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I've been looking for efficient Zero Cross Detector circuits and came to conclusion that transistor based ones are not the best solution as transistor itself spends some time between saturation and off state causing it to heat up. Then I thought that comparator based circuit would be better option and found circuit shown underneath.
I recon this circuit has something to do with Schmitt Trigger, however I do not fully understand it. Can someone please explain why present resistor values are used?
Does anyone know of any better ways to detect zero crossing?
If you want to switch on in the zx (i.e. not use it as a dimmer), get an optocoupler with built in zx circuit (something like the MOC3061 or MOC3061) and be done with it.
I do need to use it for dimmer like operation, hence I need ZX to cut equal parts off of each side of the AC cycle.
I just got an idea: If I use ZX to eliminate (switch off) 1 cycle out of 10, would it work as a dimmer and I had a bulb 10% less bright (or skip (switch off) every other cycle to get 50% brightness), or would I get bulb flickering?
I originally thought it was for your soldering iron, in which it would have worked that way, as long as you balance the phases, i.e. positive, then negative, then... You get it - this to avoid saturating any cores along the way.
I do intend to use it for soldering iron, light bulb was just an example. It's good to know that omitting cycles can be used for temp control.
To get a pulse at each zero cross of the mains, don't use any circuits that uses a transformer for isolation (and the raw mains for the triac, as this will not hit the ZX of the mains.Also, don't use capacitive reactance to lower the voltage, as this will introduce phase errors as well.Go purely Ohmic (as far as it's possible - metal film resistors like the SFR-series is LASER trimmed by cutting a spiral pattern along the length of it, so will be somewhat inductive).A two part resistive dropper with a VDR at the node connecting the resistors would be prudent and remember to check the resistors max. working voltages (200V is pretty normal for 0.25W resistors, but do check it, rather than collect the insurance when your home is burnt down).
Usually, there's no reason to re-invent the wheel, but designing a circuit from the ground up will be a good way to learn - but do get a review before connecting it to the mains
I will use microcontroller as LCD will be there as well. Also, pin going low interrupt will be used to get ZX at every cycle rather than half cycle with pin toggle interrupt.
Why do I need varistor in AC voltage divider setup?
Another concern I have. Whether I use comparator based circuit or not, I cannot isolate microcontroller from mains.
Then I though, why wouldn't I use clamp diodes already integrated to microcontroller. By looking for more info I found AVR182: Zero Cross Detector application notes. It seems to be much simpler circuit.
Is there something i do not forsee?
Quote from: newInRobotics on November 24, 2011, 09:21:33 AMAnother concern I have. Whether I use comparator based circuit or not, I cannot isolate microcontroller from mains. If isolation matters to you, you can use optocoupling for the ZX pulse.
How is the temperature feedback going to be generated and brought into the controller?
Using the internal protection diodes is not good practice, but you can add an extra set and a resistor externally.However, I think it limits you in respect to how you can power the circuit.
Do you have an estimate of the current consumption of the total circuit?
What I meant was, I will need to power up microcontroller and comparator circuit, to do that I am going to use AC-DC 5V adapter. No matter if I use opto as an isolator, common 5V supply will create direct path from mains if something goes wrong.
By K-thermocouple and cold junction compensator chip (or something similar).
Quote from: Soeren on November 25, 2011, 02:39:53 PMUsing the internal protection diodes is not good practice, but you can add an extra set and a resistor externally.However, I think it limits you in respect to how you can power the circuit.How's that?
Quote from: Soeren on November 25, 2011, 02:39:53 PMDo you have an estimate of the current consumption of the total circuit?I haven't given a thought about it yet. Does it change anything if my circuit draws 100mA or 300mA?
Quote from: newInRobotics on November 25, 2011, 06:43:14 PMWhat I meant was, I will need to power up microcontroller and comparator circuit, to do that I am going to use AC-DC 5V adapter. No matter if I use opto as an isolator, common 5V supply will create direct path from mains if something goes wrong.I'm not sure I follow you?
I've attached a rough circuit showing direct paths for AC to hit microcontroller. Are there any dangers there? Is it possible to isolate AC from DC in this circuit?
Yes, there are dangers - you cannot drive the TRIAC this way.
Yes, it can be isolated... Do I have to mention optocouplers once more?
I understand that opto can be used between comparator and uC, however they share common power supply, so how is isolation possible without having separate supplies feeding comparator and uC? Is there something I do not understand?
Forget about the comparator and just use an optocoupler (plus one for triggering the TRIAC of course) Need a schematic?
I see it now It's actually quite simple, however, a lot of power is wasted by R1.
Another thought that comes to head is that opto LED will be brightest at peaks of AC wave, meaning that it will pulsate (like fully rectified AC wave) at 100Hz, hence opto-transistor will pulsate output voltage to uC as well (instead of sharp ON and OFF),
this will cause early TRIAC triggering as uC thinks that everything below certain voltage (that is higher than 0V) is equal to 0V (pin low); say anything below 0.6V is considered to be "pin low", that means TRIAC will be triggered at 22V or -22V rather than at 0V. To overcome this I could use Schmitt Trigger after optoisolator,
however it does not solve R1 heat dissipation.
Eh? I thought you were in the US with 60Hz mains?
No, I'm in UK with 50Hz 240VAC
Thanks for all the info above, helped me to understand it better and also inspired to look around a bit more. Found circuit, called Isolated High Quality Mains Voltage Zero Crossing Detector, that uses small amount of current to do the same job. I've attached it below.
At least in sim it seams to work fine. And, before putting it together, I ask someone to verify it
As it stands, the ZX is from 40V to -40V and v.v. While this is of no consequence to your circuit, could you try changing D5 to a schottky device and check how much it decrease the width of the pulse?
Could you check the voltage over R3 and D1..D4 (just one of them) as well?(And post the numbers).
Right, I've had a look for what's available on eBay, and found 1N582, if I place it instead of 1N4148 low pulse width drops from 1.24ms to 570us. It's all in sim, hence I cannot say how accurate it is.
Voltage goes from -548mV to 6.46V.
What sim are you using?
Quote from: newInRobotics on Yesterday at 19:10:29 Voltage goes from -548mV to 6.46V.This is the drop over one of the diodes, right?
Could I persuade you to measure the drop over R3 as well?
Multisim 11.0 the name of the sim is
That's a drop over one of the diodes and R3 together.
Quote from: Soeren on December 06, 2011, 08:31:18 AMCould I persuade you to measure the drop over R3 as well?No probs It goes from 4.893V to 5.827V (see attached).
How is this useful?
I wouldn't be surprised if you could vary the ESR of the cap in the sim (or else you can change it in the parameter file for the cap), if you want to play around with it (if you have some spare time and want to understand some of the finer points of the circuit).
Have you started building the physical circuit for your temperature controlled soldering iron?
I think that in sim You get "perfect" cap, hence no ESR, however, adding resistor in series with positive terminal should do the trick, am I right?
According to the table attached - for 25V 10uF cap worst case scenario ESR is 1.5ohm.
Not yet, why do You ask?
You also mentioned that voltage from divider can be reduced to reduce pulse width, what exactly did You mean?