SR NAND LATCH CIRCUIT PROBLEM!

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

Reply to
Myauk
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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

Reply to
Myauk

On Wed, 16 Jul 2008 18:25:08 -0700 (PDT), Myauk put finger to keyboard and composed:

I've had to add buffer amps to compensate for faulty digital gates in unobtainable ASICs, so I think I understand what you are trying to do. What is it about your SR NAND latch that prevents you from replacing the faulty gate? Is it part of a custom chip?

- Franc Zabkar

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Reply to
Franc Zabkar

d text -

Thank you so much for your sharing

Actually, it is just a 4093 IC using all four gates inside it, with resistors and capacitors mounted around it. It is easy to replace the gates if they are faulty.

I have some experience in power supplies and PWM circuits but trying to repair the alram cards is new experience. This is my first time trying to repair the alarm cards.

Can I assume that it happened because of the faulty gates? I will try replacing the gates today.

Can anybody suggest me some idea for testing such SR NAND latch circuits?

Best Regards

Reply to
Myauk

On Thu, 17 Jul 2008 02:35:44 -0700 (PDT), Myauk put finger to keyboard and composed:

I have repaired one alarm with a faulty CMOS gate. That's the limit of my experience. :-)

Coincidentally, my faulty part was also a 4093:

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I would think that CMOS inputs would be more prone to damage because they are connected to the outside world.

Your testing method seems fine to me, ie apply all combinations of 1s and 0s to the inputs and test for the correct outputs.

- Franc Zabkar

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Please remove one \'i\' from my address when replying by email.
Reply to
Franc Zabkar

Thanks for the suggestion and answers.

Best Regards

Reply to
Myauk

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