Mains input 16f628A

[Larynx1]

Hi everyone,

I need to monitor for mains power failure using an input to 16f628.

I have been looking at using the transformerless circuits on Microchip appnote AN954.

My intention is to build the resistive 5v supply and put the output through a resistor/transistor arrangement into RB0

My question is this, firstly is it safe and secondly would I have to connect the 0v of my micro to neutral line?

thanks in advance for any help

 

[Nigel Goodwin]

It really depends entirely on what the circuit is doing - as that method would leave it all entirely live.

A safer method would be to use a transformer, and power the circuit safely from that, taking a 'mains' feed from the output of the bridge rectifier but before a single rectifier feeding the reservoir capacitor. This gives you the option os detecting zero crossing as well, and is perfectly safe.

 

[Larynx1]

Hi Nigel,

Thanks for your comments, attached is what I'm trying to do

 

[Nigel Goodwin]

Nothing attached?.

 

[ericgibbs]

Nothing attached?.

hi Nigel,
Its so faint, its almost invisible!

Reposted for the OP.

E.

 

[Nigel Goodwin]

Right

Obviously the emitter of the transistor would need connecting to the -ve of the electrolytic - but with only a 470ohm dropper resistor there's going to be an almighty BANG and clouds of smoke

The OP also hasn't addressed the problem of the entire circuit been live to the mains.

 

[alec_t]

Just plug in a wall-wart (e.g. redundant phone charger) and use the micro to monitor its output. Cheap, and much safer than the transformerless method.

 

[Larynx1]

Thanks for the advice chaps, I think I'll stick with the 'wall wart'

 

[mvs sarma]

You would surely be safe with an isolated PSU. Non-isolated are only for such applications where the human could not reach the device without disconnecting power. Otherwise it could be FATAL.