Hello All,
I am trying to repair a pressure alarm unit, the pressure transmitter will send 4-20 mA to the unit. The unit receives the current and let it pass through a resistor (178 ohms) to convert to voltage, then it uses an op amp-amplifier stage to amplify the signal across the resistor. The output of the amp is coupled to comparator circuits. The output of comparator circuits is connected to SR NAND LATCH.
The Reset input of low active SR NAND LATCH circuit is pulled high by a resistor and a capacitor. (Power-on reset)
The problem i found in the LATCH circuit of a typical alarm card is that when the Set input is low (as the fault "low" signal comes from the comparator unit), the circuit has high output which drives the LED light, but when the Reset input is low (i connected the reset intput to ground to give it a low signal and I also made sure that there is high signal for "Set" input), the output does not go low.
But, if I add an additional resistor (1k ohms) between the output and ground, the latch circuit works normally.
On the other hand, i fear that adding 1k ohms resistor at the output might affect other operations of the alarm cards when it is installed in engine control room of the ship.
Any suggestions?
Best Regards Aung Ko Ko Thet