Zero Crossing Hardware question

sadlpx · Joined: 19 Feb 2004 Posts: 23

This is a hardware question but i did not know where else to ask. I'm working on a AC diming project with multipule channels (ok christmas lights). I need to detect the zero crossing point of the AC line. I've read Picref-4 but gut feel says AC into PIC with just a 20m resistor is suicide. Any help or comments appreciated.

RLScott · Joined: 10 Jul 2007 Posts: 465

A 20M resistor is safe. Even if the input voltage were 4800 volts, the resistor would only allow 240 microamps of current, which is easily absorbed by the protection diodes in the PIC. These diodes will ensure that the voltage at the pin never goes more than about half a volt outside of the power supply rails. If you are worried, then add extra protection diodes off your own - Schottky diodes - which will start conducting before the diodes in the PIC.

Robert Scott
Ypsilanti, Michigan

Ttelmah · Guest

The key question, is the _nature_ of the resistor, and the board it is on. The current at 20M, is very low, but you need to be thinking 'what happens if it fails'. This is why you will often see such droppers done with two series resistors (this way if one fails 'short circuit', you only get a double current). Oher thing is to add a SCR crowbar on the 5v rail, at (say) 5.2v.
Personally though, I'd just use an opto-isolator. Somewhat larger resistor feeding the LED on this. Use an AC input type, and it'll go off, when the AC input is at zero. Just what you want, and safe...

Best Wishes

SET · Joined: 15 Nov 2005 Posts: 161 Location: Glasgow, UK

Forum wasnt taking my posts for some reason.. Crying or Very sad

Anyway, was just going to say the same. you really should use 2 resistors for fault tolerance. And optos are cheap - put the LED side inside a bridge rectifier driven from the mains signal (via dropper resistors) and you get a nice zero-cross edge either side of the mains zero point.

sadlpx · Joined: 19 Feb 2004 Posts: 23

Thanks for the help.
1) Are you recomending 2x by 20M in parrallel or 2x 40m ?
2) Did not know there was an AC input optoisolator, agree that is the best solution. got a part number.

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

Ttelmah · Guest

There are literally hundreds of AC input opto's around. KB814, KB354NT, HCPL-3700, TIL195, H11AA1 etc. etc..
Many of them are nearly as cheap as the normal versions now.
As a comment, by 'bigger resistor', when referring to this, I am talking wattage. Even drawing only a small mA level, the total power rises somewhat, compared to the direct input drive...

Best Wishes

libor · Joined: 14 Dec 2004 Posts: 288 Location: Hungary

PICdawg · Joined: 13 Dec 2007 Posts: 17

As the others have said, I'd highly recommend using an AC input, phototransistor optocoupler for isolation purposes. Cheap part, readily available from DigiKey, Mouser, etc. I built a PIC based holiday light controller a couple weeks ago and used the H11AA1. Put a 15K 1/2 W resister on each side of the input diodes and connect to the AC line. Put a 10K pullup on the output side from the collector to Vdd, ground the emitter, and run the collector point into the PIC for nice 120hz pulse. I used the ext interrupt pin B0.

Also, do not use a zero-cross type triac for your output photocoupler or load switch. You won't be able to do any dimming!

Mine has been running 24/7 and it works great!

Guest · Posted: Mon Dec 17, 2007 3:55 pm

If this is for a product you should find out what regulatory requirements are first. In North America it's UL/CSA and in Europe it's EN's.

These specifications WILL impose limits on your optocoupler package possibilities.

If this is just for a one off or a limited run lab equipment then be careful and make sure you ground the chassis very well.

HTH - Steve H.