Post by thread:[OT] Triac firing, zero cross detection.

I have to control 8 triacs for dimming incandescent lamps with the minimum possible cost. I have been doing some research but there are still some aspects I would like if someone could help me with. Follows the list. - What is the best discrete zero crossing circuit ? I have found many, ranging from a single resistor, a resistor and a zener up to a full wave bridge with clamping diodes and a transistor. Do they all work the same in practice ? I guess they do not have to be very precise because I would not be able to fire the triac exactly at zero crossing anyway ! Is the optocoupler circuit better ? I do not thing so, specially in my case where I do not need isolation from the power line. - How to fire the TRIAC without a Opto-diac ???? I have found many circuits and explanations again, but none has fully convinced me ! The circuits range from a single resistor from the microprocessor to a push-pull stage with transistors and a capacitor to the triac gate. I will not be using a logic gate triac, so I know that I need more current to drive the triac. Do I have to fire it with positive current in one semi-cycle and negative on the other in respect to MT1 ?? What happens if I am using 220v with 2 phases instead of a phase and a neutral connection ? My power supply will be a capacitor coupled supply, without a transformer. Sorry to disturb with the OT question but I was not able to find these answers anywhere else and I have already digged all the ap-notes from teccor, motorola and many web sites. It seems that everyone prefers to use opto-couplers ! It does not make sense to me if I do not need the isolation. The isolation will be in the connection betwenn the controller board and the power board. Best regards, Alexandre Guimaraes spam_OUTalexgTakeThisOuTiis.com.br

-----Original Message----- From: Alexandre Guimaraes [SMTP:.....alexgKILLspam@spam@IIS.COM.BR] Sent: Monday, 6 March 2000 15:30 To: PICLISTKILLspamMITVMA.MIT.EDU Subject: [OT] Triac firing, zero cross detection. I have to control 8 triacs for dimming incandescent lamps with the minimum possible cost. I have been doing some research but there are still some aspects I would like if someone could help me with. Follows the list. - What is the best discrete zero crossing circuit ? I have found many, ranging from a single resistor, a resistor and a zener up to a full wave bridge with clamping diodes and a transistor. Do they all work the same in practice ? I guess they do not have to be very precise because I would not be able to fire the triac exactly at zero crossing anyway ! Is the optocoupler circuit better ? I do not thing so, specially in my case where I do not need isolation from the power line. Uhm, lets see, you are using a PIC computing device to fire a triac at zero crossing but can't put a delay/advance on top of the zero crossing you detect to get the firing exactly at the zero crossing. Are you sure you understand what PICs do? - How to fire the TRIAC without a Opto-diac ???? I have found many circuits and explanations again, but none has fully convinced me ! The circuits range from a single resistor from the microprocessor to a push-pull stage with transistors and a capacitor to the triac gate. I will not be using a logic gate triac, so I know that I need more current to drive the triac. Do I have to fire it with positive current in one semi-cycle and negative on the other in respect to MT1 ?? There are things out there called logic level triacs that only need a small resistor between the PIC and the triac. But you think they are too small (bet you that some company out there is making just what you want), so you need to use a resistor and a transistor to switch about 10V to gate a power triac. Note the word 'trigger' - it matters not which way the cycle is going; you just hold the trigger on for long enough for the triac to lock on and it will stay on until current flow ceases at the next zero crossing. Note the phrase 'current flow ceases' - if there is inductance in the load (or the cables leading to the load) the current will lag the voltage and flow will not cease at the next zero crossing so the triac will not switch off. You have to add a snubber circuit in real world conditions - all it would take is for a roady to leave 2 turns unrolled in a short extension lead to create enough inductance to keep the triac on without a snubber. What happens if I am using 220v with 2 phases instead of a phase and a neutral connection ? Nothing - just treat both lines as active to earth and insulate accordingly My power supply will be a capacitor coupled supply, without a transformer. Now you could have problems if the power supply is drawn from a different pair in the 3 phase cable (like 225V instead of 5V). Sorry to disturb with the OT question but I was not able to find these answers anywhere else and I have already digged all the ap-notes from teccor, motorola and many web sites. It seems that everyone prefers to use opto-couplers ! It does not make sense to me if I do not need the isolation. The isolation will be in the connection betwenn the controller board and the power board. Best regards, Alexandre Guimaraes .....alexgKILLspam.....iis.com.br

