is activated.
SW1 - Input Selection
SW3 - Phase/Triggering
Selection
LD3-SCR1 LD4-SCR2 LD5-SCR3 LD2-Demand