Help with triac phase control - Page 1
Forum section: electronics / projects

author: alah
author_rank: Contributor
timestamp: October 31, 2017, 01:27:15 pm
content: Hi,ALL.The attatched circuit is from Fairchild AN 3006. My questions:Why rectify AC and use an opto to generate the trigger pulse train?Something to do with line synchronization?Whats the difference if they had configured the 555 as an astable oscillator?

author: ebclr
author_rank: Super Contributor
timestamp: October 31, 2017, 01:33:44 pm
content: Is zero crossing detector, forget this circuit, have easier ways to do that

author: Ian.M
author_rank: Super Contributor
timestamp: October 31, 2017, 01:43:31 pm
content: The advantage of the AN3006 circuit is its isolated, so you can easily connect a control voltage to the 555 to change the firing angle or even replace the 555 with a MCU and control the firing angle digitally. For the classic TRIAC lamp dimmer circuit, you have to use a pot with a plastic shaft for safety, and it isn't possible to use a control voltage or a MCU. Also its more likely to misbehave with a motor load.

author: Zero999
author_rank: Super Contributor
timestamp: October 31, 2017, 02:09:44 pm
content: The delay on the 555 timer is also dependant on the voltage on pin 5, so it can also be used as a voltage controlled dimmer.If the 555 timer is powered from a transformer+rectifier, then the H11L1 can be skipped and replaced with a comparator, or even a transistor, to implement the zero-crossing detection.

author: oldway
author_rank: Super Contributor
timestamp: October 31, 2017, 04:43:43 pm
content: The rectified voltage and the optocoupler are used to synchronize the 555 ... It needs pulses at 100 (or 120Hz) to do it.As Hero 999 wrote, this synchronization can be done from the 555's power supply.This circuit is isolated from the network and much safer than uninsulated circuits.

author: ebclr
author_rank: Super Contributor
timestamp: November 01, 2017, 03:51:39 am
content: You can change the pot for an LDR and is also isolated, a simple white led will control phase optically isolated, simple and cheap

author: Ian.M
author_rank: Super Contributor
timestamp: November 01, 2017, 06:36:46 am
content: .... and extremely non-repeatable as the LDR sensitivity changes with previous illumination history, declines with age and LED output also declines with age. N.B. analog output optocouplers comprising a LED and LDR are commercially available (at a price) under the trade name 'VACTROL'.

author: ebclr
author_rank: Super Contributor
timestamp: November 01, 2017, 07:37:42 am
content: I believe that accuracy isn't thing here, also the 555 capacitor is probably worse than the LDR on long therm

author: Ian.M
author_rank: Super Contributor
timestamp: November 01, 2017, 07:48:11 am
content: Read this Vactrol datasheet and weep!http://denethor.wlu.ca/pc300/optoisolators/analogoptoisolatorintroduction.pdfThe LDR resistance is highly sensitive to many things other than its current illumination level.One approach that could make it fairly consistent would be to use a batwing illumination pattern LED, and a matched pair of LDRs, one on each side of the isolation barrier equally illuminated by the LED, then servo the LED current with an OPAMP to close the loop using the input side LDR. Poorer but equivalent results could be achieved by two single Vactrols with their LEDs in series, one for output, and the other for feedback.

author: Zero999
author_rank: Super Contributor
timestamp: November 01, 2017, 09:20:40 am
content: Quote from: ebclr on November 01, 2017, 07:37:42 amIt depends on the capacitor. A polypropylene film capacitor with a tolerance of 5% and much lower drift, than an LDR will be much cheaper, than an LDR.

author: oldway
author_rank: Super Contributor
timestamp: November 01, 2017, 11:27:19 am
content: Your circuit has an horrible hysteresis ... At a minimum, it is necessary to use the modified schema.But insulated circuit is far safer....What kind of load ? Resistive ? Inductive ?

author: strawberry
author_rank: Super Contributor
timestamp: November 01, 2017, 12:44:39 pm
content: How about thishttps://www.eevblog.com/forum/projects/triac-power-load-regulator-project/msg1057089

author: Zero999
author_rank: Super Contributor
timestamp: November 01, 2017, 01:12:01 pm
content: Quote from: strawberry on November 01, 2017, 12:44:39 pmThat link just leads to the main page of the forum. Try this:https://www.eevblog.com/forum/projects/triac-power-load-regulator-project/

author: ebclr
author_rank: Super Contributor
timestamp: November 01, 2017, 03:54:18 pm
content: Do homes in Europe have isolated switches?

author: Zero999
author_rank: Super Contributor
timestamp: November 01, 2017, 06:55:53 pm
content: Quote from: ebclr on November 01, 2017, 03:54:18 pmWhat do you mean by isolated switches?

author: alah
author_rank: Contributor
timestamp: November 06, 2017, 03:17:49 pm
content: Quote from: ebclr on October 31, 2017, 01:33:44 pmI already had this circuit but the values I had didnt control the lower limit; yours does, thanksQuote from: Hero999 on October 31, 2017, 02:09:44 pmI need to limi components cost; 1 buck is worth 45.000,00 and going up,up ,up.This is jus for practice and understanding what I consider the most complex component; and after reading so many ANs, it gets confusing.My goal is ON/OFF MC control of a 3hp freezer compressor, but Im sure that should be another post ( topic ).Sorry for my late response; global comm. is bad and I do some reseach and testing before replyng THANKS to ALL.

author: alah
author_rank: Contributor
timestamp: November 07, 2017, 02:03:43 pm
content: Hi, I found a nice AN; its Littlefuse AN1003; simple circuits, some eliminate histeresis.Another question: Can the load be a bridge rectifier so as to control he rectified and filtered voltage? Suggestions, please.