Mains input 16f628A

L

Larynx1

New Member

• Feb 10, 2013

• #1

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

Super Moderator

Most Helpful Member

• Feb 10, 2013

• #2

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.

 

L

Larynx1

New Member

• Feb 10, 2013

• #3

Hi Nigel,

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

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Feb 11, 2013

• #4

Nothing attached?.

 

ericgibbs

Well-Known Member

Most Helpful Member

• Feb 11, 2013

• #5

Nigel Goodwin said:

Nothing attached?.

Click to expand...

hi Nigel,
Its so faint, its almost invisible!

Reposted for the OP.

E.

 

Attachments

• 240-5.png

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Nigel Goodwin

Super Moderator

Most Helpful Member

• Feb 11, 2013

• #6

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

Well-Known Member

Most Helpful Member

• Feb 11, 2013

• #7

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.

 

L

Larynx1

New Member

• Mar 12, 2013

• #8

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

 

mvs sarma

Well-Known Member

• Mar 12, 2013

• #9

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.