APF Spectrograph Electronics
Board Designs
APF Hollow Cathode Lamp Current Monitor Board,
EL-3520
Schematic: schematics/APF/boards/APF_HCL_lamp_monitor.pdf
Page last updated: February 8, 2012
The EL-3514 is made up of two identical circuits designed specifically to monitor currents on the order of three to twenty milliamperes. This range is typical of the hollow cathode calibration lamps used at the Automatic Planet Finder telescope. (These currents are too small for the normally used Lamp Power Monitor EL-1384). Any failure of the lamp circuit such as a broken wire or burned out lamp will cause this circuit to generate a positive 5 volt output signal.
The lamp current passes through a 47 ohm resistor, R1 (R3) and the voltage developed across that resistor is sampled by one stage of a LM324AM low power quad operational amplifier configured as a comparator. When the signal on the negative input of the comparator rises above the 25mV reference voltage (set by divider R5 & R6), the comparator output goes to ground, indicating that current is flowing through the lamp. The signal is sent out on pin J1:3 (J1:4) where it sinks current in an optically isolated Galil input circuit. When the Galil circuit senses a LOW signal, it indicates that the lamp is ON.
In the absence of current through the 47 ohm resistor, the output of the comparator rises to 3.7V. When the Galil input goes HIGH, it indicates that the lamp is OFF.
Board Layout, EL-3520 sheet 2
Simplified drawing
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