Post by thread:Zero cross over detector

Use a small bridge rectifier, where the output goes through a resister into an optocoupler. The LED in the optocoupler will be on for mist of the wave, and only off just before each zero crossing, turning back on just after the zero crossing. -Adam Andy Bassam wrote: {Quote hidden}> > Hi all > > I have written the coding to detect zero cross over, it does this by > using a square wave derived from a 60 V ac input. If anyone > could point me in the way of a suitable circuit to output this square > wave. I have a few circuits, but there wasn't a nice clean square > wave outputted. > > Thanks > > Andy B

> Use a small bridge rectifier, where the output goes through a resister > into an > optocoupler. The LED in the optocoupler will be on for mist of the wave, > and > only off just before each zero crossing, turning back on just after the > zero > crossing. > or just an optocoupler like the H11AAx, which has two diodes, designed for AC detection. But either way, you'll still have a somewhat rounded signal when the input voltage is close to the threshold of the diode. If you really want a nice square wave, try a comparator/logic gate/transistor after the opto to clean up the signal. Although I don't see why it can't work with just the opto coupler... Phil Eisermann H:(440) 284-3787 (spam_OUTmazerTakeThisOuTix.netcom.com) O:(440) 329-4680 (.....peisermaKILLspam@spam@ridgid.com)

part 0 2577 bytes content-type:text/plainI thought about something like Souto's circuit, but there is a problem. The sine wave is small, lets say 8 to 12 Vac to a 5V power supply. It means that the transistor will conduct when the sine wave is above 0.8 or 1.0 Volt, so it will exist a large gap meaning "zero crossing" and not just a narrow pulse. 1V into a 10V sine wave means aprox 5.7¡ down and up, or 11.4¡ per zero crossing, as it happens each 180¡, it means 6.33% of the time. It means the transistor will be off (collector up) during 527.56µs from the 8.33ms period between zero crossings. The zero crossing circuit should get a pulse as close as possible from the real zero volts at the AC line. The above slack window will represent a AC line window with a maximum of 10Vac (in a 120Vac line), what can not be considered zero crossing. Of course that timing circuits of 263µs could be implemented after the transistor collector goes up, but it will always be ac voltage dependent, and never precise. The other problem at your circuit, it gets only one side of the sine wave, it would require another diode at least. Using the opto-coupler idea, driven directly by the ac line, it means that the opto emitter would start to lite around 2 to 3 Volts, so it would represent 3/117, or 1.46¡ x 2 = 2.92¡, or just 1.62% of the time. This ac line zero crossing slack would be exactly the necessary voltage to lite the LED. Remember that the LED would be attached to the ac line via a resistor, and if this resistor value is high, the LED would not lite when ac line is only 3V requiring a higher voltage, if the resistor is small enough to do it, when the ac line reaches high value will fry the LED. Protecting the LED with zeners or something like that would always require a low value resistor, so it will ask for a power resistor (and zeners). The big problem here, is that to sense the zero crossing we are trying to measure "when it is not zero", what is wrong. Now, if you double rectify a sinewave from the output of a power transformer, you will have a double half sine wave, and exactly at the zero crossing you will have a sharp edge, or... high frequency pulse. Using a simple RC filter it is possible to recognize exactly when the double sinewave reverts state, so it is exactly zero crossing, or in this circuit you can call it zero peak. I don't have the minimum idea how to calculate what will be the peak pulse width, so its equivalent frequency.... anybody? Wagner Attachment converted: wonderland:hfp.gif (GIFf/JVWR) (00018872)

Wagner Lipnharski wrote: > The above slack window will represent a AC line window with a maximum > of 10Vac (in a 120Vac line), what can not be considered zero crossing. > Of course that timing circuits of 263µs could be implemented after the > transistor collector goes up, but it will always be ac voltage > dependent, and never precise. Considering the risetime at this point in the waveform, I don't think the variation with voltage will be that much (in fact it will be closely proportional). The question to be asked is: What is this for? Usually for mains phase control switching. The time delays can be corrected in software; indeed I think a software PLL to correct for interference and general jitter is the way to go (but haven't tried it yet). And it is *not* a good idea to attempt to fire the Triac within 10V of zero-crossing in either direction anyway. Part of the circuit design is to avoid this very thing. -- Cheers, Paul B.