Simple Zero Crossing Detector

[edbfmi1]

Hello all,
I have something that is driving me crazy.
I am trying to detect the Zero Crossing of the AC Power line.
I have the circuit designed with a 6.2M Ohm resistor coming from the AC line into PIN BO with diodes between VDD and VSS. This gives a fairly nice 0 to 5VDC square wave on PIN B0

I am using PCM V4.140, and a PIC16F716.

The problem I am having is the program is only interrupting when the RB0 interrupt is positive. If I put a breakpoint in the interrupt routines if-else statement it never interrupts in the "else" portion.

I stripped the program to the bare bones.

Any help would be appreciated.

[PCM programmer]

The external interrupt will only occur on one edge, either the positive or
negative edge. You should call the ext_int_edge() function to select the
desired edge. If you don't call it (as in your program), it defaults to the
positive edge.

[temtronic]

I'll assume the 'AC power line' is either 120 or 240 volts. If so you really should NOT tie the PIC directly to it rather use an optocoupler type device for isolation. You could also use a small transformer for this purpose. This is a safety issue. Should (when !) that resistor fails and blows up the PIC, it'll scare you and bam... the DVM you had in your hand is across the room going through the window in far less time that it takes you to read this.


Jay

[Ttelmah]

There are quite a lot of circuits that use direct coupling like this.

First thing is though that you need to design the circuit so if a part fails it will fail safe. Simple thing like having a PCB fuse built in the line feeding the resistor, so that if this fails (they do...), the fuse will blow. Chose the clamp diodes to well exceed the rating of the fuse. Also look carefully at the voltage rating of the resistor (most are _not_ mains rated....).

Then though the critical thing. If designing a circuit like this, when working with it, you should be powering it through an isolation transformer. In many countries this is a _requirement_ when working on non-isolated circuits. After it is designed, it can then be encapsulated in a way that meets the insulation requirements for a non isolated circuit, and be 'legal', but this won't save you when designing it....

Then have you done anything like a high voltage 'working' course?. This will teach you basic techniques that reduce the chance of death. Things like avoiding jewellery when working, and putting one hand in your pocket when touching anything, so that there is not an electrical path across your chest, if something does go wrong.

Working safely with mains kit, needs to be done 'both eyes open', with a lot of thought, if it is not going to jump out and 'bite' you.....

To detect both edges, either have the interrupt routine reprogram the detection edge depending on the polarity of the signal, or use the interrupt on chance ability that does detect both edges.

[edbfmi1]

Thanks for all the input.
This was just a rough design for proof of concept. The final design does have optical isolation to tie it to the AC mains.

As for the delay_cycles(1);