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Old 3rd November 2012, 11:01 AM   #1
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Default DC earth fault detection

i have a isolated 24DC power supply with earth fault indication.(ie,is the +ve or -ve line touch the earth)

for that i used 2 relays and connected the earth to coil and other pin to +ve and -ve lines
Quote:

-12V .. RELAY1 ...... RELAY2 .. +12V
-VE__/\/\/\/\/\____/\/\/\/\/\__ +VE
................ ... |
.................. EARTH


the problem is that the voltage 24 is divided into half and get a 12V across the earth and power lines.

later i used comparator which compare the earth and power lines then also i get the same problem.
then i use optocoupler with a microcontroller (atmega168 which i have now). i gave a isolated powersupplay to uC and connect the earth to gnd of uC.
then i pull up one pin(D,4) and using two optocoupler ,i swithched the +VE and -VE to that pin alternately(15 sec)
my logic is that when +VE or _VE line touch the earth ,the pin (D,4 )become pulldown and i make two outputs,the system works. but the problem is
that when i swiching the +ve lined to pin (D,4) i get a voltage around (12-24)V between -ve and earth.
likewise ,when i switch -ve next time i get same between +ve and earth.
is any body has to any idea to solve this problem.
atleast reduse the voltage below 3V while checking.
how can it possible.
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Old 3rd November 2012, 12:20 PM   #2
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Location: San Antonio
I admit I'm not sure what you are doing, but even a casual look at your diagram shows that 12V is precisely what should be measured between earth and either end of the supply. By isolated I'm guessing you mean it has no reference to earth, so I'm interested to see if someone has a solution.
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