<x-flowed>At 01:30 AM 3/6/00 -0300, Alexandre Guimaraes wrote: > I have to control 8 triacs for dimming incandescent lamps with the >minimum possible cost. I have been doing some research but there are still >some aspects I would like if someone could help me with. Follows the list. > > - What is the best discrete zero crossing circuit ? I have found many, >ranging from a single resistor, a resistor and a zener up to a full wave >bridge with clamping diodes and a transistor. Do they all work the same in >practice ? I guess they do not have to be very precise because I would not >be able to fire the triac exactly at zero crossing anyway ! Is the >optocoupler circuit better ? I do not thing so, specially in my case where I >do not need isolation from the power line. What you need to evaluate is how noise near the zero crossing will affect you. Phase control dimmers are inherently noisy and the glitches caused by very low or very high phase angles (firing near the zero crossing) will seriously mess up many zero cross detectors. Then you can decide what suits your purpose. > - How to fire the TRIAC without a Opto-diac ???? I have found many >circuits and explanations again, but none has fully convinced me ! The >circuits range from a single resistor from the microprocessor to a push-pull >stage with transistors and a capacitor to the triac gate. I will not be >using a logic gate triac, so I know that I need more current to drive the >triac. Do I have to fire it with positive current in one semi-cycle and >negative on the other in respect to MT1 ?? What happens if I am using 220v >with 2 phases instead of a phase and a neutral connection ? My power supply >will be a capacitor coupled supply, without a transformer. I generally fire direct coupled triacs in quadrants 1 & 2 (positive gate drive for both positive and negative load current. How you drive the triacs depends upon your operating temperature (triacs at -40 require MUCH gate current) and hou much load you might be connecting (low current loads may require continous gate drive). My standard light chasers use a picket fence type pulse stream at about 250 mA but with a fairly low duty cycle (100 uSec pulses). This works reliably in the cold. Hope this helps. dwayne Dwayne Reid <EraseMEdwaynerspam_OUTTakeThisOuTplanet.eon.net> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 16 years of Engineering Innovation (1984 - 2000) * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. </x-flowed>

> Uhm, lets see, you are using a PIC computing device to fire a triac > at zero crossing but can't put a delay/advance on top of the zero crossing > you detect to get the firing exactly at the zero crossing. Are you sure you > understand what PICs do? I am quite sure I understand what PIC's do. I have been working with microprocessors for more than 15 years :-) What I meant is that the detection would not be exactly at the zero crossing because you would always have some difference induced by inductive loads and noise. I guess it would happen in any one of the circuits. That could not be compensated in software because it would not be constant. > There are things out there called logic level triacs that only need > a small resistor between the PIC and the triac. But you think they are too > small (bet you that some company out there is making just what you want), so > you need to use a resistor and a transistor to switch about 10V to gate a > power triac. The problem with logic level triacs is the price. They are nice for a hobby project but not for production where price is a major problem. > What happens if I am using 220v > with 2 phases instead of a phase and a neutral connection ? > > Nothing - just treat both lines as active to earth and insulate > accordingly I do not have a earth connection !!! Is is not mandatory to have earth connections in houses installations in Brazil !! The outlet output may have just 2 connections with 2 power phases. I hate it but it is true. > My power supply > will be a capacitor coupled supply, without a transformer. > > Now you could have problems if the power supply is drawn from a > different pair in the 3 phase cable (like 225V instead of 5V). That is not going to be the problem. It is going to be connected to the same phases of the load always. Thanks for the fast answer. Best regards, Alexandre Guimaraes alexgspam_OUTiis.com.br

