If the distortion regime on indian supply lines are the same like in
Romania, any semiconductor devices ( ie optocouplers ) will crash after a
while. I suggest you a much simple detection mechanism: three relays
connected from phase to neutral. Al closed contacts in series to your MCLR
pin used as IO input. Resistor to vcc, series end to gnd.
If one of the phase fails the io pin see high level.
Take care about the incomplete failure of a phase ( ie: 30% voltage or
less can be still active, because of inductive 3-phase loads, also the
phase can be differet from 120 degree in malfunction regim )

Vasile



On Thu, 31 Jan 2002, Jeethu Rao wrote:

> I have designed a 16f628 based timer for 3 phase loads (using a contactor).
> Now, here in India, its quite a common occurance that 1 or 2 phases in the
> 3 phase supply fail. And it could quite easily kill the 3 Phase Motors.
> I want to design a system for detecting phase failiure and to signal the PIC
> about this. I've got only one Input pin left on the PIC (MCLR!). Any
> suggestions
> or Ideas ?
>
> I thought of using a 3 input AND gate fed by each of the phase via a 1 meg
> resistor and
> a 1N4007 diode. and connecting the output of the AND gate to the PIC. Is
> this ok, or do
> I need to use 3 opto couplers for isolation?
>
> TIA,
>
> Jeethu Rao
>
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