Review Zero-Cross Detector Please

[Olihou]

If MOC3031 is used, there is no longer any need to build a separate zero-crossing detector for the PIC ?

 

[philba]

correct if we are talking about triac control since the moc3031 has zc detection built in. If you want to do phase control of the triac, you would need to use a non-ZC version - moc3011, for example, and do zero crossing detection to know when to turn on the triac.

 

[dknguyen]

If MOC3031 is used, there is no longer any need to build a separate zero-crossing detector for the PIC ?

It's not. It's so that oven knows when 30 minutes is 30 minutes of cooking instead of 3 seconds.

I just plan to use the type of control where the thing is on for several full cycles and then off for a few cycles. It's just easier that way if it's not a light where blinking doesn't matter.

 

[em2006]

It won't make any difference to the current consumption, one LED conducts when the current flows in one direction and the other LED conducts when the current flows in the oth direction...

Yes, I'm wrong. There is not any difference, so ljcox's idea is very good.
I'll take out the two 1N4007 diodes to simplify the circuit.
Thanks.

 

[carmenus]

Hi guys I was reading this forum posts.Can you tell me if there is a opto triac like the MOC family were the zero circuit detection is connected to a pin of the ic so I can use it as a zero detection to the pic.I need to control a triac to use it as a dimmer and I want zero detection.I want to control the pic circuit with 5 volts dc as usuall and then turn on the triac with an opto triac so I will have some type of isolation.I dont like to mess with mains since in my country we have 220AC 50HZ and I dont want to die young.

 

[dknguyen]

Why don't you just use one of the zero cross circuits that appeared earlier? Once you pass the 220V through a transformer so it's only 5V it shouldn't be a problem. Dunk the whole 220V bit in high voltage insulation if you're worried.

If I knew of one I'd be using that instead of this circuit.

Either way, there is going to be something that connects directly to the 220V line whether it's an IC or a transformer. In both cases, on the output voltage has been lowered enough to be safe and the dangerous side of the circuit is still on the input side. The same hazard exists in both cases. With the IC you might acidentally touch the wrong part of the IC or something since it's so close to the "safe" side of the circuit. WIth a transformer you can at least stick it a fair distance away so there is a wider zone between the high and low voltage parts of the circuit.

 

[carmenus]

yes i decided i will opt for a transformer.It is much safer and i will not need to rectify any thing.

 

[carmenus]

Hello again.About the ZCD.Im using the circuit Nigel Godwin posted earleir in this forum.From what I can understand it is sending positive signal to the pic until the positive side of the sine wave will start so there is the ZCD.So I need to make my interrupt on the rising edge am I right.?Then after I fire the triac should I keep it on or just turn it off immediatly and it will turn of at the next ZC.

 

[dknguyen]

If I am not mistaken, NIgel's circuit is not necessarily a zero cross detection circuit. It's an AC mains oscillator circuit. It sends out a pulse where 2 pulses = 1/60th of a second. But these pulses are not necessarily at the zero cross. It depends on the base voltage when the transistor turns on. LIke if the transformer steps down the voltage to 5V or something, and the transistor turns on at 1V, then the half cycle might already be well under way and the ZC long gone. I guess you can adjust this by adjusting the base resistor...not so much if you use a FET.

But for triggering TRIACS:
Once the triac is turned on, it will stay on until the next zero crossing. When it reaches the zero crossing it will turn off until triggered again. You can only control when the TRIAC turns on, not when it turns off.

THis means that once you trigger the TRIAC at ZC, you can then control it in one of two ways:

-continue to send the pulse for as long as you want it on throughout all halfcycles. The TRIAC will automatically go off at the end of the half cycle but since the pulse is still being sent it will automatically turn back on again at the ZC. When you want it off, remove the pulse and it will turn off at the next ZC

-pulse it at every zc that you want it on for.

 

[carmenus]

So which of the circuits you suggest is the best to have the best ZCV?

 

[dknguyen]

I'd just do some testing on the circuit before actually using it. Maybe tune the resistors. And figure out the delay time (if there is one) from the rising edge at the actual zero cross. Then compensate for this in your uC.

 

[carmenus]

Dont you know about a simple schematic to get an accurate ZV?

 

[dknguyen]

SOrry, not my specialty...that's why I started this thread! (And am using a ZC Triac Driver than getting my uC to do it).

I'll say this though, the less the transformer steps down, the closer to actual ZC the transistor will switch. So maybe instead of stepping down to 5V or something maybe step down to 30V or 40V (and find a transistor that can handle these peak gate/base voltages but switches at something like 2 or 3V). For 40V peak, 3V isn't that far into the half cycle, but for 3V in a 6V peak is halfway through the half cycle.

 

[Jay.slovak]

Looks rather over complicated?, how about this, which is the normal way of doing it. The voltage regulator isn't anything to do with it, but would normally be there to feed the micro.

One could also use PICs internal voltage comparator (with internal bandgap voltage reference) to further simplify the design. A resistor divider on input of the comparator would protect the comparator at >5V and allow for some tunning.

 

[ucramola]

also see AN521 on microchip.com "Interfacing to AC Power Lines"