Hi Alex, following some european tradition for residential installations, Brazil implemented the Grounded-Neutral solution, not that it is perfect, but it works with restrictions. Since the Neutral Wire current in residential installation is not that big, and does not generate several volts drops at the wire, then grounded-neutral works as acceptable. In south of Brazil, we already use the 3 pole outlet (with ground pin), since it offers much better ground safety, please observe that the words "mandatory" and "safe" aren't the same. Even the smallest computer (PC's) room (in the south) use the regular 3 pin (ground, phase, neutral). Of course, some cities still using those ugly old 220V installations with only Phase-Phase, have a floating neutral (non existant), so there is not a polarized pin at a 2 pin outlets. Alexandre Guimaraes wrote: > I do not have a earth connection !!! Is is not mandatory to have earth > connections in houses installations in Brazil !! The outlet output may have > just 2 connections with 2 power phases. I hate it but it is true.

Hello Alexandre, > - What is the best discrete zero crossing circuit ? I have found many, > ranging from a single resistor, a resistor and a zener up to a full wave > bridge with clamping diodes and a transistor. Do they all work the same in > practice ? I guess they do not have to be very precise because I would not > be able to fire the triac exactly at zero crossing anyway ! Is the > optocoupler circuit better ? I do not thing so, specially in my case where I > do not need isolation from the power line. It really depends on your application, if you give more datails, I could help you better. I've already made some prototypes using just a 2M resistor between the phase and the pic input, and it worked very well. > - How to fire the TRIAC without a Opto-diac ???? I have found many > circuits and explanations again, but none has fully convinced me ! The > circuits range from a single resistor from the microprocessor to a push-pull > stage with transistors and a capacitor to the triac gate. I will not be > using a logic gate triac, so I know that I need more current to drive the > triac. Do I have to fire it with positive current in one semi-cycle and > negative on the other in respect to MT1 ?? What happens if I am using 220v > with 2 phases instead of a phase and a neutral connection ? My power supply > will be a capacitor coupled supply, without a transformer. Sorry, can't help you much here because my circuits are copyrighted. I think the optos are unnecessary. Best regards, Brusque

<x-rich><color><param>0100,0100,0100</param>This is what I have found from the web: SC03_TECHN_PUBS_1.pdf from philips. 1863.pdf - Aplication note; MICROCONTROLLER AND TRIACS ON THE 110/240V MAINS from SGS Thomsom (ST). <color><param>7F00,0000,0000</param>> I have to control 8 triacs for dimming incandescent lamps with the {Quote hidden}> minimum possible cost. I have been doing some research but there are > still some aspects I would like if someone could help me with. Follows > the list. > > - What is the best discrete zero crossing circuit ? I have found > many, > ranging from a single resistor, a resistor and a zener up to a full > wave bridge with clamping diodes and a transistor. Do they all work > the same in practice ? I guess they do not have to be very precise > because I would not be able to fire the triac exactly at zero crossing > anyway ! Is the optocoupler circuit better ? I do not thing so, > specially in my case where I do not need isolation from the power > line. > > - How to fire the TRIAC without a Opto-diac ???? I have found many > circuits and explanations again, but none has fully convinced me ! The > circuits range from a single resistor from the microprocessor to a > push-pull stage with transistors and a capacitor to the triac gate. I > will not be using a logic gate triac, so I know that I need more > current to drive the triac. Do I have to fire it with positive current > in one semi-cycle and negative on the other in respect to MT1 ?? What > happens if I am using 220v with 2 phases instead of a phase and a > neutral connection ? My power supply will be a capacitor coupled > supply, without a transformer. > > Sorry to disturb with the OT question but I was not able to find > these > answers anywhere else and I have already digged all the ap-notes from > teccor, motorola and many web sites. It seems that everyone prefers to > use opto-couplers ! It does not make sense to me if I do not need the > isolation. The isolation will be in the connection betwenn the > controller board and the power board. > > Best regards, > Alexandre Guimaraes > @spam@alexgKILLspamiis.com.br ============================== Ruben Jvnsson AB Liros Elektronik Box 9124, 200 39 Malmv, Sweden TEL INT +46 40142078 FAX INT +46 40947388 KILLspamrubenKILLspampp.sbbs.se ============================== </x-rich>